The overload contacts are opened by excess current flowing through the heater, located in the power circuit (fig. 7-51). The size of the heaters to be installed is determined by the full-load current draw to the motor. Magnetic starters are manufactured by many different companies. Information for the proper size of heater is given on the cover of the starter.

HEATERS AND HORSEPOWER.- Listed in table 7-4 is a typical horsepower and heater table for motors of different size and voltage. To determine the

Table 7-4.- Horsepower rating and heater table

heater number, we must know the horsepower and voltage and if the motor is single or three-phase. Once we have that information, we look at table 7-4, view A, and find the full-load motor amperage. Using the chart from table 7-4, view B, we can find the heater number for this motor. For example, we want to know the number of a heater for a 5-horsepower, 230-volt ac, single-phase motor. Checking table 7-4, view A, we find that the motor draws 28 amps. Referring to table 7-4, view B, we find heater number 42227 has an amperage range from 26.0 to 28.3. This is the heater we should use. Also in the table you will find the maximum fuse size and the amperage at which the heater will open the control circuit. Remember that each manufacturer has its own heater table to be used with its across-the-line starters.