Power and Voltage Requirements

The selection of voltage is affected by the size, character, and 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. Some small motors can be supplied by direct current (dc) or single-phase ac at nominally 120 volts. Large three-phase, ac motors above 5 horsepower generally operate satisfactorily at any voltage between 200 and 240. The general use of combined light and power circuits increases the use of 240 and 208 volts for general power application.

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.

Before any part of the system can be designed, the amount of power to be transmitted, or the electrical load, must be determined. Electrical loads are generally measured in terms of amperes, kilowatts, or kilovolt-amperes. In general, electrical loads are seldom constant for any appreciable time but fluctuate constantly. In calculating the electrical load, you must determine the connected load first. The connected load is the sum of the rated capacities of all electrical appliances, lamps, motors, and so on, connected to the wiring of the system. The maximum demand load is the greatest value of all connected loads that are in operation over a specified period of time. Knowledge of the maximum demand of groups of loads is of great importance because it is the group maximum demand that determines the sizes of conductors and apparatus throughout the electrical system.

The ratio between the actual maximum demand and the connected load is called the DEMAND FACTOR. If a group of loads were all connected to the supply source and drew their rated loads at the same time, the demand factor would be 1.00. There are two main reasons why the demand factor is usually less than 1.00. First, all load devices are seldom in use at the same time and, even if they are, they will seldom reach

Figure 5-2.\200.kW portable generator.

maximum demand at the same time. Second, some load devices are usually slightly larger than the minimum size needed and normally draw less than their rated load. Since the maximum demand is one of the factors determining the size of conductors, it is important that the demand factor be established as closely as possible.

The demand factor varies considerably for different types of loads and services. Demand factors for military structures are given in table 5-2.

Example: A machine shop has a total connected load of 50.3 kilowatts. The demand factor for this type of structure is taken at 0.70. The maximum demand is 50.3 0.70 = 35.21 kilowatts. Table 5-2.\Demand Factor

The application of the demand factor in designing electrical facilities is a major item in reducing initial costs. Knowing the demand factors for various types of buildings and installations is also beneficial when existing facilities are rearranged because increased serving capacity is often not required, even though the connected load is greatly increased. Similarly, increasing the size of feeder conductors because of a major load addition to the circuit may not be required.