Insulation systems are rated by standard NEMA classifications to maximum allowable operating temperatures.
Example of Temperature Tolerance Class F :
40°C + 105°C + 10°C = 155°C
In general, a motor should not operates with temperatures above the maximum. Each 10°C rise above the rating may reduce the motor lifetime by one half. Temperature Tolerance Class B is the most common insulation class used on most 60 cycle US motor. Temperature Tolerance Class F is the most common for international and 50 cycle motors.
**IEC insulation winding classes parallel those of NEMA and in all but very rare cases, use the same letter designations.