Transcript Step 1

Grounding, etc
1
https://www.youtube.com/watch?v=awrUxv7B-a8
Single Phase Motors
2
The 10 Worst Grounding Mistakes You'll Ever Make
Aug 1, 2008David Herres | Electrical Construction and Maintenance
Why common errors in residential, commercial, and industrial wiring can lead to fire and electric shock hazards.
Proper grounding and bonding prevent unwanted voltage on non-currentcarrying metal objects, such as tool and appliance casings, raceways, and
enclosures, as well as facilitate the correct operation of overcurrent
devices. But beware of wiring everything to a ground rod and considering
the job well done. There are certain subtleties you must follow to adhere to
applicable NEC rules and provide safe installations to the public and
working personnel. Although ground theory is a vast subject, on which
whole volumes have been written, let's take a look at some of the most
common grounding errors you may run into on a daily basis.
3
1. Improper replacement of non-grounding receptacles. Dwellings and nondwellings often contain non-grounding receptacles (Photo 1). It's not the
NEC's intent to immediately replace all noncompliant equipment with each
new edition of the Code. In fact, it's perfectly fine to leave the old “two
prongers” in place. But because an intact functioning equipment ground is
such an obvious safety feature, most electricians tend to replace these old
relics whenever possible.
Photo 1. This non-grounding receptacle is typical of those found in older homes across the country.
There are several ways you can complete this upgrade, many of which are
erroneous and strictly against the Code. For example, never apply the
following non-NEC-compliant solutions:
4




Hook up a new grounding receptacle on the theory that this is a step in the right
direction. This can lead future electricians and occupants to believe they are fully
protected by a non-functioning ground receptacle.
Connect the green grounding terminal of a grounded receptacle via a short
jumper to the grounded neutral conductor. This practice is totally noncompliant
and dangerous because when a load is connected, voltage will appear on both
the neutral and ground wires. Therefore, any noncurrent-carrying appliance or
tool case will become energized, causing shock to the user, who is typically
partially or totally grounded.
Run an individual ground conductor from the green grounding terminal of a
grounded receptacle to the nearest water pipe or other grounded object. This
“floating ground” presents various hazards. It is likely that this ground rod of
convenience will have several ohms of ground resistance so that, in case of
ground fault within a connected tool or appliance, the breaker will not trip —
and exposed metal will remain energized.
Run an individual ground conductor back to the entrance panel and connect it to
the neutral bar or grounding strip. This solution is somewhat better, but still
noncompliant. Any grounding conductor must be within the circuit cable or
raceway. One objection is that an individual conductor could be damaged or
removed in the course of work taking place in the future.
5
What are the correct ways to handle this type of situation, when you find yourself
working with non-grounded receptacles?



The best approach is to run a new branch circuit back to the panel, verifying
presence of a valid ground. Because this procedure usually involves fishing
cable behind walls or, in some cases, removing and then replacing wall finish,
it's not always feasible unless a total rewiring job is being performed.
Another possibility is to replace the two-prong receptacle with a GFCI. Hook up
the two wires and leave the grounding terminal unattached. Included with the
GFCI is a sticker that says, “No equipment ground.” This sticker must be in
place so that future electricians and users are not misled. The thinking behind
this strategy is that even though the tool or appliance case is not grounded,
the GFCI will provide enhanced safety. It's important to note that a GFCI
functions properly without the presence of a grounding conductor. The device
compares current flowing through the hot and neutral conductors and trips if a
difference of more than 5 milliamps is detected.
Non-grounding receptacles are still manufactured. If replacement is necessary
(and acquiring a ground is not feasible), installation of a new non-grounding
receptacle is a way to go.
6
2. Installation of a satellite dish, telephone, CATV, or other low-voltage
equipment without proper grounding.
If you look at a number of satellite dish installations in your neighborhood, a
certain percentage will inevitably not be grounded at all. Of those that are
grounded, there is still a high probability many are not fully compliant. For
example, the grounding electrode conductor could be too long, too small, have
unlisted clamps at terminations, have excess bends, or be connected to a single
ground rod but not be bonded to other system grounds.
For NEC purposes, a satellite dish is an antenna, and installation requirements
are found in Chapter 8, Communications Systems. Article 810, Radio and
Television Equipment, details the installation requirements. Part II deals with
receiving Equipment — Antenna Systems. This type of equipment, which
includes the satellite dish, must have a listed antenna discharge unit, which can
be either outside the building or inside between the point of entrance of the
lead-in conductors and the receiver — and as near as possible to the entrance
of the conductors to the building. The antenna discharge unit is not to be
located near combustible material and certainly not within a hazardous
(classified) location.
The antenna discharge unit must be grounded. The grounding conductor is
usually copper; however, you can use aluminum or copper-clad aluminum if it's
not in contact with masonry or earth. Outside, aluminum or copper-clad
aluminum cannot be within 18 inches of the earth.
7
Photo 2. Grounding means for a satellite dish must be located at the point
of entrance to the building. In this particular installation, the grounding
conductor is integral with the coax from the dish, but the installer did not
bond it to other system grounds.
8
The grounding conductor can be bare or insulated, stranded or solid, and
must be securely fastened in place and run in a straight line from the
discharge unit to the grounding electrode (Photo 2). If the building has an
intersystem bonding termination, the grounding conductor is to be
connected to it or to one of the following:


Grounding electrode system.
Grounded interior metal water piping system within 5 feet of point of
entrance to the building.

Power service accessible grounding means external to the building.

Metallic power service raceway.

Service equipment enclosure.

