I am hooking a 400 kw generator 480v 3 ph pri. To a buck boost transformer with 480 to 12470v then feeding utility lines as cogen. Is there a rule for sizing the feeder cable from the gen. to the transformer?
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If your located in the USA, the rules are listed in the National Electric Code.

Can be reviewed at NFPA (http://www.nfpa.org).

Who is the electrician or design engineer ? They should be doing the calcualtions for this.