Transcript 346N_090904_commercial_electrical

Quiz in Progress
1)Homework 1 was?
A.
B.
C.
D.
E.
Easy
Hard
Easy except for problem 1
Easy except for problem 2
Easy except for problem 3
Selecting Electrical Systems
• MLO/MCB
• Aluminum wiring
2) Why do we need a transformer for
panel D?
A.
B.
C.
D.
E.
Lower voltage power is more efficient
Lower voltage power reduces peak demand
Lower voltage power is required for duplexes
Lower voltage power is safer
Transformer manufacturers lobbied to make
transformers required under the NEC.
Panel D
• Duplexes (receptacles)
• Main Lugs Only (remote
protection)
• Phase total 60 kVA
• 14 kVA (neutral)
• 30 poles
Panel D
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Duplexes (receptacles)
Main lugs only (remote protection)
3 Phase total 60 kVA, 14kVA (neutral)
30 poles
60 kVA / (208V √3 ) = 167A
200A panel (standard size)
Neutral 14000/120 = 117A
Text Table 12.2, NEC, Tao and Janis (2001) Table 11-2
• Hot wires should be 250 MCM, Neutral #2/0 aluminum
• 2½ inch conduit
3) How do you calculate current flow
through a neutral conductor in a 3 system?
A.
B.
C.
D.
I =√3 E P
I =P/ (√3 E )
I =P/ E
I= E P
4) Why is neutral load not 1/3 of hot
load?
A. Because not all power goes through the
neutral conductor
B. Because there can be different amounts of
current on each hot conductor
C. Because the neutral is oversized for safety
reasons
D. A. and B.
E. A., B., and C.
Selecting Electrical Systems
• MLO/MCB
• Aluminum wiring
5) What is the current in the hot conductors
upstream of the transformer (I1)?
•
A.
B.
C.
D.
E.
E1 = V1 = 480V, E2 = V2 = 208V, I2 = 200A
86 A
100 A
462 A
Can’t be determined
149 A
Transformer capacity
• 200∙208∙√3 = 72 kVA ~ 75 kVA
• 200∙208 / 480 = 86 A
• 100 A switch
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Neutral 117*120/277 = 50 A
Three #2 and one #6 aluminum (could use #4)
1¼ inch conduit (probably actually 1½ inch)
100A fuse - could use 90, but likely same price
Selecting Electrical Systems
• MLO/MCB
• Aluminum wiring
6) What is lighting panel hot conductor
current?
A.
B.
C.
D.
65kVA/(480V∙√3) = 78 A
65kVA/(480V) = 135 A
65kVA/(277V∙√3) = 135 A
65kVA/(277V) = 235 A
7) Does the neutral conductor carry more
or less current than the hot conductor?
A. More current in neutral
B. Less current in neutral
C. No current in neutral
Lighting Panel
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100A MLO 20 pole spaces
Hot: 65kVA/(480V∙√3) = 78 A - #2 Aluminum
Neutral: 22 kVA /277V = 79 A - #2 Aluminum
100 A three pole fused switch
1½ inch conduit
Selecting Electrical Systems
• MLO/MCB
• Aluminum wiring
8) The power panel has no neutral and
therefore has how many wires in the
conduit at point 4
A.
B.
C.
D.
E.
3 hot + 1 neutral from panel = 4
3 hot + 1 ground = 4
3 hot = 3
3 hot + 1 ground + 1 neutral = 5
1 hot = 1
Power Panel
• No neutral? – all three pole circuits
• 33 poles required – chose 42 standard size and
allows for expansion
• 165kVA/(480 √3) = 198 A
• Choose 225A switch (could use 200 A)
• 3 300 MCM aluminum wires – 2 inch conduit
Wireway
• 198A + 86 A + 79 A = 363A
• 400A would allow for some expansion, some
would probably go to 600A
• 400A three-pole switch
• Six 250 MCM (unlikely to find conductors big
enough to just have 3) and one #3/0 aluminum
• Neutral current = 130A
Summary Panel Sizing
• Find panel power usage and neutral usage (we
will learn how to calculate in a minute)
• Size panel conductors and associated conduit
and switches
NEC summary (Handout)
1.
2.
3.
4.
5.
Minimum capacity
Grounding necessary
Only approved wires (voltage/temperature/location)
200, 210, 215, 220 demand calculations
Conduits, raceways required in commercial buildings
(usually rigid)
6. 310 - Number of conductors in raceway
7. Restrictions (to residential) on use of NM and NMC
conductors
8. Connections and splices must be approved and inspectable
9. Need overcurrent/overvoltage/and overload protection
10. Need disconnect switches (also 11.)
12. Not too many wires in a conduit
Branch Circuits (Handout)
• From panel to loads
• Balance loads on each phase of three phase
panel
• NEC 430
Design Guidelines
1. No wire smaller than 14 AWG in residential
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12 AWG for commercial (2)
3. Continuous load per 120 V circuit limited to
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1.2kW for 15 A and 1.5kW for 20 A breaker
4. Continuous load for heavy duty circuits
Demand (kW) Conductor Size (AWG)
2
10
2.5
8
3
6
5. Do not exceed branch circuit rating by:
80 %
70 %
50 %
Portable appliances
Inductive lighting
Fixed appliances
6. Panelboard to first lighting outlet
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Use next highest conductor size if >75 ft
Only allowed >100 ft if <2% voltage drop (7)
8. Panelboard to first receptacle
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If >100ft, use bigger than 10 AWG conductors
Can’t use branch circuit for convenience outlets
and display lighting
Ref: Tao and Janis (2001)
Ref: Tao and Janis (2001)