What is meant by “Class” in insulation?

The insulation rating is the maximum allowable winding (hot spot) temperature of a transformer operating at an ambient temperature of 40°C. Insulation systems are classified by the temperature rating. The following table summarizes the different insulation systems available.

Insulation Rating	Insulation Class	Average Winding Temperature Rise	Hot Spot Temperature Rise	Maximum Winding Temperature
Class 105	A	55 degree C	65 degree C	105 degree C
Class 150 or 130	B	80 degree C	110 degree C	150 degree C
Class 180	F	115 degree C	145 degree C	180 degree C
Class 200	N	130 degree C	160 degree C	200 degree C
Class 220	H	150 degree C	180 degree C	220 degree C

Note: the maximum acceptable temperature rise based on an average ambient of 30 degree C during any 24 hour period and a maximum ambient of 40 degree C at any time.