Journeyman Electrician Practice Exam

What is the maximum initial standard rating for non-time-delay fuses protecting a 1 hp, three-phase, 208-volt AC motor?

1. 10 amperes

2. 15 amperes

3. 20 amperes

4. 25 amperes

The correct answer is: 15 amperes

For non-time-delay fuses protecting a 1 hp, three-phase, 208-volt AC motor, the National Electrical Code (NEC) provides guidelines on sizing overcurrent protection devices. For three-phase motors, the full-load current can be calculated using standard formulae or tables provided in the NEC. A 1 hp motor typically has a full-load current of about 3-4 amps at 208 volts. The NEC specifies that non-time-delay fuses used for motor protection can be sized at 150% of the motor's full-load current for protection against overcurrents. This results in the sizing process categorizing motors into different ranges based on horsepower ratings. Using these guidelines, when you calculate 150% of the full-load current for a 1 hp motor, it typically brings the allowable rating up to around 5-6 amps. However, for practical purposes and common standards observed in installations, fuses are often sized to the nearest standard rating. In this instance, the closest and most appropriate standard rating for non-time-delay fuses protecting this motor would be 15 amperes. This value ensures adequate protection while accommodating potential inrush currents during startup, as non-time-delay fuses react immediately to overcurrents.