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How to Calculate Motor Service Factors

by Dwight Chestnut

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.

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.

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