HOLT Electricians Practice Exam

If you have a thermally protected single-phase motor rated at 1.5 horsepower, what should the overload device rating be?

1. 15.0 amperes

2. 20.0 amperes

3. 25.0 amperes

4. 30.0 amperes

The correct answer is: 20.0 amperes

To determine the appropriate overload device rating for a thermally protected single-phase motor rated at 1.5 horsepower, it is essential to understand the relationship between horsepower and current draw. In a typical scenario, a single-phase motor's full-load current can be calculated using standard reference tables or formulas based on the motor's horsepower rating. For a 1.5 horsepower single-phase motor, the full-load current is generally around 15 amps at 115 volts or about 8 amps at 230 volts. The National Electrical Code (NEC) guideline typically recommends setting the overload device at 125% of the full-load current to ensure adequate protection. When calculating 125% of the full-load current, the result aligns closely with the 20.0 amperes figure. This rating provides a suitable margin that allows for normal operating conditions recognizing that there may be slight fluctuations in current draw without tripping the overload device, while still providing enough protection against overload conditions. This principle is primarily why 20.0 amperes is an appropriate choice for an overload device in this context, offering protection while accommodating the motor's operational characteristics.