HOLT Electricians Practice Exam

For short-circuit protection, what rating is suitable for a 3-phase motor with a total of 10 horsepower?

1. 100 amperes

2. 125 amperes

3. 150 amperes

4. 175 amperes

The correct answer is: 150 amperes

To determine the suitable short-circuit protection rating for a 3-phase motor with a total of 10 horsepower, we can reference standard practices regarding motor current ratings. Typically, the current for a 3-phase motor can be calculated using the formula: **Motor Current (Amps) = (HP × 746) / (Voltage × √3 × Power Factor)** Assuming a common voltage level (for example, 460 volts) and a power factor of 0.8, the current would be approximately 12.4 amps for a 10 HP motor. However, for short-circuit protection, motor overload protection devices like circuit breakers or fuses are chosen based on their ratings to ensure they can handle startup surges and sustain normal operating conditions without nuisance tripping. The National Electrical Code (NEC) provides guidelines suggesting that the short-circuit protection for motors should generally range from 125% to 175% of the motor's full-load current. Hence, in practice, for a 10 HP motor, a short-circuit protection rating of up to 150-175 amperes is reasonable to accommodate any inrush current while remaining safe from overload conditions. Given this context, a rating of 150 amperes