Chapter 10 The Service Entrance
187
Copyright Goodheart-Willcox Co., Inc.
Making a Single-Phase
Connection on a Three-
Phase System
A more versatile three-wire, 120/208 volt,
single-phase circuit can be formed by connecting
two circuit wires to a double-pole breaker as
previously described and adding a grounded
conductor to the circuit. A circuit such as this
can be used for 120-volt receptacles and lighting
outlets. It can also supply 120/240 volts for elec-
tric ranges, counter cooktops, and other devices
designed for that purpose.
Three-phase, three-wire 208-volt circuits
are derived by connecting the circuit wires
to a three-pole circuit breaker. These circuits
are specifi cally designed to operate special
three-phase devices such as heaters and various
heavy-duty motor-driven equipment.
Last, but not least, a four-wire, three-phase,
120/208-volt circuit is connected in the same
manner as the three-wire, 208-volt circuit with
the addition of a grounded wire connected to
the neutral bus. This type of circuit is extremely
useful when extending an electrical system to
other circuit panels.
Review Questions
Answer the following questions using the informa-
tion provided in this chapter.
1. List fi ve major components of a service
entrance.
2. The minimum service entrance rating for
new buildings is
_______________
amperes.
Figure 10-38. Main breaker and panel circuit connections for a wye-connected three-phase, four-wire service. Hot
bus bars are marked A, B, and C.
Wild leg
Grounded conductor
(neutral)
3 pole
main
breaker
Single-phase 120 V circuit
Single-phase 240 V circuit
Three-phase 240 V circuit
Enclosure
cabinet
Bond to cabinet
Ground
Single-phase
208 V circuit
N
N
A C
B
Neutral bus bar