Using the rule-of-thumb method, if a motor nameplate shows 54 full-load amps, what is the locked rotor current?

To determine the locked rotor current using the rule-of-thumb method, a common practice is to multiply the full-load amps by a specific factor, typically ranging from 5 to 7 for standard motors. For a motor nameplate showing 54 full-load amps, using the factor of 6 provides a reasonable estimate of the locked rotor current.

Calculating this gives:

Locked Rotor Current = Full-load Amps × Factor

Locked Rotor Current = 54 × 6 = 324 amps

This estimate is particularly useful in electrical applications for sizing circuit breakers and conductors, as it indicates the maximum current drawn by the motor at startup before it reaches its full operating condition. This value helps in understanding the potential starting current stresses that may be placed on electrical components and helps to ensure the proper selection of protective devices.