HOLT Electricians Practice Exam

What size fuse is required for a 50 horsepower, 460 volt, 3 phase motor with a motor nameplate current rating of 60 ampere?

1. 60 ampere

2. 70 ampere

3. 75 ampere

4. 80 ampere

The correct answer is: 75 ampere

To determine the appropriate fuse size for a 50 horsepower, 460 volt, 3-phase motor with a nameplate current rating of 60 amperes, you need to consider the manufacturer's recommendations and the National Electrical Code (NEC) guidelines. In this case, it's essential to recognize that when selecting fuse sizes for motors, a rule of thumb is to use a fuse rated at 125% of the motor's full-load current. This approach helps to prevent nuisance blowing during normal motor startup conditions, which typically draw higher inrush currents. Calculating 125% of the nameplate current provides the following: 1. Multiply the nameplate current (60 amperes) by 125% (or 1.25): 60 A x 1.25 = 75 A. Given that the closest standard fuse size to this calculated value is 75 amperes, option C is appropriate because it meets the guideline of protecting the motor while allowing for normal variations during operation. Selecting a fuse larger than this, such as 80 amperes, might not offer adequate protection for the motor during overload conditions. Similarly, lower options like 60 or 70 amperes would not comply with the 125% rule and could result