HOLT Electricians Practice Exam

The branch circuit short-circuit protection device for a 10 horsepower 230 volt single-phase wound rotor motor shall not exceed what rating?

1. 110 amperes

2. 125 amperes

3. 115 amperes

4. 120 amperes

The correct answer is: 125 amperes

In determining the appropriate branch circuit short-circuit protection device rating for a 10 horsepower 230 volt single-phase wound rotor motor, the key is to understand the National Electrical Code (NEC) guidelines regarding motor circuits. For continuous duty motors, the NEC specifies that the short-circuit and ground-fault protective device can typically be sized based on the motor's full-load current (FLC). The FLC can be calculated using the formula: FLC = (Horsepower × 746) / (Voltage × √3 × Efficiency × Power Factor) However, for fractional and small industrial motors, a simplified approach is often used based on standard sizing charts provided in NEC and manufacturer's guidelines. For a single-phase 10 horsepower motor at 230 volts, the FLC under typical conditions is approximately 50 amps. When applying NEC guidelines, specifically in Article 430 regarding motors, the circuit protective device can be set at a maximum of 125% of the motor’s FLC. Calculating 125% of 50 amps results in 62.5 amps. However, for larger motors like a 10 hp, the maximum branch circuit protection must also be considered alongside standard ratings. While one might initially think smaller ratings apply, in this