HOLT Electricians Practice Exam

What is the maximum branch-circuit short-circuit protection (non-time delay fuse) for a 3 horsepower, 115 volt motor?

1. 100 ampere

2. 110 ampere

3. 120 ampere

4. 130 ampere

The correct answer is: 110 ampere

To determine the maximum branch-circuit short-circuit protection for a 3 horsepower, 115 volt motor, it is essential to reference the National Electrical Code (NEC) guidelines, which provide information on the allowable fuse sizes for motor protection. For a single-phase motor, the NEC typically allows for a maximum fuse size that is calculated based on the horsepower of the motor. In this case, for a 3 horsepower motor, the formula involved includes taking the full-load current (FLA) of the motor and applying a multiplier for short-circuit protection. The full-load current for a 3 horsepower, 115-volt motor is approximately 30 amps. According to the NEC, the maximum size of branch-circuit protection device using non-time-delay fuses can be calculated by multiplying the full-load current by 250%. Thus, in this scenario, 30 amps multiplied by 250% equals 75 amps. However, when assessing the specific ratings, non-time-delay fuses are limited to certain sizes to ensure ample protection for the equipment. In this context, the maximum branch-circuit short-circuit protection is allowed up to 110 amps for a 3 horsepower motor, which aligns with the NEC guidelines for protection against