Grounding electrode conductor or its metal enclosure.
9
If this grounding conductor is installed within a metal raceway, you must bond
the metal raceway to it at both ends. For this reason, if raceway is deemed
necessary for extra protection, UL-listed PVC (rigid non-metallic conduit) is
generally used. The grounding conductor must be no smaller than 10 AWG
copper.
Where separate electrodes are used, you must connect the antenna
discharge unit grounding means to the premises power system grounding
system by a 6 AWG copper conductor. Needless to say, grounding a satellite
dish goes well beyond simply driving a ground rod at the point of entrance.
Grounding for CATV is slightly different. Typically, CATV is brought into the
building via coaxial cable, which has a center conductor, insulating spacer, and
outer electrical shield. Because of the spacer, capacitive coupling is
diminished so that the cable provides a high-quality signal for data, voice, and
video transmission. Improper grounding of coaxial cable used for CATV is very
common.
There is no antenna discharge unit as required for satellite dish installation.
Instead, the shield of the coaxial cable is connected to an insulated grounding
conductor that is limited to copper but may be stranded or solid. The
grounding conductor is 14 AWG minimum so that it has current-carrying
capacity approximately equal to the outer shield of the coaxial cable.
10
The major distinguishing characteristic is that for one- and two-family homes
the grounding conductor cannot exceed 20 feet in length and should
preferably be shorter. If a grounding electrode such as the Intersystem
Bonding Termination is not within 20 feet, it is necessary to drive a ground
rod for that purpose. However, even after this dedicated grounding means is
established, in order to be NEC-compliant, the installation must have a
bonding jumper not smaller than 6 AWG or equivalent, which is connected
between the CATV system's grounding electrode and the power grounding
electrode system for the building. Omitting this jumper is a serious Code
violation, second only to no grounding at all. You must bond all system
grounds, antenna, power, CATV, telephone, and so on with a heavy bonding
jumper.
3. Non-installation of GFCIs where required. Recent Code editions have
mandated increased use of GFCIs. In dwelling units, GFCIs are required on all
125V, single-phase, 15A and 20A receptacles in: bathrooms; garages;
accessory buildings with a floor at or below grade level not intended as a
habitable room, limited to storage, work and similar areas; outdoors;
kitchens along countertops; within 6 feet of outside edge of laundry, utility,
and wet bar sinks; and boathouses. In other than dwelling units, GFCIs are
required on all 125V, single-phase, 15A and 20A receptacles in bathrooms,
kitchens, rooftops, outdoors, and within 6 feet of the outside edge of sinks.
11
Other areas requiring the use of GFCIs include: boat hoists, aircraft
hangars, drinking fountains, cord- and plug-connected vending machines,
high-pressure spray washers, hydromassage bathtubs, carnivals, circuses,
fairs (and the like), electrically operated pool covers, portable or mobile
electric signs, electrified truck parking space supply equipment, elevators,
dumbwaiters, escalators, moving walks, platform lifts/stairway chairlifts,
fixed electric space heating cables, fountains, commercial garages,
electrical equipment for naturally and artificially made bodies of water,
pipeline heating, therapeutic pools and tubs, boathouses, construction
sites, health-care facilities, marinas/boatyards, pools, recreational
vehicles, sensitive electronic equipment, spas, and hot tubs.
4. Improperly connecting the equipment-grounding conductor to the
system neutral. You must connect a grounded neutral conductor to
normally noncurrent-carrying metal parts of equipment, raceways, and
enclosures only through the main bonding jumper (or, in the case of a
separately derived system, through a system bonding jumper). Make this
connection at the service disconnecting means, not downstream. When
you buy a new entrance panel, a screw or other main bonding jumper is
usually included in the packaging.
12
Attached to it are instructions stipulating that it is to be installed only when the
panel is to be used as service equipment.
It's a major error to install a main bonding jumper in a box used as a subpanel
fed by a 4-wire feeder. It's also wrong not to install it when the panel is used as
service equipment. Improper redundant connection of grounded neutral to
equipment-grounding conductors can result in objectionable circulating current
and presence of voltage on metal tool or appliance casings. You should connect
grounded neutral and equipment-grounding conductors at the service
disconnect. Then separate them — never to rejoin again. Additional optional
ground rods may be connected anywhere along the equipment-grounding
conductor but never to the grounded neutral.
5. Improperly grounding frames of electric ranges and clothes dryers. Prior to
the 1996 version of the NEC, it was common practice to use the neutral as an
equipment ground. Now, however, all frames of electric ranges, wall-mounted
ovens, counter-mounted cooking units, clothes dryers, and outlet or junction
boxes that are part of these circuits must be grounded by a fourth wire: the
equipment-grounding conductor.
An exception permits retention of the pre-1996 arrangement for existing
branch-circuit installations only where an equipment-grounding conductor is
not present. Several other conditions must be met. If possible, the best course
of action is to run a new 4-wire branch circuit from the panel. If you must keep
an old appliance, be sure to remove the neutral to frame bonding jumper if an
equipment-grounding conductor is to be connected.
13
6. Failure to ground submersible well pumps. At one time, submersible well
pumps were not required to be grounded because they were not considered
accessible. However, it was noted that workers would pull the pump, lay it on
the ground, and energize it to see if it would spin. If, due to a wiring fault, the
case became live, the overcurrent device would not function, causing a shock
hazard. The 2008 NEC requires a fourth equipment-grounding conductor that
you must now lug to the top of the well casing. Many people assume that in
a 3-wire submersible pump system one wire is a “ground.” In actuality,
submersible pump cable consists of three wires (plus equipment-grounding
conductor) twisted together and unjacketed. Yellow is a common 240V leg,
black is run, and red is start, which the control box energizes for a short
period of time. Prior to the new grounding requirement, everything was hot.
7. Failure to properly attach the ground wire to electrical devices. Wiring
daisy-chained devices in such a way that removing one of them breaks the
equipment grounding continuity is a common problem. The preferred way to
ground a wiring device is to connect incoming and outgoing equipmentgrounding conductors to a short bare or green jumper. The bare or green
insulated jumper is then connected to the grounding terminal of the device.
14
8. Failure to install a second ground rod where required. A single ground
rod that does not have a resistance to ground of 25 ohms or less must be
augmented by a second ground rod. Once the second ground rod is
installed, it's not necessary for the two to meet the resistance
requirement. As a practical matter, few electricians do the resistance
measurement.
Figure. Non-overlapping effective resistance areas reduce net resistance.
You cannot use a simple ohmmeter because that would require a known
perfect ground. Special equipment and procedures are needed, so it's
common practice to simply drive a second ground rod. You must locate
them at least 6 feet apart. Greater distance is even better (Figure). If both
rods and the bare ground electrode conductor connecting them are directly
under the drip line of the roof, ground resistance will be further diminished.
This is because the soil along this line is more moist. Ground resistance
15
greatly increases when soil becomes dry.
9. Failure to properly reattach metal raceway that is used as an equipmentgrounding conductor. When equipment is relocated, replaced, or removed
for repair, many times equipment ground paths are broken. If these
connections are not fixed, there's an accident waiting to happen (Photo 3).
Setscrews, locknuts, and threads should be fully engaged and continuity tests
performed before equipment is put back into service. Dirt and corrosion can
also compromise ground continuity.
Photo 3. Standard locknuts or bushings shall not be the sole connection for grounding purposes.
NEC Article 250.4 requires that electrical equipment, wiring, and other
electrically conductive material likely to become energized shall be installed
in a manner that creates a low-impedance circuit from any point on the
wiring system to the electrical supply source to facilitate the operation of
overcurrent devices.
16
10. Failure to bond equipment ground to water pipe. Improper connections
are often seen in the field. Screw clamps and other improvised connections
do not provide permanent low impedance bonding. The worst method
would be to just wrap the wire around the pipe or to omit this bonding
altogether.
Photo 4. Someone used a water pipe clamp to improperly connect a ground wire to this ground rod.
In a dwelling, a conductor must be run to metallic water pipe, if present, and
connected with a UL-listed pipe grounding clamp (Photo 4). This bonding
conductor is to be sized according to Table 250.66, based on the size of the
largest ungrounded service entrance conductor or equivalent area for parallel
conductors.
Herres is a licensed master electrician in Stewartstown, N.H.
17
Motor Calculations: Motors and
Branch-Circuit Conductors
Overcurrent and short-circuit protection aren’t the same for
motors
18
The best method for providing overcurrent protection for most circuits is to use
a circuit breaker that combines overcurrent protection with short-circuit and
ground-fault protection. However, this isn't usually the best choice for motors.
With rare exceptions, the best method for providing overcurrent protection in
these cases is to separate the overload protection devices from the short-circuit
The best method for providing overcurrent protection for most circuits is to use
a circuit breaker that combines overcurrent protection with short-circuit and
ground-fault protection. However, this isn't usually the best choice for motors.
With rare exceptions, the best method for providing overcurrent protection in
these cases is to separate the overload protection devices from the short-circuit
and ground-fault protection devices (Fig. 1).
Motor overload protection devices like heaters protect the motor, the motor
control equipment, and the branch-circuit conductors from motor overload and
the resultant excessive heating (430.31). They don't provide protection against
short-circuits or ground-fault currents. That's the job of the branch and feeder
breakers, which don't provide motor overload protection. This arrangement
makes motor calculations different from those used for other types of loads.
Let's look at how to apply Art. 430, starting at the motor.
19
Overload protection. Motor overload devices are often integrated into the
motor starter. But you can use a separate overload device like a dual-element
fuse, which is usually located near the motor starter, not the supply breaker.
Fig. 1. Overcurrent protection is generally accomplished by separating the overload protection from the short-circuit and
ground-fault protection device.
If you use fuses, you must provide one for each ungrounded conductor (430.36
and 430.55). Thus, a 3-phase motor requires three fuses. Keep in mind that
these devices are at the load end of the branch circuit and that they don't
20
provide short-circuit or ground-fault protection.
Motors rated more than 1 hp without integral thermal protection and motors
rated 1 hp or less that are automatically started [430.32(C)] must have an
overload device sized per the motor nameplate current rating [430.6(A)]. You
must size the overload devices no larger than the requirements of 430.32.
Motors with a nameplate service factor (SF) rating of 1.15 or more must have an
overload protection device sized no more than 125% of the motor nameplate
current rating.
Fig. 2. When working with motors that have a service factor rating of 1.15 or higher, size overload protection
devices no more than 125% of the motor nameplate rating.
21
Let's look at Fig. 2 and work through a sample calculation.
Example No. 1: Suppose you use a dual-element fuse for overload
protection. What size fuse do you need for a 5-hp, 230V, single-phase motor
with a service factor of 1.16 if the motor nameplate current rating is 28A?
(a) 25A
(c) 35A
(b) 30A
(d) 40A
The overload protection shall be sized according to the motor nameplate
current rating [430.6(A), 430.32(A)(1), and 430.55].
You also have to consider another factor: nameplate temperature rise. For
motors with a nameplate temperature rise rating not over 40°C, size the
overload protection device no more than 125% of the motor nameplate
current rating. Thus, 28A×1.25=35A [240.6(A)]
22
Fig. 3. Size the overload protection device of a motor with a nameplate temperature rise rating of 40°C or less at
no more than 125% of the motor nameplate current rating.
23
Let's look at Fig. 3 and work through another example problem.
Example No. 2: Again, suppose you're using a dual-element fuse for the
overload protection. What size fuse do you need for a 50-hp, 460V, 3-phase
motor that has a temperature rise of 39°C and motor nameplate current
rating of 60A (FLA)?
(a) 40A
(c) 60A
(b) 50A
(d) 70A
The overload protection is sized per the motor nameplate current rating,
not the motor full load current (FLC) rating. Thus, 60A×1.25=75A. Overload
protection shall not exceed 75A, so you need to use a 70A dual-element
fuse [240.6(A) and 430.32(A)(1)].
Motors that don't have a service factor rating of 1.15 or higher or a
temperature rise rating of 40°C and less must have an overload protection
device sized at not more than 115% of the motor nameplate ampere rating
(430.37).
24
Fig. 4. Refer to Table 310.16 when selecting the proper size conductor to
serve a single motor.
25
Sizing branch-circuit conductors. Branch-circuit conductors that serve a single
motor must have an ampacity of not less than 125% of the motor's FLC as listed
in Tables 430.147 through 430.150 [430.6(A)]. You must select the conductor size
from Table 310.16 according to the terminal temperature rating (60°C or 75°C)
of the equipment [110.14(C)]. Let's reinforce this concept by working through a
sample calculation. Refer to Fig. 4.
Example No. 3: What size THHN conductor do you need for a 2-hp, 230V, singlephase motor?
(a) 14 AWG
(c) 10 AWG
(b) 12 AWG
(d) 8 AWG
Let's walk through the solution:
Step 1: Conductor sized no less than 125% of motor FLC
Step 2: Table 430.148 shows the FLC of 2-hp, 230V, single-phase as 12A
Step 3: 12A × 1.25 = 15A
Step 4: Per Table 310.16, you need to use 14 AWG THHN rated 20A at 60°C
26
The minimum size conductor the NEC permits for building wiring is 14
AWG [310.5]. However, local codes and many industrial facilities have
requirements that 12 AWG be used as the smallest branch-circuit wire. So
in this example you might need to use 12 AWG instead of 14 AWG.
Fig. 5. Short-circuit and ground-fault protection devices are designed for fast
current rise, short-duration events. On the other hand, overload protection devices
27
are designed for slow current rate, long-duration situations.
Branch-circuit protection for short-circuits and ground-faults.
Branch-circuit short-circuit and ground-fault protection devices protect
the motor, motor control apparatus, and conductors against short circuits
or ground faults. They don't protect against an overload (430.51) (Fig. 5).
The short-circuit and ground-fault protection device required for motor
circuits isn't the type required for personnel (210.8), feeders (215.9 and
240.13), services (230.95), or temporary wiring for receptacles (527.6).
Per 430.52(C), you must size the short-circuit and ground-fault protection
for the motor branch circuit — except those that serve torque motors —
so they're no greater than the percentages listed in Table 430.52.
When the short-circuit and ground-fault protection device value that you
find in Table 430.52 doesn't correspond to the standard rating or setting
of overcurrent protection devices as listed in 240.6(A), use the next
higher protection device size [430.52(C)(1) Ex. 1].
28
Did that statement stop you? Does it strike you as incorrect? That's a common
response, but remember, motors are different than other system
components. Motor overload protection devices, such as heaters and fuses,
protect the motor and other items from overload. The short-circuit and
ground-fault protection doesn't need to perform this function. Therefore,
oversizing won't compromise protection. Undersizing will prevent the motor
from starting.
Use the following two-step process to determine what percentage from Table
430.52 you should use to size the motor branch-circuit short-circuit groundfault protection device.
Step 1: Locate the motor type on Table 430.52.
Step 2: Select the percentage from Table 430.52 according to the type of
protection device, such as non-time delay (one-time), dual-element fuse, or
inverse-time circuit breaker. Don't forget to use the next higher protection
device size when necessary.
Let's see if you have this concept down with a short quiz. Of the following
statements, which one is true? Use Table 430.52 to look up the numbers.
29
1. The branch-circuit short-circuit protection (non-time delay fuse) for a 3-hp,
115V, single-phase motor shall not exceed 110A.
2. The branch-circuit short-circuit protection (dual-element fuse) for a 5-hp,
230V, single-phase motor shall not exceed 50A.
3. The branch-circuit short-circuit protection (inverse-time breaker) for a 25hp, 460V, 3-phase synchronous motor shall not exceed 70A.
Let's address each question individually. We'll be referring to 430.53(C)(1) Ex. 1
and Table 430.52.
1. Per Table 430.148, 34A×3.00=102A. The next size up is 110A. So this is true.
2. Per Table 430.148, 28A×1.75=49A. The next size up is 50A. So, this is also
true.
3. Per Table 430.150, 26A×2.50=65A. The next size up is 70A. This is also true.
30
Remember the following important principles:



