How to Determine the Maximum Rating for a Circuit Breaker for a 30-HP Motor?

Calculating the Circuit Breaker Rating

In order to determine the maximum rating permitted for a circuit breaker used to protect a 30-horsepower, 208-volt, 3-phase squirrel-cage motor, we need to follow a specific calculation process.

First, we need to translate the motor rating from horsepower to watts. Remember that 1 horsepower is equivalent to 745.7 watts, so for a 30-horsepower motor, the power in watts would be 30 * 745.7 = 22371 watts.

Next, assuming a phase to phase voltage of 208V and a unity power factor (cos=1), we can calculate the rated current using the formula: Rated Power (watt) = 3 * Voltage * Current * Power Factor.

If 208V is the voltage from phase to neutral, the formula becomes: Rated Power (watt) = 3 * 208V * Current * 1, which simplifies to Rated Power (watt) = 3 * 208V * Current.

By rearranging the formula, we find that the rated current for the motor is 22371 watts / (3 * 208V) = 62.169A.

Now, in order to calculate the maximum capacity of the circuit breaker, we need to multiply the rated current by 1.2 and 1.25 to account for typical installation practices. Therefore, the needed maximum rating for a circuit breaker is 62.169A * 1.2 = 74.6A and 62.169A * 1.25 = 77.7A.

Any circuit breaker between the aforementioned ratings of 74.6A to 77.7A will be adequate for protecting the 30-horsepower, 208-volt, 3-phase squirrel-cage motor.