500 MCM is 380a @ 75c
I don't see a real problem. If you can get in under the feeder tap rules it is a slam dunk and you can probably use 240.4(B) if it comes down to it.
From a practical sense, you are really only providing short circuit protection in the 400a breaker since overload protection will come with the 300a. That is how they rationalize the "tap rules".
Last edited by gfretwell; 08/19/07 09:57 PM.