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NEMA Insulation Classes

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Electrical insulation systems are rated by standard NEMA (National Electrical Manufacturers Association) classifications according to maximum allowable operating temperature:

NEMA Insulation Classes
Temperature Tolerance Class Maximum Operation
Temperature Allowed
Allowable Temperature Rise at full load
1.0 service factor motor 1)
Temperature Rise
1.15 service factor motor 1)
oC oF oC oC
A 105 221 60 70
B 130 266 80 90
F 155 311 105 115
H 180 356 125 -
  • T(oF) = [T(oC)](9/5) + 32

1) Allowable temperature rises are based upon a reference ambient temperature of 40oC. Operation temperature is reference temperature + allowable temperature rise + allowance for "hot spot" winding.

Example Temperature Tolerance Class F:

40 oC + 105 oC + 10 oC

    = 155 oC

In general a motor should not operate with temperatures above the maximum. Each 10 oC rise above the rating may reduce the motor lifetime by one half. It is important to be aware that insulation classes are directly related to motor life.

Example - a motor operating at 180oC will have an estimated life of

  • only 300 hours with Class A insulation
  • 1800 hours with Class B insulation
  • 8500 hours with Class F insulation
  • tens of thousands of hours with Class H insulation

Temperature Tolerance Class B is the most common insulation class used on most 60 cycle US motors. Temperature Tolerance Class F is the most common for international and 50 cycle motors.

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