HOLT Electricians Practice Exam

What is the maximum branch-circuit short-circuit protection for a 25 horsepower synchronous motor, 460 volt, 3-phase?

1. 60

2. 70

3. 80

4. 90

The correct answer is: 70

To determine the maximum branch-circuit short-circuit protection for a 25 horsepower synchronous motor operating at 460 volts in a 3-phase system, one must refer to the National Electrical Code (NEC) guidelines. Specifically, NEC Table 430.52 provides the necessary information for motor branch-circuit protection. For motors up to 100 horsepower, the NEC typically allows for short-circuit protection to be based on a percentage of the motor's full-load current. In the case of a 25 horsepower motor, you calculate the full-load current using the formula for three-phase motors: Full-load current = (Horsepower × 746) / (√3 × Voltage × Efficiency × Power Factor). Assuming a standard efficiency and power factor for synchronous motors, the full-load current for a 25 HP motor at 460V can be estimated to be around 30-32 amperes. According to NEC guidelines, the maximum allowed short-circuit protection for a motor of this size is often set at 250% of the motor's full-load current for circuit breakers, which leads to set values. For a 25 HP synchronous motor, the maximum short-circuit protection typically recommended is 70 amperes, making this the appropriated