1ph to 3ph upgrade, back feeding existing panels?

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ihope

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I have a situation where I need to upgrade the existing service from 200A, 240V, 1ph to 400A, 208V, 3ph. I'm curious to know how people handle feeding the existing 1ph panelboards when you change phase like this. Would it be possible to keep the original 200A, 1ph main panel and back feed it from a 2-pole breaker from a new 400A, 3ph panel? The service is for an office which mostly has receptacle and lighting loads. Thanks for any help sent my way!
 
I have a situation where I need to upgrade the existing service from 200A, 240V, 1ph to 400A, 208V, 3ph. I'm curious to know how people handle feeding the existing 1ph panelboards when you change phase like this. Would it be possible to keep the original 200A, 1ph main panel and back feed it from a 2-pole breaker from a new 400A, 3ph panel? The service is for an office which mostly has receptacle and lighting loads. Thanks for any help sent my way!

You can do that. Just be mindful that any mutiwire circuits that are already there, the neutral will now be a current carrying conductor and you may technically run into some derating issues. See 310.15(B)(5).
 
You can do that. Just be mindful that any mutiwire circuits that are already there, the neutral will now be a current carrying conductor and you may technically run into some derating issues. See 310.15(B)(5).

I should add that also, be certain that any 240 volt load you have is suitable for 208 volt.
 
Is the reason for the upgrade is mainly new three phase loads? A delta service may be what your needing, it would be the easiest way to refeed the existing loads without worrying about voltage changes and load balancing on the old service. But if your new loads are mostly 120 volt, then this would not be the best option.
 
:) Thanks so much for your answers, Texie and Hillbilly1. We need to upgrade to 3 phase because the majority of the HVAC system is being upgraded and the units we want are only available with 3 phase power.
 
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