Dry transformer >112.5 KVA indoors

[chevy1969pu]

Installing a transformer (Dry type) indoors in an industrial/manufacturing facility. Reviewing NEC section 450.21(B) requires all dry transformers above 112.5 KVA installed indoors to be installed in a transformer room. This must be a fire resistant room with a 1 hour fire rating. There is an Exception to this section which will allow enclosed dry transformers (w/ventilating openings) to be installed without a transformer room if the insulation class is 155 or higher.

Does this insulation class refer to the temperature rating in degree C??

 

[charlie b]

Re: Dry transformer >112.5 KVA indoors

Yes.

 

[chevy1969pu]

Re: Dry transformer >112.5 KVA indoors

Thanks Charlie,

That's what I thought, just wanted to make sure.

Mike

 

[jim dungar]

Re: Dry transformer >112.5 KVA indoors

Dry type transformers are usually in manufacturer catalogs with their temperature rise listed not the insulation temperature rating.

The insulation rating for a transformer takes into account an design ambient temperature (typically 40C), an internal hot spot temperature difference (usually 30C), and the average conductor temperature rise.

So, what is the NEC really asking for?

Transformer insulation ratings:
normal dry type transformers
Class B -> 80C rise - 150C ultimate
Class F -> 115C rise - 185C ultimate
Class H -> 150C rise - 220C ultimate

specialty transformers
Class A -> 55C rise - 105C ultimate