ECN Forum Forums General Topics of the Trade General Discussion Area What changed, the wire or the breaker?

Yes, it is current carrying conductor but it only carries the unbalanced load on the single phase winding of the transformer. There is no way all 4 conductors can be full loaded at the same time. 310-15(b)(4). If the conductor ampacity needs to be adjusted because of ambient temperatures then the neutral size would also need to be adjusted.