Inverse-Time Circuit Breaker Sizing for Motors

What size inverse-time circuit breaker is required for a feeder supplying a 13hp. 208v motor and a 30.5hp. 208v motor?

a) 40 amp

b) 50 amp

c) 60 amp

d) none of these

Answer:

The correct answer is (d) none of these.

When determining the size of an inverse-time circuit breaker required for a feeder supplying a 13hp, 208V motor, and a 30.5hp, 208V motor, it is essential to consider the total horsepower and current needs of the motors.

First, calculate the total horsepower by adding the two motor capacities together. In this case, the total horsepower is 13hp + 30.5hp = 43.5hp.

Next, convert the total horsepower to watts by multiplying it by 746 watts per horsepower. The total watts will be 43.5hp * 746 watts/hp = 32499 watts.

Then, determine the total current by dividing the total watts by the voltage, which is 32499 watts / 208 volts ≈ 156.25 amps.

Lastly, select the next available standard breaker size above the calculated total current. In this case, none of the provided options (a) 40 amp, (b) 50 amp, or (c) 60 amp are suitable since they are below the required 156.25 amps. Therefore, the correct answer is (d) none of these.

It is crucial to select the appropriate size of the inverse-time circuit breaker to ensure the safe and efficient operation of the motors. Improper sizing can lead to electrical hazards and malfunctions. Understanding the calculations involved in determining the breaker size is essential for maintaining the electrical system's integrity.