Hi Master66
The 2 sets of 500's would give you a 760 amp service but it is permissible to use a 800 amp breaker to protect them. If your project requires a full 800 amp service or the calculated load is greater than 760 amps then you would need to use 600's. If you use 1 set of 500's for a 400 amp service then your load cannot exceed 380 amps but a 400 amp breaker is ok. The load on the conductors can not exceed the ratings shown in table 310-16 but if the ratings of the conductors do not correspond to a standard rating of fuse or circuit breaker the next higher rating can be used. This only applies if the rating is 800 amps or less. I would say that if the project calls for a 400 amps then you need to use 600's, 2 sets of 600's for 800 amps and 2 sets of 350's for 600 amps.
