NATE Air Conditioning and Heat Pumps Practice Exam

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The NEC approves a ____ dual-element time-delay fuse as overcurrent protection for a 20-A motor?

20-A
30-A
40-A
60-A

The correct answer is: 60-A

In the context of protecting a 20-A motor, the National Electrical Code (NEC) allows for a dual-element time-delay fuse rated higher than the motor's full-load current to accommodate for the inrush current that occurs when the motor starts. Dual-element fuses are specifically designed to handle temporary overloads without blowing immediately, making them ideal for motor applications. Using a 60-A dual-element time-delay fuse provides more than sufficient protection and does not exceed the allowable limit for overcurrent protection in this scenario. The selection includes a margin to ensure that the fuse does not blow during the motor's normal start conditions, while still providing effective protection against sustained overloads or short circuits. Therefore, a 60-A fuse is suitable as it aligns with NEC requirements for motors of this amperage. The other fuse ratings would not provide the same level of overcurrent protection without risk of nuisance tripping or an insufficient margin for starting currents. This makes the 60-A fuse the most appropriate choice.