Transcript Objectives - Goodheart

CHAPTER
10
The Service
Entrance
Objectives
• List the components of the service entrance.
• List service drop clearances specified in the NEC.
• List eight basic guidelines when locating a residential
service entrance.
• Calculate the size of service entrance needed based on
the power needs of the dwelling.
• Select proper conductors and components for the
service entrance.
• Size and install conductors between the meter socket,
main disconnect, and service panel.
• Provide proper grounding for a service entrance.
• Explain the purpose of a step-down transformer and
indicate where it is used.
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Permission granted to reproduce for educational use only.
Service Entrance
• All conductors, devices, and fittings that carry
electricity from power company’s transformer to
the consumer
• Service entrance components:
–
–
–
–
–
Service entrance conductors
Meter socket, pan, or enclosure
Service entrance panel
Grounding system
Fittings, fasteners, and hardware
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Permission granted to reproduce for educational use only.
Service Entrance Location
• Service drop—Overhead service entrance
conductors
(OSHA)
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Permission granted to reproduce for educational use only.
Service Entrance Location (cont.)
• Service lateral—Buried service entrance
conductors
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Permission granted to reproduce for educational use only.
Selecting Service Entrance Location
• Underground service conductors installed in a
straight line
• Service conductors should be kept as short as
practical to minimize voltage drop
• Service conductors must enter building as near
as possible to the service panel
• Service disconnect must be at or near point of
entry
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Permission granted to reproduce for educational use only.
Selecting Service Entrance Location (cont.)
• Service panel must be located in a central and
accessible area
• Service equipment must be protected from
physical damage, water, and dust
• Service equipment cannot be placed in
bathrooms, storerooms, closets, or damp
basements
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Permission granted to reproduce for educational use only.
Number of Services
• Generally, a structure can have only one service
• Some exceptions:
To supply fire pump or emergency system
To supply a large load demand
Large structures
Service from a generator or solar photovoltaic
source
– Multiple voltage/phase requirements
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–
–
–
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Permission granted to reproduce for educational use only.
Service Disconnects
• For multiple occupancies buildings, a maximum
of six disconnects can be connected to a single
service drop or lateral
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Permission granted to reproduce for educational use only.
Code Alert
• Sizing Service Entrance Conductors
Reprinted with permission from NFPA 70-2011, National Electrical Code, Copyright  2010, National
Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and official
position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Service Drop
•
•
•
•
•
•
Service drop mast
Insulator
Service head
Drip loop
Meter socket
Meter
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Permission granted to reproduce for educational use only.
Temporary Wiring
• Provides electrical power during construction
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Permission granted to reproduce for educational use only.
Temporary Wiring (cont.)
• Installation requirements:
Locate in a safe place
Keep conductors overhead if possible
All receptacles require GFCI protection
Protect wiring from physical damage
Lamps require protective covers
Weatherproof housings required when exposed to
weather
– Inspect frequently
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–
–
–
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–
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
Service Disconnect
• Service equipment must include a disconnect
that disconnects service from building’s wiring
system
• Common types:
– Main disconnect switch
– Main circuit breaker in service panel
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Permission granted to reproduce for educational use only.
Service Grounding
• Grounded (neutral) service conductor connected
to neutral bus bar in service panel
• Neutral bus bar is connected to grounding bus
bar
• Grounding bus bar is connected to grounding
electrode
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Permission granted to reproduce for educational use only.
Grounding Electrode Conductor
• Connects grounding bus bar in service panel to
grounding electrode
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Permission granted to reproduce for educational use only.
Above-Roof Clearances
(OSHA)
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Permission granted to reproduce for educational use only.
Above-Grade Clearances
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Permission granted to reproduce for educational use only.
Building Opening Clearances
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Permission granted to reproduce for educational use only.
Transformers
• Convert voltage and current
• Primary winding
• Secondary winding
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Permission granted to reproduce for educational use only.
Transformer Equations
Voltage of Primary
T urnsof Primary

Voltage of Secondary T urnsof Secondary
PrimaryVoltage Secondary Voltage

SecondaryCurrent
PrimaryCurrent
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Permission granted to reproduce for educational use only.
Transformer Example
• Primary: 12 V, 20 A
• Secondary: 120 V
• Find secondary
current
PrimaryVoltage Secondary Voltage

SecondaryCurrent
PrimaryCurrent
12 V 120 V

x
20 A
120 x  12  20
x
12  20 240

2A
120
120
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Permission granted to reproduce for educational use only.
Transformer Types
• Step-up transformer—Voltage increases from
primary to secondary
• Step-down transformer—Voltage decreases from
primary to secondary
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Permission granted to reproduce for educational use only.
Phases
• Single-phase
• Two-phase
• Three-phase
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Single-Phase, Three-Wire System
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Three-Phase, Four-Wire Wye System
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Three-Phase, Four-Wire Delta System
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Permission granted to reproduce for educational use only.