Chapter 10 The Service Entrance
187
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
Write your answers on a separate sheet of paper.
Do not write in this book.
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 B
C
Neutral bus bar