Electrical Power Systems Technology

Single-phase and Three-phase Distribution Systems 261

number of wires required. These systems, shown in Figure 10-4, are the

three-phase three-wire system, the three-phase three-wire system with neu-

tral, and the three-phase four-wire system. The primary winding connection

is not considered here. The three-phase three-wire system, shown in Fig-

ure 10-4A, can be

used to supply

motor loads of

240 volts or 480

volts. Its major

disadvantage is

that it only sup-

plies one volt-

age, as only three

hot lines are sup-

plied to the load.

The usual insu-

lation color code

for these three

hot lines is black,

red, or blue, as

specified in the

NEC.

The disadvan-

tage of the three-

phase three-wire

system may be

partially over-

come by add-

Figure 10-4. In-

dustrial power

distribution sys-

tems: (A) Three-

phase, three-wire

system, (B) Three-

phase, three-wire

system with neu-

tral, (C) Three-

phase, four-wire

system