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I have a main panel in my garage, with an existing sub-panel mounted right next to it. I want to add a new sub-panel at the opposite side of my house and need to run three #8 THHN and a #10 THHN into the existing sub-panel. There is an attic above the existing sub-panel and I can get to the top-plate to drill through it. I would like to minimize drywall repair. I'm trying to figure out what the best way is to run these THHN conductors into the panel. I'm planning to do this with a homeowners electrical permit, and want to follow the 2017 NEC.

I would be a tiny bit (41.3%) over 40% conduit fill if I put the 3 #8 and 1 #10 into a 1/2" FMC, but 3/4" FMC would be be best. I will have a junction box in the attic, so the total FMC run would only be about 5 ft (37" from the top of the panel to the bottom of the ceiling). I was hoping to attach the FMC conduit connector to the FMC and lower it through the hole in the top plate and feed it into the panel. But I would need to drill a very large hole in the top plate to be able to fit a FMC connector through it. I found these "Arlington 38AST 1/2" SNAP²IT Connectors with Insulated Throat" connectors that I think you can mount from the panel side, but they appear to be limited to 1/2" FMC.

Another issue is securing of the FMC. I think I need to secure it within 12" of the panel, which would mean opening up the drywall. Is there any way around this? I'm not set on using FMC, but it seemed like the thinnest wall conduit I could feed from the attic.

What is the best approach to do this with minimal drywall repair?

Thanks

Existing layout

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    Considering it's a garage and the wall is already loaded up with all kinds of ugly junk, I don't see why you want to minimize drywall repair. If you had to achieve a perfect finish and match the paint or paint the whole wall ... MAYBE ... but you really don't. A reasonable finish and some white paint will be invisible next to all that hardware. It'll be a lot easier to open the wall. Assuming the #10 is ground, you use EMT instead of FMC and you don't even need the #10.
    – jay613
    Oct 4, 2022 at 15:09
  • Or ..... if you really don't want to damage the wall ... why not use 8/3 NM instead of FMC? I think there is an appropriate strain relief you can install from inside the panel having fished the cable through. But the one I've used is only good up to 10/3, I'm not sure if there's an internally installed strain relief for 8/3.
    – jay613
    Oct 4, 2022 at 15:37
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    To point out the obvious, you've already got 6 unfinished holes in that photo. Neatly cutting the wall open to work on it would be an improvement by most measures. Oct 4, 2022 at 15:53
  • FMC run would only be about 5 ft (37" from.... Is it 5' or 37"? 5' = 60" and 37" is 3'1", so there's a bit of difference. I guess 37" is more "about 5 feet" than it is "about a mile", but... ;)
    – FreeMan
    Oct 5, 2022 at 13:01

2 Answers 2

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Did you compute the conduit fill with an insulated ground, or a bare ground? The bare ground will most likely give you the square inches you need to fit in 1/2".

If you can use non-flex EMT conduit, that's the ground... so 1 less wire.

I for one do not understand the purpose of the drywall here. If the goal is to "fancy the place up", all it's done is put lipstick on a pig due to the many ugly conduits. FMC and smurf are the ugliest conduits. The FMC between panels is vexing. Not only that, if the drywall being added caused the panel covers to be lifted off the panel by drywall thickness, that's a code violation - you can't inset a panel into drywall even slightly. It's not like a junction box where 1/4" is allowed.

If to comply with a firewall requirement, it totally doesn't due to the numerous openings. The drywall should be fit into the stud bays so it's between the conduits and the far wall.

This is utility space. Every house needs utility space unless it's an Amish house. It needs to be usable to do its job, and there's nothing wrong with it looking like utility space.

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  • Yeah, Square-D says out and out that panelboards have no business being in fire-rated assemblies... Oct 5, 2022 at 3:02
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It seems you have a plan in mind for this project. I question trying to do this without opening the drywall above that panel. Typically there is a lot of wiring going in and out of any panel- usually from the top. Trying to drill from the attic through the wall framing plates and fish a conduit into that box seems risky. I would suggest opening the drywall above the box.

Also it seems there are several wires in conduit and a junction box that protrude from the wall above that sub panel. It appears that one conduit goes over to the main panel and 2 go into the j-box (maybe an LB- I cannot tell from the picture). My first question is why do they come out of the wall - is there something above them that keeps them from going straight up to the attic as you are planning to do with your new wiring ? That j-box seems to have a conduit going up into the attic space- have you looked into where that terminates up above ? Is it possible you could use those access points for your new wiring ?

If those existing access points are not a viable option, I would definitely open the drywall above the panel you want to add wiring to- just too risky without doing that.

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  • Kyle, somebody in the past installed the sub-panel and used the liquidtight conduits to avoid the vertical stud between the panels. The larger conduit is the feeder for the sub-panel. The junction boxes 'intercepts' some of the NM circuits that used to go into the main panel and 'divert' them to the sub-panel. I'm not crazy about this setup and will eventually put in one large new panel. Right now I'm trying to just feed the new subpanel. You are right that I should probably just open up the drywall Oct 4, 2022 at 15:46

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