NEMA Insulation Classes
Insulation systems are rated by standard NEMA classifications according to maximum allowable operating temperatures
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Insulation systems are rated by standard NEMA (National Electrical Manufacturers Association) classifications according to maximum allowable operating temperatures as follows:
| 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: 40oC + 105oC + 10oC = 155oC.
In general a motor should not operate on temperatures above the maximums. Each 10oC rise above the ratings may reduce the motor's lifetime by one half.
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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