Just What is a Service Factor?

This is a question we regularly receive on the Hotline. Understanding how the service factor affects an electric motor can be confusing, but it is not as complicated as it may seem. This issue of the Franklin AID explains the term service factor and its relationship to AC submersible motors.

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.”

How does Service Factor (S.F.) Apply To A Motor?

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

 Franklin Electric submersible motors have service factors in agreement with NEMA guidelines for “pump motors”. Table I lists the service factors and horsepower ratings for Franklin’s 60 Hz submersible motors. 50 Hz submersible motors generally have a service factor rating of 1.0.

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.

What Do I Check If The Amps Are Higher Than Nameplate Max Amps?

If the amperage reading of your motor is higher than ameplate service factor maximum amps, you should check the following:

  • Voltage within 10% of nameplate voltage
  • Proper cooling flow past the motor as recommended by Franklin’s AIM manual
  • Use of recommended drop cable size
  • Phase (current) imbalance less than 5% at S.F. load on three-phase motors
  • Specified pump performance matches the application
  • Insulation resistance is good and there is no ground fault in the system

If any of these criteria are not met, the system should be corrected. If you have a voltage supply problem or phase imbalance still exists after rotation of the leads, contact the power company. Correct any deficiencies in cable sizing and motor cooling as well as a ground fault or low insulation resistance condition. If there is a concern related to matching the pump and the application, consult with the pump distributor or manufacturer.


Service factor is a multiplier that can be applied to a motor’s nameplate horsepower to determine the permissible loading under specified installation conditions. By understanding service factor and its relation to motor amps and proper installation conditions, you should be able to get the longest life out of your system and avoid service calls from your customer.