HOLT Electricians Practice Exam

For a 5 horsepower, 230 volt, single phase motor with a nameplate current rating of 28 ampere, what size fuse is required if a dual element fuse is used for overload protection?

1. 20 ampere

2. 30 ampere

3. 35 ampere

4. 40 ampere

The correct answer is: 35 ampere

In determining the appropriate size of the fuse for a 5 horsepower, 230-volt single-phase motor with a nameplate current rating of 28 amperes, it is essential to consider the characteristics and requirements for motor protection. When selecting a fuse for overload protection, particularly a dual element fuse, the National Electrical Code (NEC) typically recommends using a fuse that can accommodate the starting current of the motor, which is usually much higher than the motor's running current. Motors, especially when starting, can draw significantly more current than their rated amperage, often exceeding 5 to 7 times the normal operating current. In this case, the motor's nameplate indicates a running current of 28 amperes. A dual element fuse is designed to allow for this momentary overload condition during start-up while still providing protection against prolonged overloads. For a motor with a running current of 28 amperes, the fuse should be sized adequately to handle the inrush current without blowing immediately. A common practice is to allow up to 125% of the full-load current for fuse sizing. By calculating 125% of 28 amperes, you get 35 amperes. This value accounts for the inrush current that occurs when