How to Calculate Motor Service Factors

by Dwight Chestnut
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A service factor, or "SF," is a factor representing the capacity at which a motor can operate without overloading or damaging its core or components. The reference point is a service factor of 1, which would mean a motor cannot operate over its rated horsepower, or "hp," without overheating. Similarly, for example, a service factor of 1.25 says the motor can operate 25% over its rated hp without damage or overheating.

Step 1

Determine the horsepower of your motor. If you do not know it, reference the manufacturer's specifications.

Step 2

Refer to the National Electrical Manufacturers Association table, or "NEMA table", for service factors. The table relates service factors to hp and revolutions per minute, or "RPMs" . Calculate or determine your service factor by choosing the service factor that corresponds to your hp and various RPM levels. For example, according to the table, if you have a 1HP motor and you operate it at 3600 RPM continuously, your service factor is 1.25.

Step 3

Calculate your operating or "effective" HP level. Multiply your motor horsepower by the service factor. For example, if you have a 1HP motor and your service factor is 1.25, then you can operate safely at an HP = 1.25HP without overheating or damaging the motor.