You must size the conductors at 125% of the motor FLC [430.22(A)].
You must size the overloads no more than 115% to 125% of the motor
nameplate current rating, depending on the conditions [430.32(A)(1)].
You must size the short-circuit ground-fault protection device from
150% to 300% of the motor FLC [Table 430.52].
If you put all three of these together, you can see the branch-circuit
conductor ampacity (125%) and the short-circuit ground-fault protection
device (150% to 300%) aren't related.
This final example should help you see if you've been paying attention.
31
Fig. 6. Although this example may bother some people, the 14 AWG THHN conductors and motor are
protected against overcurrent by the 16A overload protection device and the 40A short-circuit
protection device.
32
Example No. 4: Are any of the following statements true for a 1-hp, 120V
motor, nameplate current rating of 14A? Refer to Fig. 6.
(a) The branch-circuit conductors can be 14 AWG THHN.
(b) Overload protection is from 16.1A.
(c) Short-circuit and ground-fault protection is permitted to be a 40A
circuit breaker.
(d) All of these are true.
Walking through each of these, you can see:
(a) The conductors are sized per 430.22(A): 16A×1.25=20A; Table 310.16
requires 14 AWG at 60°C.
(b) Per 430.32(A)(1), overload protection is sized as follows: 14A
(nameplate)×1.15=16.1A.
(c) Short-circuit and ground-fault protection is determined based on
430.52(C)(1): 16A×2.50=40A circuit breaker.
Therefore all three statements are true.
33
The 16A overload protection device protects the 14 AWG conductors from
overcurrent, while the 40A short-circuit protection device protects them from
short circuits. This example illustrates the sometimes confusing fact that when
you're doing motor calculations, you're actually calculating overcurrent and
short-circuit protection separately.
Motor calculations have long been a source of confusion and errors for many
people. Understanding what makes these calculations different should help
you do your motor calculations correctly every time. Next month we'll look at
sizing motor feeders in Part 2
34
Motor Calculations Part 2: Feeders
Part 1 of this two-part series explained how to size overload protection devices
and short-circuit and ground-fault protection for motor branch circuits.
Understanding the key point of that article, which was that motor overload
protection requires separate calculations from short-circuit and ground-fault
protection, clears up a common source of confusion and a point of error. But
another source of confusion arises when it comes to sizing short-circuit and
ground-fault protection for a feeder that supplies more than one motor. Let's
look again at branch-circuit calculations and then resolve the feeder issues so
your calculations will always be correct.
Branch-circuit conductors and protection devices
Per 430.6(A), branch-circuit conductors to a single motor must have an
ampacity of not less than 125% of the motor full load current (FLC) as listed in
Tables 430.147 through 430.150. To illustrate this, let's size the branch-circuit
conductors (THHN) and short-circuit ground-fault protection device for a 3-hp,
115V, single-phase motor. The motor FLA is 31A, and dual-element fuses for
short-circuit and ground-fault protection are in use (Fig. 1).
35

Per Table 430.148, the FLC current is 34A.

