The copper coming from "something" will be at least 1/2 inch thick, probably more so that it doesn't lose voltage. We're probably talking a 7/0 cable where 4/0 would be bare minimum.
Probably larger than that. If they’re 200m from the transformer, 400A service requires (3) 1000kcmil cables per leg. 100m drops it to (3) 350kcmil which is still pretty beefy