Figure 3-2.Multiple-transfer switches.
The electrical loads to be supplied power, volt-
age, phase, frequency, and duty cycle requirements
govern the selection of generating equipment.
Probable load deviation, probable life of the
installation, availability of fuels, and availability of
skilled personnel are other important factors.
Power and Voltage Requirements
The selection of voltage is affected by the size, the
character, and the distribution of the load; length,
capacity, and type of transmission and distribution
circuits; and size, location, and connection of
generators. Practically all general-purpose lighting in
the United States and at United States overseas bases is
120 volts. The lighting voltage may be obtained from a
three-wire, 120/240-volt, single-phase circuit or a
120/208-volt, three-phase, four-wire circuit.
Electrical plants at advanced bases serve a varied
load of lighting, heating, and power equipment, most
of which demand power day and night. The annual
load factor (the ratio of average power to peak power)
of a well-operated active base should be 50 percent or
more with a power factor (explained later in this
chapter) of 80 percent or higher. If the load is more
than a few hundred feet from the power source, a
high-voltage distribution system may be required.
If several generators are to serve primary
distribution systems, they should generate the same
voltage to avoid the need for voltage transformation.
The number of phases required by the load may differ
from that produced by the generator. As loads usually
can be divided and balanced between phases, most
generators of appreciable size are wound for three-
Small motors can be supplied by single-phase ac at
normally 120 volts. Large three-phase, ac motors
above 5 horsepower generally operate satisfactorily at
any voltage between 200 and 240. The use of combined
light and power circuits will be accomplished by the
use of 240- or 208-volt systems.
Computation of the Load
As mentioned earlier in this chapter, there are
various factors that must be taken into consideration in
the selection of the required generating equipment.
The following technical data will help you in
computing the load.