What is the formula used to calculate the size for a feeder overcurrent protective device with given loads?

The formula used to determine the size of a feeder overcurrent protective device accounts for both continuous and noncontinuous loads to ensure the system operates safely and effectively. By multiplying the continuous load by 1.25, you are applying a necessary safety factor that takes into account the prolonged duration that continuous loads are operating. Continuous loads are defined as loads that will run for three hours or more, and they can lead to heat buildup in conductors and devices if not accounted for properly.

Adding the noncontinuous load, which can be operated for shorter periods and doesn't necessarily contribute to the same prolonged heating effects, completes the calculation for the total load that will be placed on the system. This ensures that the overcurrent protective device is sized adequately to handle both types of loads under normal operating conditions, thus minimizing the risk of tripping the device unnecessarily while also ensuring safety by preventing overheating and potential failures.