Serfice Factor

The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage. The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0.
A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.
NEMA Service Factor at Synchronous Speed (RPM) for drip proof motors:

Power (HP)	Service Factor - SF
	Synchronous Speed (RPM)
	3600	1800	1200	900
1/6, 1/4, 1/3	1.35	1.35	1.35	1.35
1/6	1.25	1.25	1.25	1.25
3/4	1.25	1.25	1.15	1.15
1	1.25	1.15	1.15	1.15
1 1/2 and up	1.115	1.15	1.15	1.15

Example - Service Factor

A 1 HP motor with a Service Factor - SF = 1.15 can operate at

1 HP x 1.15
    = 1.15 HP

without overheating or otherwise damaging the motor if rated voltage and frequency are supplied to the motor.
Insulation life and bearings life are reduced by the service factor load.