I am working on a site that has one existing building with a 100 single phase mb panel. I have to upgrade the incoming service to 3 phase 4wire 400 amp service and reconnect the 100 single phase. They will be right next to each other. Would it be better to put a trough off the ct cabinet then install a 400 amp main breaker panel and tap in the trough for the 100amp panel or can I use a 400 amp disc. switch with double lugs and feed the 100 amp panel from the 400amp disconnect switch (only tap two legs). I looking for it to be less labor intensive even it cost more in material. They are not looking for seperate meters but they do not mind if there is. I dont see a multi meter stack that can handle this. I cant find a small 3R 400 amp panelboard that will accept a 3p 400 and 2p 100. I have to feed a new 400 amp panel in the new building ,What do you think?
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