HOLT Electricians Practice Exam

What is the recommended feeder protection for a 25 horsepower 208 volt 3 phase motor and three 3 horsepower 120-volt motors?

1. 200 ampere

2. 225 ampere

3. 250 ampere

4. 300 ampere

The correct answer is: 225 ampere

To determine the recommended feeder protection for a 25-horsepower 208-volt three-phase motor and three 3-horsepower 120-volt motors, it is necessary to calculate the total load and apply the National Electrical Code (NEC) guidelines for feeder sizing. First, calculate the full-load current for the 25-horsepower motor. For a three-phase motor, this can be found using a standard formula that takes into account the horsepower, voltage, and efficiency. The full-load current for a 25-horsepower motor operating at 208 volts can be approximately calculated as: \[ I = \frac{HP \times 746}{\sqrt{3} \times V \times pf} \] Assuming a power factor (pf) of 1 for simplicity, we find: \[ I \approx \frac{25 \times 746}{\sqrt{3} \times 208} \approx 68.6 \text{ Amps} \] Next, for the three 3-horsepower motors at 120 volts, the same calculation applies: \[ I_{single} = \frac{3 \times 746}{120} \approx 18.65 \text