Standard for OA/FA/FOA TRANSFORMER, REF. Sizing Secondary conductors

[sarawakmiri]

Have searched all over for written standard that specifically addresses sizing the Secondary conductors for OA/FA/FOA, 55 / 65 C. TRANSFORMER.
I have been unable to find anything. Logic would suggest the max load would be fixed for the OA Rated KVA.
And the FA/ FOA RATING IS FOR AMBIENT CONDITIONS and the long life of the Transformer.
This must be common in Industrial applications.
Do you size the Secondary conductors for the FOA 65 C. Rated KVA.
Thank you for your experience and any technical reference you can give.

 

[dpc]

In the US, the National Electrical Code (NEC) requires that the secondary feeder be sized based on the "nameplate rating" of the transformer. Electric utilities in the US are not required to follow the NEC so can use whatever logic they want for the most part as David Beach has already explained to you.

I'm not sure why you keep saying that the FA and FOA rating are for ambient conditions. This depends on the design intent. The transformer can be operated continuously at the FA or FOA rating as long as the ambient temperature basis is not exceeded. To extend the life of the transformer, many engineers design their system such that normally the transformer operates up the OA rating, but can be run at FA ratings for high load conditions.

If a transformer is purchased with FA ratings, it is generally false economy to base conductor sizing on the OA rating. The day may come when the extra capacity is needed and it would be poor engineering practice to be limited by the conductor size.