For the sum of multiple motor loads, how is the largest motor treated in calculations?

In load calculations for multiple motor loads, it is standard practice to treat the largest motor differently than the others. The largest motor is added to the total load calculation at 125% of its full load current. This approach is rooted in the need to account for potential starting currents and peak demands that significantly exceed normal operating conditions.

The rationale behind adding the largest motor at 125% is to provide a safety margin that accounts for the starting inrush current of the motor when it is turned on. Unlike smaller motors, which can be safely factored in at their full load rating, the largest motor’s characteristics warrant an increased consideration due to its substantial influence on the overall demand and system performance. This allows the calculation to ensure that there is adequate capacity in the electrical service and feeder conductors to handle these brief, high demands without compromising system reliability.

By using this methodology, designers can create a more resilient electrical system that can handle peak loads effectively. It also aids in preventing overheating and potential equipment failure due to undersized conductors or circuits that cannot handle the starting load of larger motors.