If the calculated load on the service is under 200A, it's not a problem to have 200A service conductors supplying (2) 200A service disconnects. See 230.90(A) Exception 3.
Cheers, Wayne
What is your load calculation? That 4/0 UG service conductor can feed up to six service disconnects of any rating as long as the calculated load is not more than the 180 ampacity of the 4/0 conductor.
If you add a tap to the load side of the 200 amp service device you still can have whatever you want on the resulting feeder as it is all still protected by the 200 amp main, this presuming you still have a load calculation of 200 or less.
I'm not sure what the load calculation is. I've never done one. The main house, which has the existing 200 amp exterior disconnect, is just a single story 1200ft² house with gas stove, gas dryer, and central air, and 95% efficiency gas boiler with circulating gas fired hydronic heat, nothing fancy.
Apartment below(650ft²) since on its own meter will have electric stove, electric dryer, one 4ft minisplit AC with heat pump. Possible electric heat if heat pump doesnt work out and possible electric domestic hotwater. If I can get an electric unit that will both heat domestic water and run electric heated hydronic heat, I may do that instead of running straight electric fin tube. But either way, this apartment is gonna be spray foamed and well insulated and 90% below ground, the electric heat is just a backup if heat pump cant keep up. I don't think it would need too much heat regardless
The thing is, I'm confused about how to present this to the inspector at this point or should I even get him involved since power company says I can do the fire underwriters myself. But I wanted the electrical inspector to sign off on it anyway since the apartment may eventually get a certificate of occupancy. Inspector and I had briefly spoken about this underground line and he said 4/0 is not adequate for 400amp. I've even spoken to the power company and they said the same nor did I expect them to know or care what NEC says about 4/O supplying two 200 amp services. I'm not sure if they will even go for it even if inspector signs off on it. Would save me a ton of work though and money. Should I just show the inspector my load calculations?
So the way I see it, I have 3 choices:
1) Switch 200 amp exterior disconnect breaker(supplies main house) to 100 amp and switch 200 amp apartment panel main breaker to 100 amp
2) See if power company and inspector will go for the existing 4/O feeding two 200 amp services.
3) Just run a second set of 4/O.
