What formula is used to determine the feeder overcurrent device size?

The formula for determining the feeder overcurrent device size is based on the sum of continuous load current and non-continuous load current. This approach ensures the device can handle both the regular continuous loads and any additional non-continuous loads that may occur during operation. Continuous loads are those that are expected to run for three hours or more without interruption, while non-continuous loads operate for shorter durations and can vary significantly.

By considering both types of loads, the overcurrent protection device is sized to effectively manage the potential maximum current flow, thus preventing overheating and ensuring the safety and reliability of the electrical system. This method also aligns with the National Electrical Code (NEC) requirements, which provide guidelines to ensure that electrical equipment is adequately protected from overcurrent conditions.