User manual

Motor Service Factor
7.2 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.
z 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:
NEC section 430-34 permits further modifications if the service factor is not sufficient to start the motor:
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.
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7 - THEORY OF OPERATION
Service factor 1.15 or more
1.25
Motor temp. rise 40°C or less 1.25
All others 1.15
Motor Overload Multiplier
Service factor 1.15 or more
1.40
Motor temp. rise 40°C or less 1.40
All others 1.30
Motor Overload Multiplier