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# NEMA Insulation Classes

## Electrical insulation systems rated by standard NEMA classifications to maximize allowable operating temperatures.

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

 Temperature Tolerance Class Maximum Operation Temperature Allowed Allowable Temperature Rise at full load 1.0 service factor motor 1) Allowable 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.

## Related Topics

• ### Electrical

Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors.

## Related Documents

• ### Dielectric Strength of Insulating Materials

The dielectric strength of a material is the ability to the material to act as an insulator.
• ### Electrical Motors - Frame Dimensions

Electrical motors NEMA frame dimensions.
• ### Electrical Motors - NEMA Frame Assignments

NEMA frame assignments.
• ### NEC - National Electrical Code

National Electrical Code.
• ### NEMA - National Electrical Manufacturers Association

National Electrical Manufacturers Association.
• ### NEMA Enclosure Standards for Electric Motors

NEMA enclosure standard for electrical motors.

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## Citation

• The Engineering ToolBox (2004). NEMA Insulation Classes. [online] Available at: https://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html [Accessed Day Month Year].

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