34A×125%=43A.

Per Table 310.16 (60°C terminals [110.14(C)(1)(a)]), the conductor must
be a 6 AWG THHN rated 55A.
Fig. 1. Don’t make the mistake of using a motor’s FLA nameplate rating when
using the short-circuit and ground-fault protection devices. You must use the FLC
36
rating given in Table 430.148.
Per the motor FLC listed in Table 430.52, size the branch-circuit short-circuit and
ground-fault protection devices by using multiplication factors based on the type
of motor and protection device. When the protection device values determined
from Table 430.52 don't correspond with the standard rating of overcurrent
protection devices listed in 240.6(A), you must use the next higher overcurrent
protection device. To illustrate this, let's use the same motor as in the previous
example.

Per 240.6(A), multiply 34A×175%

You need a 60A dual-element fuse.
To explore this example further, see Example No. D8 in Annex D of the 2002 NEC.
Once you've sized the motor overloads, branch-circuit conductors, and branchcircuit protective devices, you're ready to move on to the next step.
Motor feeder conductor calculations
From 430.24, you can see that conductors that supply several motors must have
an ampacity not less than:



125% of the highest-rated motor FLC [430.17], plus
The sum of the FLCs of the other motors (on the same phase), as determined
by 430.6(A), plus
The ampacity required to supply the other loads on that feeder.
37
Fig. 2. Motor feeder conductors shall be sized not less than 125% of the largest
motor FLC plus the sum of the FLCs of the other motors on the same phase.
38
Use Fig. 2 and solve the following problem.
Example No. 1. For what ampacity must you size the feeder conductor if
it supplies the following two motors? The terminals are rated for 75°C.

One 7.5-hp, 230V (40A), single-phase motor

One 5-hp, 230V (28A), single-phase motor
(a) 50A
(b) 60A
(c) 70A
(d) 80A
Let's walk through the solution.

The largest motor is 40A.

40A×1.25+28A=78A.

80A is the closest selection that's at least 78A.
39
What size conductor would give us this ampacity
(a) 2 AWG
(b) 4 AWG
(c) 6 AWG
(d) 8 AWG
Per Table 310.16, a 6 AWG conductor rated at 75°C provides 65A of
ampacity, so it's too small. However, a 4 AWG conductor provides 85A of
ampacity, which will accommodate the necessary 78A. Therefore, you
need to size this feeder conductor at 4 AWG.
Next, we have to determine what size overcurrent protection device
(OCPD) we must provide for a given feeder.
40
Fig. 3. To size overcurrent protection devices for each feeder, start by determining the
ampacities required for each motor and move on from there.
41
Example No. 2. Using a slightly more complex example, try sizing the feeder
conductor (THHN) and protection device (inverse-time breakers, 75°C
terminal rating) for the following motors (Fig. 3):

Three 1-hp, 120V, single-phase motors

Three 5-hp, 208V, single-phase motors

One wound-rotor, 15-hp, 208V, 3-phase motor
Refer to 240.6(A), 430.52(C)(1), Table 430.148, and Table 430.52. Start by
determining the ampacities required for each size of motor, then walk
through each step until you arrive at the correct OCPD size.
1-hp motor: FLC is 16A.
16A×250%=40A
5-hp motor: FLC is 30.8A.
30.8A×250%=77A (Next size up is 80A.)
15-hp motor: FLC is 46.2A.
42
Fig. 4. Each motor’s FLC will come into play when sizing the conductor.
43
46.2A×150% (wound-rotor) 569A (Next size up is 70A.)
Now, let's look at the feeder conductor. Conductors that supply several motors
must have an ampacity of not less than 125% of the highest-rated motor FLC
(430.17), plus the sum of the other motor FLCs [430.6(A)] on the same phase (Fig.
4).
Continuing with this example, add up all the ampacities, multiplying the highest
rated motor by 125%. Thus:
(46.2A×1.25)+30.8A+30.8A+16A=136A.
Table 310.16 shows you need 1/0 AWG THHN because at 150A it's the smallest
conductor that accommodates the 136A of ampacity we're working with. When
sizing the feeder conductor, be sure to include only the motors that are on the
same phase. For that reason, these calculations only involve four motors.
You must provide the feeder with a protective device with a rating or setting not
greater than the largest rating or setting of the branch-circuit short-circuit and
ground-fault protective device (plus the sum of the full-load currents of the other
motors of the group) [430.62(A)]. Remember, motor feeder conductors must be
protected against the overcurrent that results from short circuits and ground
faults but not those that result from motor overload. When sizing the feeder
protection, be sure to include only the motors that are on the same phase.
44
Fig. 5. In this example, the largest branch-circuit fuse or circuit breaker allowed for Motor 1 is 70A.
45
Refer to Fig. 5 for this sample motor feeder protection calculation.
Example No. 3. What size feeder protection (inverse-time breaker) do you need for
the following two motors?

5-hp, 230V, single-phase motor

3-hp, 230V, single-phase motor
(a) 30A breaker
(b) 40A breaker
(c) 50A breaker
(d) 80A breaker
Let's walk through the solution.
Step 1: Get the motor FLC from Table 430.148.

A 5-hp motor FLC is 28A.

A 3-hp motor FLC is 17A.
46
Step 2: Size the branch-circuit protection per the requirements of 430.52(C)(1),
Table 430.52, and 240.6(A)

