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I'm working with an installer on a solar system that can power essential loads when the grid is down, and this requires moving all essential load circuits (refrigerator, boiler/furnace, etc) to a new special subpanel near the main that is powered when the grid is down. I want to be well informed while we discuss it.

My home is unique, it has two attached buildings (all within one building envelope) and each building has its own heating system, so the "back" building's boiler is off of that building's subpanel.

My essential question: Can I run a home run circuit for the back building's boiler (currently on that building's subpanel) all the way back to next to the main, where the essential load subpanel will be? Is it okay to bypass the building's subpanel so that this essential load can be powered when the power is out?

I can't otherwise set the boiler up as an essential load as I'd have to make the whole subpanel "essential" and it is too big (70A) to do that.

FreeMan
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Jon
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    If they are attached in such a way that code would consider them a single building, you're fine. In the case of a separate outbuilding there might be a problem. One potential diagnostic - does the subpanel have grounding rods? Those are required for "separate building" and optional for "same building." As such generally never seen when in the same building. – Ecnerwal Feb 23 '22 at 22:51
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    Will this be an automatic transfer switch? – Harper - Reinstate Monica Feb 24 '22 at 00:16
  • No separate grounding rod, definitely same building. I think we're good! Thank you Ecnerwal. – Jon Feb 24 '22 at 00:47
  • I believe it is an automatic transfer switch as part of the system, yes. – Jon Feb 24 '22 at 00:48
  • A common wall or roof is required to have multiple feeders to multiple sub panels. How is the system going to isolate the main from the utility? This would be my next question. If you have a whole house transfer I can see that would be easy but a 70a sub is quite a large load for many systems even gas and diesel generators that’s a big system if that is just for the heat. What about everything else? – Ed Beal Feb 24 '22 at 02:35
  • I believe the interrupt is for the solar system and the essential loads only. It's an Enphase Sunlight Only Backup, see page 3 if interested: https://enphase.com/download/system-use-cases-iq8-microinverters – Jon Feb 24 '22 at 15:47

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You can do it if the buildings are "attached".

The minimum definition of "attached" is "has a breezeway between them".

So it sounds like you are well within that definition.

Harper - Reinstate Monica
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