I am uncertain as to when this section can be sited as a reason for utilizing a breaker with a rating above the ampacity of the conductors being protected. Say, for instance, the ampacity (after all derating) of a set of service conductors is 380A. Since, according to 240.6(A), standard breaker size jumps from 350 to 400A, can the conductors with an ampacity of 380A feed a 400A service?

The 380 amp conductors may be protected by a 400 amp over current device but the maximum calculated load on these conductors can not exceed 380 amps.

For service over current protection and conductor coordination you must look in services because the rules are diferent.

230.90.A

This does not allow for selection of the next higher size of over current protection.

tom

Rounding up to the next standard size is permitted. Take a look at 230.90(A) exception 2 and if you have the handbook, see the commentary following the exception.

Tom