182
Section 2 Installation
building that is supplied. See Figure 10-32.
These transformers are designed to supply the
following service ratings:
Phases
Phase refers to the angle between various
generated ac currents. The term three-phase
indicates three separately derived ac currents
that are “out-of-step” from each other by 120
electrical degrees.
Two-phase current occurs when two alter-
nating currents are generated 90° apart. The
typical single-phase current exists when
one current is produced at regular intervals.
Figure 10-33 illustrates this important variance
in electrical power generation.
Single-phase, two-phase, and three-phase
are abbreviated as 1φ, 2φ, and 3φ. Single-phase
and three-phase current are used commonly in
electrical generation. Two-phase has little prac-
tical application today.
Three-Wire,
Single-Phase System
The most common residential service is a
three-wire, single-phase system. This consists of
two ungrounded conductors and one grounded
conductor. See Figure 10-34. One ungrounded
conductor is usually black and the other red.
The grounded conductor is usually yellow. This
system will provide 120 V and 240 V, depending
No. of Wires Voltage
2
Phase Comments
120
These
installations
are no longer
made
Most common
residential
service
Three
Single
Single
Three
3
4
120/240
120/208
4
120/240
Generator station
Transmission lines
Substation Substation
Local pole
transformer
Consumer
Load
Step-up Step-down Step-down Step-down
11,500 V 23,000 V 4,800 V 2,400 V 120 V/240 V
Figure 10-31. Power companies rely on transformers to effi ciently transmit electricity over long distances. A chain
of transformers links the generating plant with the consumer.