Journeyman Electrician Practice Exam

What is the minimum ampere rating a general-use fused switch must have for a 25 hp, 480-volt, three-phase AC motor with a FLA of 32 amperes?

1. 25 amperes

2. 32 amperes

3. 39 amperes

4. 45 amperes

The correct answer is: 39 amperes

To determine the minimum ampere rating for a general-use fused switch for a 25 hp, 480-volt, three-phase AC motor with a Full Load Amperage (FLA) of 32 amperes, you can use the National Electrical Code (NEC) guidelines. According to NEC, for motor applications, the minimum fuse or breaker size typically needs to be at least 125% of the Full Load Amperage. Calculating 125% of the FLA of 32 amperes gives you 40 amperes (32 A x 1.25 = 40 A). However, since you need to select an actual fuse rating, it must comply with standard available sizes. The next standard size fuse larger than 40 amps is 45 amperes. Thus, the 39 amperes option would not suffice because it falls below the recommended rating, making it not suitable for protection against overcurrent situations. Therefore, the minimum appropriate size for the fused switch would be 45 amperes to adequately protect the motor while allowing for any surges during operation. This ensures both compliance with the NEC standards and the safety of the motor's operation as it ramps up during start-up.