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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.
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Objectives (cont.)
• 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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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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Service Entrance Location
• Service drop—Overhead service entrance
conductors
(OSHA)
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Service Entrance Location (cont.)
• Service lateral—Buried service entrance
conductors
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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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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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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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Service Disconnects
• For multiple-occupancy buildings, a maximum of
six disconnects can be connected to a single
service drop or lateral
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Sizing Service Entrance Conductors
Copyright © 2013 NFPA
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Service Drop
•
•
•
•
•
•
Service drop mast
Insulator
Service head
Drip loop
Meter socket
Meter
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Temporary Wiring
• Provides electrical power during construction
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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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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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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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Grounding Electrode Conductor
• Connects grounding bus bar in service panel to
grounding electrode
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Above-Roof Clearances
(OSHA)
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Above-Grade Clearances
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Building Opening Clearances
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Transformers
• Convert voltage and current
• Primary winding
• Secondary winding
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Transformer Equations
Voltage of Primary
Turns of Primary

Voltage of Secondary Turns of Secondary
Primary Voltage
Secondary Voltage

Secondary Current
Primary Current
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Transformer Example
• Primary: 12 V, 20 A
• Secondary: 120 V
• Find secondary
current
Primary Voltage
Secondary Voltage

Secondary Current
Primary Current
12 V 120 V

x
20 A
120x  12  20
x
12  20 240

2A
120
120
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Transformer Types
• Step-up transformer—Voltage increases from
primary to secondary
• Step-down transformer—Voltage decreases from
primary to secondary
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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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