The National Electrical Manufacturers Association (NEMA) defines service factor in section MG1 - 1.43 of their manual as: "The service factor of an alternating current (AC) motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor." The conditions under which service factor may be applied are described in NEMA MG1 - 14.36 as: "When the voltage and frequency are maintained at the value specified on the motor's nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate."

To determine the service factor horsepower of a motor, multiply the nameplate horsepower (not amperage) by the service factor. For example, if a 1/2 Hp motor has a service factor of 1.6, the motor's service factor maximum horsepower is:

(0.5 HP) x (1.6 S.F.) = 0.8 Hp.

Likewise, a 10 Hp motor with a nameplate service factor of 1.15, as shown in example 1, has a service factor If maximum horsepower of:

(10 HP) x (1.15 S.F.) = 11.5 Hp

How Does Service Factor Relate to Motor Amps?

Franklin 60 Hz motors typically show two amperage values on the nameplate and in the specification sections of the Application Installation Maintenance (AIM) manual. The first value labeled "amps" or "rated input amps" is the expected running amps when the motor is operated at rated voltage and nameplate horsepower loads. The second value labeled "SF Max Amps" or "Max A" is the expected running amps when the motor is operated at rated voltage and service factor horsepower loads.

Even though amps may increase slightly when input voltage is higher or lower than nameplate voltage, the motor amp reading remains the simplest indicator of motor load. your motor amp reading is higher than nameplate service factor amps, the horsepower load probably exceeds the service factor horsepower rating.