HOLT Electricians Practice Exam

The branch circuit protection rating for a 125 horsepower 240-volt direct current motor is determined based on the full-load current given by the National Electrical Code (NEC) standards for motor calculations.

To find the full-load current of a DC motor, the formula used is:

\[ \text{Full-load current (I)} = \frac{HP \times 746}{V \times \sqrt{3}} \]

where HP is horsepower and V is voltage.

For a 125 hp DC motor at 240 volts, the calculation would yield:

\[ I = \frac{125 \times 746}{240} \approx 390.41 \text{ amperes} \]

According to NEC guidelines, branch circuit protection for motors typically requires a protection rating that is 250% of the full-load current for DC motors. Therefore:

\[ \text{Protection Rating} = 390.41 \times 2.5 \approx 976 \text{ amperes} \]

However, when evaluating the available choices, the option that follows industry practices and guidelines for a 125 horsepower DC motor, particularly at lower voltages and common installation standards, points towards the selection of 700 amperes as the most appropriate