Master Electrician Practice Exam

What is the minimum ampere rating for an inverse time circuit breaker used as a disconnect for a 50 hp, 480-volt motor?

1. 50 amperes

2. 60 amperes

3. 75 amperes

4. 100 amperes

The correct answer is: 75 amperes

To determine the minimum ampere rating for an inverse time circuit breaker serving as a disconnect for a 50 hp, 480-volt motor, we first need to calculate the full-load current of the motor. Using the formula for three-phase motors, the full-load current can be calculated as: \[ I = \frac{746 \times \text{HP}}{V \times \sqrt{3} \times \text{Efficiency} \times \text{Power Factor}} \] Assuming standard values for efficiency (around 0.9) and power factor (usually around 0.8 for industrial applications), the calculation becomes: \[ I = \frac{746 \times 50}{480 \times \sqrt{3} \times 0.9 \times 0.8} \] This results in a calculated full-load current of approximately 60-70 amperes, depending on the precise values used for efficiency and power factor. The National Electrical Code (NEC) provides guidelines for the selection of circuit breakers and requires that the circuit breaker must be sized at a minimum of 125% of the motor’s full-load current when used as a disconnect. Therefore, using 75%