ED-GEE,
You need to find the unbalanced current first. But lets assume that you can connect a 400A single phase load on 3 phase 4 wire system, then your unbalanced load is 400A. NEC 220.61(B)(2) allows 70% on neutral current in excess of 200A. So your neutral current in this case will be 340A.
The transformer jumpers from the transformer terminals to the wye point connection carry the same current as the phases and should not be reduced in size.
Bill
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"Why not the best?"
Jimmy Carter