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As the title suggests, I need to add a few plugs -- (1) 220v and (3) 120v -- in a total of 3 rooms in my workshop. However, everything is fully finished with drywall, insulation in between studs, and 20"+ of cellulose in the attic; in short, for at least one of the plugs it would be physically impossible to reach that wall and fish wire down, and very difficult for the rest. My thought was to maybe run a large PVC conduit (1.5" to 2", with the idea for plenty of future space for pulling) from directly above the panels, in the attic space, put tee's above each room, and then supported overhead at the top of the attic ridgeline. It would run in a perfectly straight line, except where it would tee off to go into X, Y, or Z room. When building, I screwed down a 2x8 walkway the entire middle length of the attic, with direct access all the way over to the panels, so I can move around easy enough up there.

However, since I don't want to have NM/Romex just popping through a hole in the ceiling, over and across to where the plug(s) need to be on the outside of the wall, I thought to transition to EMT once it comes through the ceiling. The only reason I'm saying EMT and not continuing with PVC is because I already have EMT in different rooms, and I feel the aesthetic would be thrown off with PVC just for these runs, as well as THHN being easier to work with/pull.

A couple of questions:

  1. Should I use THHN to run through the PVC conduit, then transition to the EMT when I go through the ceiling and over to where the plug(s) need to be? The THHN would obviously be much easier to pull vs. NM, from my fairly limited experience.
  • -OR- Since everything coming off the main panel is NM right now, should I just run NM up into the attic, then into the PVC conduit, and then create a junction box before going into each room, to transition from the Romex/PVC to the THHN/EMT?
  1. If I use THHN wire and ground all the circuits to the main panels ground bar, do I need to ground every junction box separately in the EMT run, via the green screw? Or does the dedicated ground wire going back to the main panel forego the need for that?

I really appreciate any help!

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    @FreeMan I understood OP to mean running conduit along open attic, with tees to run down through ceiling and then exposed surface-mounted conduit on the walls below, so no in wall work.
    – Armand
    Aug 31 at 16:13
  • Conduits longer than 24" are only allowed to have 3-4 circuits in them, without derating the wires (bumping them a size or two). 5-10 circuit requires 1 size bump. 11-20 circuits requires 2 size bumps. So there you are using #8 for a 20A circuit. Not desired. Sep 1 at 6:41
  • @FreeMan Armand was correct in what he assumed. It is an open attic, and the conduit would run straight down the middle, with tee's to branch off, go down to individual rooms through the ceiling, and to where they need to go on the outside of the walls. Sep 1 at 12:45
  • Thanks for clearing that up. It seems I'm the only one confused about what you're doing. Since everyone else understands, I'll just keep my mouth shut from here on out. ;)
    – FreeMan
    Sep 1 at 12:54

1 Answer 1

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You can do it that way, but there are potential reasons to choose other ways to do it.

The PVC section needs a ground wire. If it's all EMT you don't need one, as the properly assembled conduit itself is a listed grounding conductor.

If you have many circuits sharing "one big fat conduit" you have to derate the wires for each circuit, which means you have to use larger and more expensive wire for each circuit sharing the single big fat conduit.

If you use multiple smaller conduits, you can avoid being at a point where derating has a practical impact on your wire sizes.

You absolutely do not want NM in conduit, barring a few edge cases where it doesn't matter much. Conduit fill for "oval" cable is based on treating it as round cable of the larger axis diamter, and it's miserable to pull. I would use EMT/THHN the whole way, with separate run of EMT for each room.

It would also be code acceptable and quite common (but my rodent reality experience advises against) to run NM without conduit in the attic, and transition to EMT/THHN in a junction box, just deleting your large PVC conduit from the plan.

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  • The only reason I'm hesitant to do straight EMT all the way from the main panel, is because when installing a lot of my initial electrical over a year ago, they weren't run all at perfect 90's, so many come from a diagonal angle into the top of the panel. I was relying on my "buddy" to guide me and teach me at that point, and apparently he did not have a lot of forethought about how it would look/working with it in the future. It would also be difficult to get EMT into the attic, without cutting out parts of the top plate above the panels. Picture: [link]i.imgur.com/zixf1nG.jpg[/link] Aug 31 at 12:26
  • You have to drill holes in the top plate for cables or conduits, so that does not seem like a huge difference between the two methods. Though with conduit, you do have the option of coming out inside the top plate an avoiding adding another hole through it.
    – Ecnerwal
    Aug 31 at 13:10
  • "If it's all EMT you don't need one, as the properly assembled conduit itself is a listed grounding conductor." My local inspector and code disagree. It's stupid, but too much effort to fight city hall. Aug 31 at 13:58
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    Run the ground wire regardless. Remember code is not a how to do it manual but a minimum specification. I have had several experiences where emt after several years where it is no longer ground. Sometimes oxidized at the joint, sometimes separated and other various reasons. This is especially important if there are electronic or other safety systems connected to those feeds.
    – Gil
    Aug 31 at 18:32
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    @Gil -- I'd say it's more "set screw connectors aren't great" than an issue with EMT generally Sep 1 at 1:55

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