HOLT Electricians Practice Exam

When feeding three motors, including one 5 horsepower motor with a service factor of 1.2, what is the feeder conductor protection device rating?

1. 70 amperes

2. 80 amperes

3. 90 amperes

4. 100 amperes

The correct answer is: 90 amperes

To determine the appropriate feeder conductor protection device rating for the three motors, you first need to calculate the full-load current of the motors, particularly the 5 horsepower motor with a service factor of 1.2. The full-load current for a 5 horsepower motor can be calculated using the formula: \[ I = \frac{P \times 1000}{\sqrt{3} \times V \times \text{Efficiency} \times \text{Power Factor}} \] Assuming a standard voltage of 480V and a motor efficiency of approximately 0.9 with a power factor of around 0.85: \[ I = \frac{5 \times 746}{\sqrt{3} \times 480 \times 0.9 \times 0.85} \] This approximation yields a full-load current of about 6.1 amperes for the 5 horsepower motor under normal conditions. Next, considering the service factor of 1.2, you multiply the full-load current by the service factor to account for increased load capacity: \[ \text{Adjusted Current} = 6.1 \times 1.2 = 7.32 \text{ amperes} \]