When should over-current devices be selected for multiple motors?

Selecting over-current devices for multiple motors is primarily guided by the need to protect the motors without unnecessary nuisance tripping during specific conditions, such as starting. The correct practice is to choose over-current protection based on the largest motor because this motor will have the highest current demand during startup. Electrical codes and standards often allow for the addition of certain percentages of the full load rating of the other motors, but the key point remains that the protection device must be adequately rated to handle the starting current of the largest motor without tripping unnecessarily.

Choosing protection based on the smallest motor can lead to under-protection, as it may not accommodate the starting current of larger motors. Selecting a device based on the total of all motor ratings might result in the protection device being too sensitive, leading to tripping during normal motor startup conditions or overloads. Lastly, considering ampacity exceeding 15A does not take into account the operational characteristics required for protection of motors, potentially putting more expensive equipment at risk.

Thus, grounding the selection of the over-current device in the specifications of the largest motor ensures both compliance with safety standards and operational efficiency.