258 Electrical Power Systems Technology
Single-phase systems can be of two major types—single-phase two-
wire systems or single-phase three-wire systems. A single-phase two-wire
system is shown in Figure 10-2A (the top diagram). This system uses a 10-
kV A transformer whose secondary produces one single-phase voltage,
such as 120 or 240 volts. This system has one hot line and one neutral line.
In residential distribution systems, this was the type most used to provide
120-volt service several years ago. However, as appliance power require-
ments increased, the need for a dual-voltage system was evident.
To meet the demand for more residential power, the single-phase
three-wire system is now used. A home service entrance can be supplied
with 120/240-volt energy by the methods shown in Figures 10-2B and 10-
2C (center and bottom diagrams). Each at these systems is derived from a
three-phase power line. The single-phase three-wire system has two hot
Figure 10-2. Single-phase power distribution systems: (A) Single-phase, two-
wire system, (B) Single-phase, three-wire system (taken from two hot lines),
(C) Single-phase, three-wire system (taken from one hot line and one grounded
neutral)