Maximum HP of 3 phase AC motor
Motors rated in horsepower probably conform to NEMA standards while motors rated in kW probably conform to IEC standards. NEMA standard MG-1 has a provision for marking a service factor on the nameplate. A service factor 0f 1.0 is used for motors that are not designed to accommodate more than brief a brief overload when the motor is started. A service factor of 1.15 is used for motors that can tolerate a 15% overload for an extended period of time. A 1.15 S.F. motor can operate continuously at 15% overload, but it would not be expected to have the same lifetime as it would if not overloaded. There are curves available that relate insulation life to temperature. Insulation temperature is affected by load, ambient temperature, altitude, frequency of starting and the presence of dirt or anything else that would impede cooling.

The USA National Electrical Code requires motors to be protected by a device that is responsive to motor internal temperature or current. The trip point is allowed to be 125% of rated current for 1.15 S.F. motors and 115% for 1.0 S.F. motors.Winding temperature detectors are supposed to protect the motor based on internal temperature, but they can allow a higher overload current than the current sensing type.

The time vs. temperature curves for current sensing overload protection devices will provide a general idea of what level of overload will cause the protection to shut the motor off.

A motor that is never subjected to overload or high ambient temperature conditions could run for 24 hours a day every day for many years. A motor that is overloaded frequently or for an extended period of time might last quite a while, but not as long. It is difficult to estimate how long.