Calculate Maximum Rating for Inverse-Time Circuit Breaker for 30-HP Motor

What is the maximum rating permitted for the circuit breaker used for a 30-horsepower, 208-volt, 3-phase squirrel-cage motor?

A. 74.6A

B. 77.7A

Answer:

The maximum rating of the circuit breaker can be calculated by first finding the full-load current of the motor using the formula given and then using the NEC rule that the circuit breaker should not exceed 250% of this current.

To find the maximum rating permitted for the circuit breaker, we need to apply a formula from the National Electrical Code (NEC) that states the circuit breaker should not exceed 250% of the full-load current of the motor.

First, we need to calculate the full-load current of the motor. Assuming the power factor is 0.85 and the motor's efficiency is 0.9, we can use the formula: I = P / (1.732 * V * pf * eff), where V is the voltage (208 V), Pf represents the power factor (0.85), and eff the efficiency (0.9). Substituting these values, we get a full load current value.

Then, we apply the NEC rule: if the resultant value is the full-load current, multiply it by 250% (or 2.5). The result will be the maximum rating of the circuit breaker for this motor.