How to route #8 THHN wires into an existing panel

Question

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

Answer 1

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.

Answer 2

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.