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#177674 - 05/09/08 11:43 AM Cable derating for transformer rise
SteveFehr Offline
Registered: 03/19/05
Posts: 1195
Loc: Chesapeake, VA
I recieved this information for a NEMA 4 dry-type transformer I required for a particular application, and was concerned the enclosure will end up essentially being an enclosed box that will just bake my cables.

[Our company] designs enclosed transformers to have a maximum enclosure temperature of 65C above ambient for readily accessible parts and a maximum temperature rise of 80C for not readily accessible parts per IEEE 57.12.01.

If 115° and 150°C rise and hotter are common ratings for transformers, how in the world is this reconciled with 90°C rated cables?

I'm having difficulty finding any solid information about this. Am I worried over nothing, will the cables wick away the heat fast enough that it's not an issue? I don't mind derating cabling, but it's difficult to derate a 90C cable for 120C ambient conditions. If I go strictly by NEC, I know I can ignore this because it's less than 2' at the end of the cable, but it still worries the hell out of me. What's the standard way to terminate to a NEMA 4 transformer?
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