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?

If your located in the USA, the rules are listed in the National Electric Code.

Can be reviewed at NFPA.

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