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Contactors are similar to relays except they are designed to carry larger currents. Ill. 1 shows examples of several sizes of available contactors for industrial applications. The smallest contactor in this figure is about 4 inches tall, and the largest is about 4 feet tall.
Ill. 2 shows a table for voltage, current, and horsepower ratings for contactors and motor starters. This table provides NEMA ratings for contactors and motor starters that range from size 00 through size 9. From this table notice that the current rating for a size 00 is 9 A, and the rating for a size 9 contactor is 2250 A. Additional ratings for total horsepower are also listed in this table. When one is selecting a contactor or motor starter, one must ensure that the contact ratings exceed the load one is going to control with them. Each of these sizes of contactors and motor starters is available with a variety of coil voltages that range from 120-600 V.
Use the table in Ill. 2 to determine the proper size of contactor to use for controlling a large electric heating element that pulls 75 A as a continuous load. The voltage for the heating element is 480 volts.
Since the heating element draws 75 A, one can notice that the size 2 contactor can safely control 45 A and the size 3 contactor can handle 90 A. Since the 75 A load falls between these two values, the larger size 3 starter must be used. Also notice that the starter size can be used for all voltages up to 600 volts.
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