5-hp: 28A×2.5=70A

3-hp: 17A×2.5=42.5A (Next size up is 45A.)
Step 3: Size the feeder conductor per 430.24(A).
The largest motor is 28A.
(28A×1.25)+17A=52A
Table 310.16 shows 6 AWG rated 55A at 60°C as the smallest conductor with
sufficient ampacity.
Step 4: Size the feeder protection per 430.62.
It must not be greater than the 70A protection of the branch circuit plus the
17A of the other motor, which is the total of all loads on that feeder.
70A+17A=87A
Choose the next size down, which is 80A.
47
How can you be safe if you're selecting the next size down instead of the next
size up? Remember, you've already accounted for all the loads, and the NEC
requires that you not exceed the protection of the branch circuit. Again, keep
in mind that you aren't calculating for motor overload protection. Motor
calculations are different from other calculations. With motor feeders, you're
calculating for protection from short circuits and ground faults, only — not
overload.
Putting it all together
Motor calculations get confusing if you forget there's a division of
responsibility in the protective devices. To get your calculations right, you
must separately calculate the motor overload protection (typically near the
motor), branch-circuit protection (from short circuits and ground faults), and
feeder-circuit protection (from short circuits and ground faults). Remember
that overload protection is only at the motor.
Any time you find yourself confused, just refer to NEC Figure 430.1. It shows
the division of responsibility between different forms of protection in motor
circuits. Example D8 in Annex D of the 2002 NEC illustrates this with actual
numbers. Keeping this division of responsibility in mind will allow you to make
correct motor calculations every time.
48
Sizing Circuit Protection and
Conductors — Part 1
Suppose you have a 60A breaker supplying a branch circuit. What size
conductors do you need for that circuit? One table in the NEC says No. 8;
another says No. 4. Which one is right? What does the note about ambient
temperature correction factors mean? And how do you know you need that
size breaker in the first place?
Let’s start with looking at the order of the calculations. The overcurrent
protection device (OCPD) — whether a breaker or a fuse — defines the
circuit. So sizing the OCPD logically comes before sizing the conductors. But
before you can size the OCPD, you have to determine what load it will supply.
49
As part of collecting data for sizing branch circuits and feeders for an
addition, Seth White, a field technician with Industrial Tests, Inc., Rocklin,
Calif., is using the power monitor on a similar installation to determine which
loads really are continuous and whether the majority are non-linear.
50
Load calculations
Loads may be continuous (operating 3 hr or longer) or noncontinuous. Your
first step in branch circuit load calculations is to characterize each load as
either continuous or noncontinuous. This distinction is critical. If you
mischaracterize just one continuous load as noncontinuous, you could
undersize the OCPD and the conductors.
Make sure you avoid the trap of assuming lighting is noncontinuous if
controlled by occupancy sensors. There’s no guarantee they won’t run less
than 3 hr. Even if the controls shut the lights off after 2 hr, sensor trips can
keep extending the runtime. Similar logic applies to other types of loads that
are on automatic control.
A good practice when performing branch circuit calculations is to make a table
with two columns — one for
continuous and one for noncontinuous. For each load you can determine that
will run less than 3 hr at a time, list it in the noncontinuous column. List all
other loads in the continuous column.
You also need to determine if the load you’re supplying is a specific-purpose
load covered by another Article in the NEC. To do that, look for your load type
in Table 210.2. If it’s a motor, it’s always covered by Art. 430. If it’s a hermetic
motor (used in air-conditioners, refrigeration units, and chillers), it’s covered
by Art. 430 as well as by
Art. 440 [440.3].
51
At this point, it seems we’re ready to multiply the total of the continuous loads
by 125% [210.19(A)] and then add that number to the total of the
noncontinuous loads. For branch circuits, it’s usually just that simple — but
not always.
Some loads operate in a mutually exclusive fashion. That’s why when you’re
sizing feeders, for example, you use the larger of the heating or airconditioning load when determining the total load.
Rarely does a branch circuit have loads that are mutually exclusive. However, it
can happen. For example, consider an industrial shop that has one portable
arc welder and 10 welding outlets. Only one outlet will be in use at any given
time (the others are then excluded by dint of not having an arc welder), so all
of these are on a single branch circuit.
52
OCPD sizing
Once you’ve correctly calculated the load, you’re ready to size the OCPD.
Breakers and fuses come in standard sizes [240.6]. To size the OCPD, follow
what’s known as the “next-size-up” rule. Look at the standard sizes that are
larger than your load, and pick the one that’s closest. Do not apply this to
motor overload protection. For motor OCPDs, refer to Art. 430. But wait. You
calculated your load in VA, and the OCPDs are rated in amps. You’ll have to
convert VA to amps. In a DC circuit, you’d just divide VA by the nominal
voltage to derive amps. For AC loads, you must also divide by the appropriate
phase factor.
For 3-phase loads, you divide the VA by the nominal voltage and by the
square root of three (approximately 1.732). If your total 3-phase load in a
480V system is 50,000VA, what size breaker do you need?
50,000VA ÷ (480V × 1.732) = 60.2A. The next size up is 70A.
Note that some types of single-phase loads are routinely supplied by a 3phase panel. Lights, for example, are single-phase loads. Just make sure you
wire the lighting system to achieve balance across the phases.
53
Conductor sizing
Because sizing conductors isn’t a one-size-fits-all process, the NEC has several
ampacity tables. Making matters a bit more complicated, the tables have
multiple temperature-rating columns. You can avoid confusion here if you
understand the logic of the tables.
After you select the right table and column in that table, you must apply the:

