HOLT Electricians Practice Exam

For a 5 horsepower, 230 volt, single phase motor and a 3 horsepower, 230 volt, single phase motor, what size feeder protection is required?

1. 70 amperes

2. 80 amperes

3. 90 amperes

4. 100 amperes

The correct answer is: 80 amperes

To determine the required size of feeder protection for both the 5 horsepower and 3 horsepower, 230 volt, single-phase motors, it is essential to consider the Full Load Amperage (FLA) for each motor. The National Electrical Code (NEC) provides guidelines for calculating the FLA based on horsepower and voltage. For a single-phase motor, the FLA can be calculated using the formula: \[ \text{FLA} = \frac{\text{Horsepower} \times 746}{\text{Voltage} \times \text{Efficiency} \times \text{Power Factor}} \] Assuming standard efficiency and power factor values, the approximate FLA for a 5 horsepower motor is around 28 amperes, and for a 3 horsepower motor, it is around 16 amperes when using typical values for efficiency and power factor. Adding these current values gives: \[ 28 \text{ amperes} + 16 \text{ amperes} = 44 \text{ amperes} \] However, feeder protection sizing also takes into account continuous load requirements. According to the NEC, feeder protection is often sized at 125% of the total load current: \[ \text{Fe