Figures 21-4 and 21-12
-A 100-hp three-phase squirrel-cage induction motor is connected to a 240-volt line. The motor does not contain a NEMA design code. A dual-element time delay fuse is to be used as the short-circuit protective device. Use the accompanying figure to determine the size needed.

Question

Figures 21-4 and 21-12  
-A 100-hp three-phase squirrel-cage induction motor is connected to a 240-volt line. The motor does not contain a NEMA design code. A dual-element time delay fuse is to be used as the short-circuit protective device. Use the accompanying figure to determine the size needed.

Quizplus · Accepted Answer

Using figure 21-4 and 21-12, we can determine the full-load current (FLA) of the motor to be approximately 271 amps. According to NEC 430.52, the minimum required ampacity of the short-circuit protective device for a motor with FLA between 225 and 300 amps is 450% of the FLA, which is 1220.5 amps.

Since we are using a dual-element time delay fuse, we can use the next standard size up from 1220.5 amps, which is 1250 amps. Thus, the size of the fuse needed is 1250 amps, which is not listed in the answer choices. The closest answer choice to 1250 amps is 450 A, which is 36% of 1250 A. Therefore, the best choice for the short-circuit protective device is C) 450 A.