Temperature correction factors [310.15(B)(2)], and

Adjustment factors [310.15(B)(3)].
Depending upon your application, you might also need to apply the correction
factors in 310.15(B)(4) through 310.15(B)(7).
The temperature correction factors are based on the expected ambient
temperature where you’re installing the conductor. Use the peak expected
temperature, not an average.
54
Dick Reese, an electrical power engineer with Industrial Tests, Inc., Rocklin, Calif., analyzes
his coordination study to determine whether the system’s new conductor size/type will
cause any unnecessary faults or nuisance trips.
55
Adjustment factors vary, depending upon the wiring method. At this point, you
should already know the type of raceway and conductor you’re using. You
should also know if a raceway or cable will carry more than three currentcarrying conductors. Those design decisions logically come ahead of sizing the
raceway and conductors, though sometimes it’s necessary to change them late
in this process.
Once you’ve applied these factors, you have your conductor ampacity number.
Now you just need to look it up in the table you selected, using the
appropriate temperature rating column. Follow it to the left to see the
minimum conductor size.
Note: You might increase the conductor size due to voltage drop or other
considerations.
56
Typical lighting circuit
Let’s apply the process we’ve learned to sizing the OCPD and conductors for
a 480V branch circuit consisting of six 400W lights.
Here’s the 7-step process:
Step 1: Characterize the loads. Regardless of any controls, characterize
these lights as continuous loads; there’s no guarantee they will be on for
less than 3 hr.
Step 2: Calculate total load. Look up the input wattage (manufacturer’s
data). For example, assume it’s 452W.
Calculate kVA. Since lights are actually single-phase loads, you can skip this
step. You do not divide the watts by 1.732 for this type of load, even if
supplied by 480V.
Step 3: Calculate amps. (452W ÷ 480V) × 6 = 5.65A.
57
Step 4: Size the OCPD. Use the next size up [240.6], which is 15A.
Step 5. Identify the table. You’re running 3 THHN conductors in EMT. After
reading the table headings, it’s obvious that Table 310.15(B)(16) is the
correct ampacity table.
Step 6. Apply temperature correction factor. The architect gave you 125°F as
the highest expected ambient temperature in the ceiling space. From Table
310.15(B)(2)(b), we see this requires us to multiply the allowable ampacity
by 0.5. Alternatively, we can multiply our load amps by 2.
Step 7. Size the conductor per the required ampacity. THHN is in the 90°
column, but your connectors are rated for 60°C. Using the 60°C column, we
find this circuit requires a conductor at least 14 AWG.
The process for calculating a feeder is different from that of calculating a
branch circuit. In Part 2, we’ll dive into that and see what to do.
58
Sizing Circuit Protection and Conductors — Part 2
In Part 1, we looked at how to size a branch circuit and walked through a simple
example. So if you’re sizing a feeder, why don’t you do it the same way? Except in
cases where a feeder is also essentially a branch circuit by dint of supplying a
single load (e.g., a large motor supplied by a feeder), you’ve already done the
branch circuit load calculations.
In normal power distribution, the purpose of a feeder is to supply power to
multiple branch circuits. The typical arrangement in an industrial facility is a 480V
feeder supplies a transformer that supplies a panel (or multiple transformers,
each supplying a panel). That transformer might supply the panel with 120V for
office loads, 277V for lighting loads, or maybe 480V for production equipment.
Each overcurrent protective device (OCPD) in that panel is supplied by the
transformer that’s supplied by that feeder.
You can follow the flow of power from the service through the feeder through the
branch circuit, but the flow of demand is in the opposite direction. The individual
loads add up on the branch circuit, and the branch circuits add up on the feeder.
This means that as part of the process of determining the load on the feeder, you
add up the branch circuit loads that feeder supplies. Because you can’t know your
feeder load until you’ve calculated your branch circuit loads, you do branch circuit
calculations first.
59
Load analysis
You need to do a bit of load analysis before sizing your feeder OCPD and
conductors. You don’t need to look at whether loads are continuous or noncontinuous, as you’ve already accounted for this in determining your total VA
for each branch circuit.
Typically, but not always, you can simply add up the VA of the branch circuits to
determine the VA on the feeder. For example, a feeder supplies 10 branch
circuits for lighting at 452VA each. That totals up to 4,520VA. But what if those
are HID lights, commonly used in high-bay applications (Photo)?
60
Make sure you know how to account for non-linear lighting loads in your calculations.
61
HID lights are an excellent choice for many reasons, but the downside is their
ballasts make them nonlinear loads. This has some ramifications.
First, you’ll probably want to derate the conductors. This is especially true if
you have long runs, and voltage drop comes into play. The good news is that
manufacturers of highly nonlinear loads like HID lighting typically provide
derating factors. And because high-bay lighting is a ceiling application, you
also must account for the higher ambient temperature using the correction
factors in Tables 310.15(B)(2)(a) and (b).
Second, this type of load means you have to treat the neutral as a currentcarrying conductor [310.15(B)(5)(c)].
Third, not all loads run at a given time. Lighting, which we’ve been talking
about, isn’t a diverse load — that is, if you turn the lights on, they all operate.
That is not true of all loads, however.
Some loads operate in a mutually exclusive fashion. That’s why, for example,
you use the larger of the heating or air-conditioning load when determining
the total load. That’s part of accounting for load diversity. Load diversity is
really an engineering decision, and you base it on the expected operation of
the equipment. Consider an industrial shop that has one portable arc welder
and 10 welding outlets. Only one outlet will be in use at any given time.
Suppose this is a 250A 220V/380V arc welder. If you didn’t factor in load
62
diversity, you’d need to add a service instead of just a single breaker.
You do this analysis to determine the actual maximum VA that will be on the
feeder. Once this is done, you can turn your attention to sizing the OCPDs and
then the conductors.
To illustrate the basics of feeder sizing, we’ll expand on the example we used
in Part 1. Now we’re sizing the feeder for six of those branch circuits. We’ll
start with the OCPD.
OCPD sizing
In this example, we can simply add up the branch circuit loads to determine
the feeder load and size the OCPD. Calculate the feeder load per Art. 220,
Parts III, IV, and V.
For our example, we multiply our 452VA load by 10 branch circuits to arrive at
4,520VA. Because lighting is a single-phase load, we divide by the voltage to
determine the current.
In Part 1, these were 480V lights, but the corporate office has just sent out a
memo stating that all lights must be 277V because the company has
contracted for special discount pricing for 277V lamps. Previously with 480V
lights, each branch circuit drew 5.65A. Now with 277V lights, it’s going to draw
9.8A.
63
If you divide your total of 4,520VA by 277, you see the feeder carries 16.31A.
Using the next-size-up rule, the OCPD is 20A. But suppose it’s a warehouse
supplied by just that one feeder, and it has four additional loads:
• Electric heaters totaling 250,000VA
• Office air-conditioner: 15,000VA
• Convenience receptacles: 10,000VA
• Temperature-controlled exhaust fans: 50,000VA
You don’t heat and cool at the same time, so applying load diversity gives us an
additional load equal to the heater load plus the convenience receptacles. Odds
are that the air conditioner and receptacles are powered by the same 120/240
transformer. Who’s to say that someone won’t add more circuits in the future?
It’s a good engineering decision to allow for the maximum transformer VA minus
the air-conditioner VA and add that number to the 250,000VA. This way, the
feeder will have to be pulled only once rather than needing to be replaced due
to a simple upgrade or small expansion project.
64
Conductor sizing
For the current-carrying conductors, use the same approach we outlined in Part
1. Whether sizing conductors for feeders or branch circuits, you work with the
ampacity tables the same way. Select a feeder conductor with sufficient
ampacity to carry your calculated load [215.2].
The complicating issue here is the neutral, which is normally the grounded
conductor. Any time the majority of the load on a feeder is nonlinear, you must
treat your neutral as a current-carrying conductor.
In any other situation, size the feeder neutral at least as large as the grounding
electrode conductor (GEC) [250.122], but also review 220.61 — because you
may need to go larger than the size of the GEC.
Sizing per 220.61 involves some load analysis and calculations. The only
downside to an oversized neutral is a slightly higher cost of materials. If a
significant part of, though not a majority of, the load is nonlinear, it often makes
sense for a small project to just treat it as a current-carrying conductor.
Sometimes, it makes no difference, and we’ll see that play out in our example.
65
Typical lighting circuit feeder
Let’s apply what we’ve learned to sizing the OCPD and conductors for a 277V
feeder supplying 10 branch circuits, each consisting of six 400W lights.
Here’s the 6-step process:
Step 1: Calculate total load. The easiest way to do this is to add up your branch
circuit VA. Remember that you characterized these as continuous and noncontinuous already, thereby satisfying the last sentence of 215.2(A)(1).
Step 2: Calculate amps. In our example, 4,520VA ÷ 277V = 16.4A. Don’t forget to
divide by the square root of three for 3-phase loads. If your feeder has a mix of
single-phase and 3-phase loads, you have a mix of calculations to perform.
Step 3: Size the OCPD. Use the next size up [240.6]. For our example, that’s 20A.
Step 4. Identify the table. You’re running 3 THHN conductors in EMT. Use Table
310.15(B)(16).
66
Step 5. Apply temperature correction factor. For the branch circuits, we had
to consider the ceiling temperature. But the feeder runs to a transformer
sitting on a poured pedestal on the floor. The warehouse will be maintained
at a maximum 80°F. From Table 310.15(B)(2)(b), we see this requires us to
multiply the allowable ampacity by 1.22. Alternatively, we can multiply our
amps by 0.82, and that gives us 13.5A.
Step 6. Size the conductor per the required ampacity. Using the 60°C column,
we find this circuit requires a conductor at least 14 AWG.
It’s interesting that for the feeder and branch circuits, you’re going to run 14
AWG for the whole installation, including the neutral. This is something to
keep in mind before delving into complex complications for sizing the
neutral.
Things get a bit turned on their head when motors are involved. In Part 3,
we’ll look at why that is and what to do about it.
67
Sizing Circuit Protection and Conductors — Part 3
Motors differ from other types of loads in one important way: The motor
needs much more current to start than to run. This temporary, but significant,
inrush current is what complicates motor circuit protection. The overcurrent
protection device (OCPD) must accommodate inrush current while still
protecting the conductors. The conductors must be able to handle and
dissipate the short-term increase in heat from that starting current. Much of
Art. 430 is concerned with getting both of these requirements right.
For other types of loads, a single device provides overcurrent protection and
overload protection. Because of inrush, motor circuits handle those functions
separately — that is, the job of protecting the conductors and the load gets
split up for motor circuits. Fault protection opens the circuit when there’s a
high level of excess current, such as from a fault or short circuit. But an
overload is a relatively small amount of excess current. OCPDs protect against
current in excess of the rated current of the equipment or ampacity of a
conductor, whether it’s a motor circuit or not. Normally, the OCPD also
handles overloads. But with motor circuits, separate thermal overloads do
that.
68
Though you can’t see the motor, this coal feeder won’t run without it. Among the many
calculations Black & Veatch engineers perform when designing a coal power station are the sizing
of circuits and protection for the motors that drive the coal feeders. They have to get those right,
69
or the plant won’t have coal to burn and thus won’t produce power.
Load analysis
As with any circuit, analyze the loads before sizing motor OCPDs and
conductors. Here are some highlights:
1. If multiple motors are on the same feeder and/or branch circuits, look at
load diversity. Do any of these operate in a mutually exclusive fashion?
2. Which loads are continuous? Noncontinuous?
3. Does the system include a variable-frequency drive (VFD)? Is it power-factor
corrected? Does it have harmonics mitigation?
4. Do you need to derate conductors for voltage drop?
5. What type of motor(s) are you installing? For example, part-winding motors
have additional requirements. Review Art. 430 Part I when making this
assessment.
6. Is this application covered by another article [Table 430.5]?
70
Sizing motor circuit conductors
Normally, we size the OCPD and then the conductors. You find this order of
calculation in the examples of Appendix D. But with motors, your second
step is to size the conductors [Table 430.1].
Not coincidentally, the requirements are in Art. 430 Part II. In Part III, you’ll
find the requirements for providing thermal protection to the motor; that’s
outside the scope of this discussion. Part II applies to motors operating at
under 600V, such as typical 480V industrial motors. If your motor operates
at over 600V, use Part XI instead.
How you proceed depends on whether these conductors are for:

Single motor. Apply 430.22. Then see if any of subsections (A) through (I)
apply to your installation.

Wound rotor secondary. Apply 430.23.

More loads than just that motor. Apply 430.24.

Combination load equipment. Apply 430.25.

Motors with PF capacitors installed. Apply 430.27.

