Journeyman Electrician Practice Exam

For a feeder supplying a continuous lighting load of 39 amperes and a motor load of 24 amperes, the minimum required ampacity is?

1. 50 amperes

2. 60 amperes

3. 79 amperes

4. 100 amperes

The correct answer is: 79 amperes

To determine the minimum required ampacity for the feeder supplying both a continuous lighting load and a motor load, it's essential to apply the National Electrical Code (NEC) guidelines for calculating the total load correctly. In this scenario, there is a continuous lighting load of 39 amperes. According to the NEC, continuous loads must be calculated at 125% of their rating when determining conductor size. Therefore, the calculation for the continuous lighting load is as follows: 39 amperes × 1.25 = 48.75 amperes. Next, there is a motor load of 24 amperes. The NEC allows for the motor load to be calculated at 100% of its rating since it is not a continuous load. Thus, the motor contributes a full 24 amperes to the total required capacity. Now, to find the total required ampacity for the feeder, the continuous load and motor load need to be added together: 48.75 amperes (for the continuous lighting load) + 24 amperes (for the motor load) = 72.75 amperes. To determine the minimum ampacity required, it is prudent to round up to a standard available ampacity rating. The closest standard size above 72.