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What is the difference between a Transformer's "temperature rise" and its "insulation class"?

Issue:
Explanation of the difference between temperature rise and insulation class specifications.

Product Line:
LV Transformers

Environment:
Applies to Low Voltage Transformers by SquareD/Schneider Electric

Cause:
Transformer Temperature Rise and Insulation Class are two different technical attributes

Resolution:

This question has come up because NEC 450-21(b) requires Transformers larger than 112.5 kVA to be installed in fire resistant construction unless the insulation class is 155 or higher.

The insulation system of a Transformer is rated in degrees Celsius at its maximum temperature rating (the Class number = the degree C of the insulation system; example, a Class 220 insulation system is rated for 220°C) Transformers have either a Class 220 (former designation "H") Class 180 (former designation "F",) Class 150 (former designation "B") or Class 105 (former designation "A") Some smaller manufacturers may also use a Class 200 (former designation "R").

The temperature rise of a Transformer is the average temperature rise that will occur in the coils during normal full load.

The insulation system <> standard temperature rise relationships are:
Class 220 <> 150 degree Celsius rise
Class 180 <> 115 degree Celsius rise
Class 150 <> 80 degree Celsius rise
Class 105 <> 50 degree Celsius rise

A Transformer with a Class 220 insulation system can be designed for a maximum temperature rise that is lower than the standard 150 degree Celsius. It can be designed for either 115 or 80 degree Celsius rise. As well, a Class 180 insulation system Transformer can be designed with 80 degree Celsius rise. Class 150 and 105 transformers are not typically designed for other than their standard temperature rise.

Since all Square D low voltage dry type Transformers larger than 112.5kVA are built with a 220 degree Celsius insulation system, exception #2 under NEC 450-21(b) applies.

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