Constant voltage DC motors. Apply 430.29.
71
If you have load diversity, you can apply a feeder demand factor [430.26]. If
you’re using feeder taps, apply 430.28.
To make things simple, let’s assume you need to size the conductors for a
single, continuous-duty 40HP motor. Those conductors must have an
ampacity of at least 125% of the motor FLC. Use the FLC from the motor
nameplate. If, for some reason, you can’t get this from the nameplate or the
motor data sheet, use the applicable NEC table (e.g., Table 430.250). It’s
best, however, to obtain the information from the motor manufacturer (and
then affix a new nameplate to the motor).
Your motor’s nameplate says its FLC is 53A. Coincidentally, this is close to
the 52A shown in Table 430.250. You now have a three-step process for
sizing the conductors:
Step 1. Identify the table. You’re running three THHN conductors in
intermediate metal conduit (IMC). Use Table 310.15(B)(16).
Step 2. Apply the temperature correction factor. Determine the maximum
ambient temperature — not for where the motor is, but for where the
conductors are running.
Suppose these will run overhead, and you know the maximum temperature
will be 110°F (43°C).
72
From Table 310.15(B)(2)(b), you see you must multiply the allowable
ampacity by 0.87. Alternatively, you can multiply 53A by 1.15, which gives
you 61A.
But what if your ceiling temperature is, say, 160°F (71°C)? In the 60°C
column, the ambient temperatures end at 55°C. In such a case, split the
run; see Annex D3(a) for an example of how to do this.
Step 3. Size the conductor per the required ampacity. Using the 60°C
column, you see this circuit requires a conductor at least 4 AWG.
OCPD sizing
Size the OCPD per 430 Part III, noting that you use the motor nameplate
current rating (FLA), not the FLC [430.6(A)].
In our example, you sized the conductors for a single motor that has 53A
FLC. Let’s assume the motor is on its own branch circuit. How do you size
the OCPD for that circuit? The answer is in 430.52. How would you know to
go there? Earlier, you referred to Table 430.1 to see what your second step
is. Go back to Table 430.1 as you continue to work out your motor
requirements. For this step, Table 430.1 directs you to Part IV.
There, you’ll read that the OCPD must be capable of carrying the starting
current of the motor [430.52(B)]. This doesn’t mean you size the OCPD for
the starting current, however. The meaning of this emerges in 430.52(C).
You need to specify an OCPD per Table 430.52.
73
When you size the overload, you use the FLA. But to size the OCPD, you use
the FLC. First, find your motor type and OCPD type in Table 430.52. Then,
multiply your FLC by the percentage of FLC required by the chart.
Using the 53A FLC of our example with an inverse time breaker, you multiply
the FLC by 2.5 for a maximum rating of 132.5A. This isn’t a standard OCPD size
[240.6(A)]. Since 430.52(C)(1) says the OCPD can’t exceed this calculated
value, do you use the next size down? If you read on just a bit, you’ll see that
Exception No. 1 lets you use the next size up. So for this branch circuit, you
need a 150A breaker.
If it turns out that the breaker trips every time you try to start the motor, then
what? Determine if the trip is due to a fault or from overload. If it’s overload,
Exception No. 2 lets you use a larger OCPD.
But there are limits to how big that OCPD can be. You may have voltage drop
or power factor problems that lead to excess starting current. In addition to
correcting these, consider a soft-start or VFD so you eliminate across-the-line
starting. A big advantage of using a soft-start or VFD is you eliminate a
common cause of cable failure. The power anomalies resulting from across
the line starting can damage loads on other feeders, not just the motor
74
system.
Multiple loads
Now let’s change our example slightly. Suppose this motor is on a branch
circuit with two other motors and four electric heaters. The loads are 40A,
27A, and 40A, respectively. How do you size the OCPD?
Unless you have a compelling reason to put these loads on the same branch
circuit, it’s generally best to put each motor on its own branch circuit.
First, analyze the loads. If that 40A motor runs an HVAC compressor, you can
disregard the 40A heaters; these are mutually exclusive loads. To see why not
to disregard the motor instead, review the calculations we just did.
Next, turn to 430.53. The first requirement is that you must use fuses or
inverse time circuit breakers. So when you use Table 430.52, ignore the two
middle columns. Apply Table 430.52 to the sum of your motor loads, then add
to the sum of the other loads.
75
Fig. 1. These are the major steps in sizing conductors and OCPDs for branch circuits.
76
Feeders
For feeder OCPD requirements, turn to Part V. The key here is the “don’t
exceed the rating” requirement of 430.52(C)(1) applies only to the largest
load.
For the conductor requirements on that feeder, turn to Part II. To
determine the minimum conductor ampacity [430.24]:
1.
Multiply the FLC of the largest motor by 125%.
2.
Add up the FLCs of the other motors.
3.
Multiply the continuous non-motor loads by 125%.
4.
Add up all of the above to the total of the non-continuous loads.
77
Avoiding confusion
Fig. 2. The major steps in sizing conductors and OCPDs for feeders are almost identical to the
branch circuit ones, but change as shown here.
78
To size conductors and OCPDs for branch circuits, follow the steps shown
in Fig. 1. The steps for feeders modify the steps for branch circuits, as
shown in Fig. 2. But if you have motor circuits, remember that the inrush
current of motors changes things:


