Which percentage is used to calculate the minimum feeder-circuit conductor size by applying the continuous load percentage?

• A. 100
• B. 125
• C. 80
• D. 75

Answer: (B)

When sizing feeders for loads that run continuously (three hours or more), you must account for the extra heat that sustained current can generate. The way to do this is to multiply the continuous load by 1.25, i.e., use 125%. This gives the required ampacity a cushion so the conductor won’t overheat under steady operation. In practice, you determine the continuous load, multiply by 1.25 to get the minimum needed ampacity, and then pick a conductor with an ampacity at or above that value. That makes 125% the correct choice for calculating the minimum feeder-circuit conductor size.