What is meant by the insulation class of a transformer?

Insulation materials are an essential part of a transformer because the stability of the insulation system has a major impact on the life of the transformer.

Insulation material manufacturers specify up to which temperature their material remains stable. The insulation system (and thus the transformer) is practically indestructible if this maximum temperature is respected.

Based on this temperature, insulation materials are classified into different insulation classes:

Insulation classe







Maximum temperature

105 °C

120 °C

130 °C

155 °C

180 °C

>180 °C

The class of the least-performing insulation material determines the class of the transformer's entire insulation system and therefore the maximum permissible heat-up of the transformer.

E.g. a transformer with insulation class B will not get hotter than 130°C at its warmest point (inside the windings), even at maximum ambient temperature and maximum load.

Does this mean that a higher insulation class always results in a better transformer?

Not necessary!

Choosing insulation materials with a higher insulation class usually implies that higher heating is expected, which may mean that the transformer is less efficient.