Technical information

8 – THEORY OF OPERATION
Motor Service Factor
117
8.2 Motor Service Factor
General
The Motor Service Factor parameter should be set to the service factor of the motor. The service
factor is used to determine the “pick up” point for the overload calculations. If the service factor of
the motor is not known then the service factor should be set to 1.00.
NOTE: The NEC (National Electrical Code) does not allow the service factor to be set above 1.40.
Check with other local electrical codes for their requirements.
The National Electrical Code, article 430 Part C, allows for different overload multiplier factors
depending on the motor and operating conditions. NEC section 430-32 outlines the allowable service
factor for different motors as follows:
Motor Overload Multiplier
Service factor 1.15 or more 1.25
Motor temp. rise 40°C or less 1.25
All others 1.15
NEC section 430-34 permits further modifications if the service factor is not sufficient to start the
motor:
Motor Overload Multiplier
Service factor 1.15 or more 1.40
Motor temp. rise 40°C or less 1.40
All others 1.30
Although the NEC does not address the effect of the ambient temperature of the motor location,
guidance can be derived by examining NEC limits. If the motor is operating in an ambient
temperature that is less than 40°C, then the overload multiplier can be increased while still protecting
the motor from exceeding its maximum designed temperature. The following curve gives the
ambient temperature versus the correction factor.
Example: If a motor operates at 0°C, then a 1.36 correction factor could be applied to the overload
multiplier. This could give a theoretical overload multiplier of 1.36 x 1.25 or 1.70. The highest legal
NEC approved value of overload multiplier is 1.40, so this could be used.