Figure 3-2. - Multiple-transfer switches.

The electrical loads to be supplied power, voltage, 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.

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- phase operation.

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.

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.