HOLT Electricians Practice Exam

For twenty-two ½ horsepower single phase 115 volt motors, what should the fuse size not exceed if dual-element fuses are used?

1. 100 amperes

2. 110 amperes

3. 120 amperes

4. 90 amperes

The correct answer is: 110 amperes

To determine the appropriate fuse size for the motors in question, we first need to calculate the total current drawn by the twenty-two ½ horsepower single-phase 115-volt motors. Each motor has a power rating of ½ horsepower, which converts to watts since 1 horsepower equals about 746 watts. Therefore, the power for one motor is: ½ HP × 746 W/HP = 373 watts. For twenty-two motors, the total power would be: 22 × 373 W = 8,206 watts. Next, using the formula for calculating current (I = P/V), where the power (P) is 8,206 watts and the voltage (V) is 115 volts, we find the total current drawn by the motors: I = 8,206 W / 115 V ≈ 71.7 amperes. When selecting fuse sizes, it’s essential to consider the type of fuse used. Dual-element fuses are designed to handle inrush currents that occur when motors start. Generally, the National Electrical Code (NEC) recommends that fuse sizes for motors can be calculated based on a percentage of the full-load current. Typically, dual-element fuses can be sized at 125%