NEMA Insulation Classes for Transformers

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.