HOLT Electricians Practice Exam

How should branch circuit protection be rated for a motor with a full-load current of 52 amperes?

1. 80 amperes

2. 100 amperes

3. 125 amperes

4. 175 amperes

The correct answer is: 175 amperes

When determining the appropriate branch circuit protection rating for a motor, it is essential to follow the guidelines established by the National Electrical Code (NEC). The NEC specifies that the branch circuit protection for motors should typically be rated based on the motor's full-load current and should account for potential inrush currents when the motor starts. For a motor with a full-load current of 52 amperes, the NEC recommends that the branch circuit protection device be sized at 125% of the full-load current for overload protection and an additional provision for short-circuit protection. Calculating 125% of 52 amperes gives us 65 amperes (52 amperes x 1.25). However, when selecting a standard breaker size, one needs to choose the next standard size over that calculated value, which would be a 70-ampere breaker in this scenario. For larger motors, the NEC allows for higher ratings based on the type of motor and application, and it also allows flexibility in sizing for branch circuit conductors as long as they are adequate for the operational capacity. The choice of 175 amperes likely considers specific motor start characteristics or a particular application where a more robust protection scheme is necessary. Brushless or high starting torque motors might