So 4/0 AL cannot be used either, then, correct? Although if the load calc for the dwelling could be used on only one of the runs, the other run could be a future panel to feed the shop and other things like pumps, sheds, etc. Am I getting that right, or am I missing something? I am about to do a voltage drop calc on the 4/0 AL, and may see that the dwelling load does not work with that size wire. Seems like that load calc must needs be extremely accurate. Competitor is telling homeowner that it can be done, and I would love to agree, but just a bit more cautious, I guess. Would love input on this further.
If you are supplying individual disconnects with individual conductors your conductor ampacity must be equal or greater than the overcurrent protection - next size up rule applies to 800 amps and under.
If you have the allowed two to six disconnecting means, then you can tap them off a conductor with ampacity equal or greater than the calculated load.
4/0 aluminum is good for 180 amps. It can be used on a circuit with 200 amp overcurrent protection as long as the calculated load is not over 180 amps, because of the next size up rule. If it were to have 181 amp of load then a larger conductor must be used.
You can not use two runs of 2/0 copper to feed 2- 200 amp circuits because of the mentioned fact that neither one of the sets will feed an entire dwelling unit.
I also don't think 310.15(B)(7) would allow paralleling two 2/0 copper conductors and calling the net result a 400 amp conductor.
You however could parallel two runs of 2/0 copper effectively making them a single 350 amp conductor, and then tap two to six disconnecting means from it as long as the calculated load is not more than 350 amps. All two to six disconnecting means could add up to well over 350 amps and it is still compliant if load calculation is 350 or less.
If you were supplying a two family dwelling you could use 310.15(B)(7) and supply each dwelling with 2/0 copper and install 200 amp service disconnect for each dwelling because each set is supplying the load of an entire dwelling unit.
Knowing the rules sometimes helps you find ways around problems. If one already had two sets of 2/0 installed and was red tagged, a way around not replacing all the conductor could be to make them so they are parallel and still feed two 200 amp disconnects as long as the load calculation is under 350 amps.
Confused yet?