If the calculated load on the conductors is less than 381 amps why is this a violation with the 400 amp situation?
In a meeting with the state back in the 90's this came up as people were using the exception 3 to 230.90(A) for where you only have one service main or disconnect, it was explained by the CMP's at the meeting that in order to use this exception you must have one set of SEC's that feed more than one set of mains or disconnects but conductors feeding each main are required to still be sized to its rating as per 230.90(A)
If we look at 230.90(A) it clearly says it:
230.90 Where Required. Each ungrounded service conductor
shall have overload protection.
(A) Ungrounded Conductor. Such protection shall be
provided by an overcurrent device in series with each ungrounded
service conductor that has a rating or setting not
higher than the allowable ampacity of the conductor.
No where in 240 is there an allowance for service conductors to be sized smaller then the OCPD, and 240.2 makes this clear:
Tap Conductors. As used in this article, a tap conductor is
defined as a conductor, other than a service conductor, that
has overcurrent protection ahead of its point of supply that
exceeds the value permitted for similar conductors that are
protected as described elsewhere in 240.4.
Which allows it for taps same as 230.90(A) exception 3 but you are still required to size each tap conductor to the size of the OCPD down stream of the tap.
so if I have a 200 amp service feeding a 200 amp main and a 100 amp out building then yes the service conductors to the split can be 3/0 but the run to the 100 amp main has to be sized for it #3's and the conductors to the 200 amp main will have to be 3/0 as long as the total caculated load doesent exceed the 3/0 service conductors.
But if I only have one main there is no allowance to reduce the size required in 230.90(A)
Maybe this is a wrong interpatation but it's what I got out of that state meeting.