New York State Master Electrician Practice Exam 2025 - Free Electrician Practice Questions and Study Guide

To determine the required fuse size for a 100HP DC motor operating at 220V with 90% efficiency, it’s essential to first convert the motor's horsepower to watts.

1 horsepower is equivalent to approximately 746 watts. Therefore, a 100HP motor translates to:

100 HP × 746 W/HP = 74,600 watts.

Since the motor has an efficiency of 90%, the actual power that it consumes from the power supply is calculated by accounting for the efficiency rate:

Power input = Output Power / Efficiency

Power input = 74,600 W / 0.90 = 82,888.89 watts.

Next, we convert this power consumption into current using the formula for power in a DC circuit:

Power (watts) = Voltage (volts) × Current (amps)

Rearranging this gives us:

Current (amps) = Power (watts) / Voltage (volts)

Substituting in the values:

Current = 82,888.89 W / 220 V = 376.77 A.

Fuses should be selected with a safety margin, typically around 125% of the calculated continuous current. This ensures that the fuse will not blow under normal operating conditions