Home » News » FAQ » What is the Transformer Class A Insulation?

Class A insulation in a transformer include.

The Materials of class Y (cotton,silk,paper,wood,cellulose,fibre etc. without impregnation or oil
immersion) impregnated with natural resin,cellulose esters,insulating oils,laminated
wood,varnished paper and cellulose-acetate film etc.

credit: Electrical Machines 3rd edition, B.J Le Roux

The classes of insulation systems (at least on NEMA) are based on temperature.

Class A is not worth talking about because it is so low.

Class B is common – uses a 80 deg C total.

Class F is usually used in large motors – 115 deg C system total.

and Class H is used in dry-type transformers and large utility generators operating up in the 15
–20kV ranges – 220 deg C system total.

Use ambient + winding temp rise + hot spot temperature to get the totals.

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