NEMA Insulation Classes for Transformers

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The capacity or rating of a transformer is limited by the temperature that the insulation can tolerate.  The life of a transformer can be extended by making sure it is not operated over and above the temperature rating of the insulation system on a continuous basis. A guiding rule of thumb would be that the useful operating life of the transformer halves for every 10°C  rise above its rated temperature. 

The insulation system of a transformer is rated in degrees Celsius at its maximum temperature rating:

The class number  = the maximum °C of the transformer insulation

NEMA (National Manufacturer’s Association) has the following thermal or insulation classification as regards transformers (dry type):


NEMA Insulation Class
NEMA Letter Designation
Ambient
Temperature
Maximum Allowable Temperature Rise
Maximum Allowable Operating Temperature
°C 
°F
°C 
°F
105
A
40°C 
50
122
105
221
130
B
80
176
130
226
155
F
105
221
155
311
180
H
125
257
180
356
220
R
150
302
220
428

Note  that a transformer with a Class 220 insulation system can be designed for a maximum temperature rise that is lower than the standard 150°C . It can be designed for either 125 or 80°C rise. Also, a Class 180 insulation transformer can be designed with 80°C rise. Class 155 and 105 transformers are not typically designed for other than their standard temperature rise

The maximum operating temperature is determined by by adding the rated ambient temperature of the device which is normally 40°C, the maximum temperature rise, and a 10°C hot-spot allowance:
Maximum operating Temperature 
= Ambient Temperature + Maximum Temperature Rise + 10°C hot-spot allowance.