Inductive loads, like the chiller motor in our example, behave differently than resistive loads such as electric heaters. Their current draw is greatest at start-up and corresponds to the existing load when running. In other words, a motor operating normally draws rated amps (RLA) at rated load, fewer amps at less-than-rated load and more amps at greater-than-rated load. It's the latter condition that necessitates overload protection.

Adding an overload protection device prevents the motor from drawing more than its rated amperage for an extended period. Basic overload devices simply open the circuit when current draw reaches the "trip" point. More sophisticated devices attempt to restore normal motor operating conditions by reducing the load, but will disconnect the motor if overloading persists.

For most overload protection devices, "trip" time is determined by the magnitude of the overload. Figure 1 illustrates a straight-line, time/current "trip" curve that shows response times for current draws greater than 110 percent of RLA. A device with these characteristics would allow our example chiller motor to draw 480 amps for 8 seconds before disconnecting it.

Continue on to Fault Current, n.