The normal functions of the OCPD are split. With motors, you have an
additional device that does the overload protection job normally done
by the OCPD.
You use multipliers for sizing the conductors (125%) and the OCPDs
[Table 430.52].
If you understand the branch circuit conductor and OCPD sizing steps, it’s
just a matter of modifying them a bit for feeders and/or motors.
79
Dwelling Unit Calculations
Apply demand factors for correct load calculations
A dwelling unit is a single unit that provides complete and independent
living facilities, according to the NEC definition found in Art. 100 (Fig. 1 ).
Fig. 1. The definition of dwelling unit, as described above, is found in Art. 100.
80
Dwelling units have special requirements for load calculations. Although most of
the actual load calculation requirements are in Art. 220, others are scattered
throughout the Code and still come into play when making certain calculations .
Keep the following considerations in mind when making dwelling unit
calculations:
Voltages. Unless other voltages are specified, calculate branch-circuit, feeder,
and service loads using the nominal system voltage [220.5(A)]. For a singlefamily dwelling unit, the nominal voltage is typically 120/240V.
Motor VA. Use motor table voltage and current values, such as 115V, 230V, or
460V — not 120V, 240V, or 480V [430.248 and 430.250]. A much more accurate
VA rating is obtained by using the motor’s rated voltage and current, which were
used in developing the Code Tables.
Rounding. Where calculations result in a fraction of less than 0.50A, you can
drop the fraction [220.5(B)].
Receptacles. You can use 15A or 20A receptacles on 20A circuits as long as there
is more than one receptacle on the circuit. For these purposes, a duplex
receptacle is considered to be two receptacles [210.21(B)(3)].
Continuous loads. A continuous load is one in which the maximum current is
expected to continue for 3 hr or more, according to the Art. 100 definition. Fixed
electric heating is one example of a continuous load [424.3(B)]. When sizing
branch circuit conductors and overcurrent devices for a continuous load,
81
multiply the load by 125% [210.19(A)(1) and 210.20(A)].
Laundry rooms. A laundry area receptacle is required [210.52(F)], at least one of
which must be within 6 ft of a washing machine [210.50(C)]. Any receptacle
within 6 ft of the outside edge of a laundry sink must be GFCI protected
[210.8(A)(7)].
Required circuits. In addition to the circuits required for dedicated appliances
and those needed to serve the general lighting and receptacle load, a dwelling
unit must have the following circuits:
A minimum of two 20A, 120V small-appliance branch circuits for receptacles in
the kitchen, dining room, breakfast room, pantry, or similar dining areas
[220.11(C)(1)]. These circuits must not be used to serve other outlets, such as
lighting outlets or receptacles from other areas [210.52(B)(2) Ex]. These circuits
are included in the feeder/service calculation at 1,500VA for each circuit
[220.52(A)].
One 20A, 120V branch circuit for the laundry receptacle(s). It can’t serve any
other outlet(s), such as lighting, and can serve only receptacle outlets in the
laundry area [210.52(F) and 210.11(C)(2)]. In your feeder/service load
calculation, include 1,500VA for the 20A laundry receptacle circuit [220.52(B)],
as shown in Fig. 2.
82
Feeder and service calculations. Occupants don’t use all loads
simultaneously under normal living conditions, so “demand factors” can be
applied to many of the dwelling unit loads in order to size the service. Some
demand factors provided in the Code are intended for use in dwellings only;
others are allowed only in non-dwellings. Therefore, be careful to apply
demand factors only as allowed by the NEC.
Fig. 2. Per Sec. 210.11(C)(2), one 20A, 120V
branch circuit is required for the laundry
area receptacles.
83
The NEC provides two dwelling service load calculation methods: the standard
method and the optional method.
Standard method for feeder and service load calculations
The standard method consists of three calculation steps:
General lighting VA load. When calculating branch circuits and feeder/service
loads for dwellings, include a minimum 3VA per sq ft for general lighting and
general-use receptacles [220.12]. When determining the area, use the outside
dimensions of the dwelling. Don’t include open porches, garages, or spaces
not adaptable for future use.
Small appliance and laundry circuits. The 3VA per sq ft rule includes general
lighting and all 15A and 20A, 125V general-use receptacles, but doesn’t
include small-appliance or laundry circuit receptacles. Therefore, you must
calculate those at 1,500VA per circuit. See 220.14(J) for details.
Number of branch circuits. Determine the number of branch circuits required for
general lighting and general-use receptacles from the general lighting load
and rating of the circuits [210.11(A)]. Although this is explained in Annex D,
Example D1(a) of the NEC, let’s look at an another example.
84
Fig. 3. Sample calculation showing how to follow the rules in Sec. 220.12
regarding general lighting and receptacles for a 2,000-sq-ft dwelling unit.
85
Question: What’s the general lighting and receptacle load for a 2,000-sq-ft
dwelling unit that has 34 convenience receptacles and 12 luminaires rated 100W
each (Fig. 3)?
The calculation is pretty simple.
2,000 sq ft x 3VA = 6,000VA.
No additional load is required for general-use receptacles and lighting outlets
because they are included in the 3VA per sq ft load specified by Table 220.12 for
dwelling units. See 220.14(J).
Now let’s work through an example to determine the number of circuits required.
Question: How many 15A circuits are required for a 2,000-sq-ft dwelling unit?
Step 1: General lighting VA = 2,000 sq ft x 3VA = 6,000VA
Step 2: General lighting amperes:
I = VA ÷ E
I = 6,000VA ÷ 120V*
I = 50A
*Use 120V, single-phase unless specified otherwise.
86
Step 3: Determine the number of circuits:
Number of circuits = General lighting amperes ÷ circuit amperes
Number of circuits = 50A ÷ 15A
Number of circuits = 3.30, or 4 circuits. Any fraction of a circuit must be
rounded up.
Optional method for feeder and service load calculations
You can use the optional method [Art. 220, Part IV] only for dwelling units
served by a single 120/240V or 120/208V 3-wire set of service or feeder
conductors with an ampacity of 100A or larger [220.82]. The optional method
consists of three calculation steps:
General loads [220.82(B)]
Heating and air-conditioning load [220.82(C)]
Feeder/service conductors [310.15(B)(6)]
Step 1: General loads [220.82(B)]
The general calculated load must be at least 100% for the first 10kVA, plus
40% of the remainder of the following loads:
General lighting and receptacles: 3VA per sq ft
Small-appliance and laundry branch circuits: 1,500VA for each 20A, 120V
small-appliance and laundry branch circuit specified in 220.52.
Appliances: The nameplate VA rating of all appliances and motors that are
fastened in place (permanently connected) or located on a specific circuit, not
87
including heating or air-conditioning..
Be sure to calculate the range and dryer at their nameplate ratings.
Step 2: Heating and air-conditioning load [220.82(C)]
Include the larger of (1) through (6):
Air-conditioning equipment: 100%
Heat-pump compressor without supplemental heating: 100%
Heat-pump compressor and supplemental heating: 100% of the nameplate
rating of the heat-pump compressor and 65% of the supplemental
electric heating for central electric space-heating systems. If the control
circuit is designed so that the heat-pump compressor can’t run at the
same time as the supplementary heat, omit the compressor from the
calculation.
Space-heating units (three or fewer separately controlled units): 65%.
Space-heating units (four or more separately controlled units): 40%.
Thermal storage heating: 100%.
Step 3: Feeder/service conductors [310.15(B)(6)]
88


400A and less. For individual dwelling units of one-family, two-family, and
multi-family dwellings, use Table 310.15(B)(6) to size 3-wire, single-phase,
120/240V service or feeder conductors (including neutral conductors) that
serve as the main power feeder. Feeder conductors aren’t required to have an
ampacity rating greater than the service conductors [215.2(A)(3)]. Size the
neutral conductor to carry the unbalanced load per Table 310.15(B)(6). Table
310.15(B)(6) can’t be used for sizing the feeder or service conductors that
supply more than a single dwelling unit.
Over 400A. Size ungrounded conductors and the neutral conductor using
Table 310.16 for feeder/services over 400A and those that do not fill all of the
requirements for using Table 310.15(B)(6). Let’s try a calculation example.
Question: What size service conductor is required for a 1,500-sq-ft dwelling unit
containing the following loads?
Cooktop: 6,000VA
Disposal: 900VA
Dishwasher: 1,200VA
Dryer: 4,000VA
Ovens (two each): 3,000VA
Water heater: 4,500VA
A/C: 17A, 230V
Electric heating (one control unit): 10kVA
89
Step 1: General loads [220.82(B)]
General lighting: 1,500 sq ft x 3VA = 4,500VA
Small-appliance circuits: 1,500VA x 2 circuits = 3,000VA
Laundry circuit: 1,500VA
Appliances (nameplate):
Cooktop: 6,000VA
Disposal: 900VA
Dishwasher: 1,200VA
Dryer: 4,000VA
Ovens (each 3 kW): 6,000VA
Water heater: 4,500VA
Total connected load: 31,600VA
First 10kW at 100%: 10,000VA x 1.00 = 10,000VA
Remainder at 40%: 21,600VA x 0.40 = 8,640VA
Calculated general load: 10,000VA + 8,640VA
Calculated general load: 18,640VA
90
Step 2: Air-Conditioning versus heat [220.82(C)]
Air-conditioning at 100% [220.82(C)(1)] vs. electric space heating at 65%
[220.82(C)(4)]
Air conditioner [Table 430.248]:
A/C VA = V x A
A/C VA = 230V x 17A
A/C VA = 3,910VA (omit)
Electric space heat: 10,000VA x 0.65 = 6,500VA
Step 3: Feeder/service conductors [310.15(B)(6)]
Calculated general load (Step 1): 18,640VA
Heat calculated load (Step 2): 6,500VA
Total calculated load = 18,640VA + 6,500VA = 25,140VA
I = VA ÷ E
I = 25,140VA ÷ 240V = 105A
91
Therefore, the feeder/service ungrounded conductor is sized to 110A, 3
AWG [310.15(B)(6)].
The Code doesn’t explain how demand factors were derived, and it’s not
essential that you understand this in order to apply them correctly. Be sure
to work on some practice calculations so you understand how to apply the
various demand factors to a dwelling unit calculation.
The standard calculation and the optional calculation methods were both
discussed in this article. These are two distinctly different calculation
methods, so be careful not to mix them. Remember that the standard
method is in Part III of Art. 220, and the optional method is contained in
Part IV. When you are evaluating the necessary loads in either type of
calculation method, follow the requirements for specific loads covered in
other Articles outside of Art. 220. Which method is better to use? On an
exam, you’ll likely be told which method to use on a specific question.
However, if the question doesn’t specify a method, use the standard
calculation. The optional method is usually faster and easier to apply, so it
has a natural advantage for daily use on the job.
92
Where to Find Dwelling Unit Code Requirements Outside Art. 220
Branch circuits — Art. 210
Areas supplied by small appliance circuits — 210.52(B)(1)
Feeders — Art. 215
Services — Art. 230
Overcurrent protection — Art. 240
Wiring methods — Art. 300
Conductors — Art. 310
Appliances — Art. 422
Electric space-heating equipment — Art. 424
Motors — Art. 430
Air-conditioning equipment — Art. 440
93
94