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I have an attached garage, insulated and drywalled. It has a single outlet on the wall, on a GFCI circuit (a few other outlets around the house are on the same circuit). It has the lights and a garage door opener on a separate circuit. I'd like to tap off the GDO circuit for another outlet on the wall.

First question is: Would this circuit arrangement be code-acceptable (county follows 2017 NEC)?

Second question, if the answer to the first is affirmative, is: What's the simplest way to accomplish this?

Ideally, I'd like to run an NM cable from the GDO outlet (above is unoccupied, uninsulated attic space) to a junction box above the wall exterior, then poke a hole in the ceiling drywall for some PVC conduit and run THHN down the face of the wall to an outlet box. Kosher or no?

cross section

In an ideal world, I'd drill through the top plate(s) and run the NM down inside the wall, but without knowing where/if there's blocking in the stud bay, I have no way to drill down through any potential blocking with one of those flex augers. Not enough headroom to operate, and also there's insulation in the way of pushing any NM down. Rather not get into a drywall repair here, either.

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  • Well, to close this out, the part of the wall I wanted to put the outlet in turned out to be uninsulated—probably because it jutted out from the front of the house by a few feet, it wasn't required to be insulated since there was no habitable space on either side. So I just drilled a hole in the top plate from the attic ('twas tight fitting myself close to the eave) and dropped a cable straight down. No horizontal blocking, was able to grab the end through the hole I'd cut for the outlet. No junction box or conduit or drywall work needed.
    – Huesmann
    Commented Dec 30, 2022 at 21:53
  • Never been to a house with wire channels?
    – JoshuaD
    Commented Jun 30 at 14:20

4 Answers 4

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It is acceptable to run NM inside a conduit, you just need to upsize the conduit enough than for individual THHN wires. So that said, you won't need a junction box to transition from NM to THHN because there will be no transition connections.

What you proposed in your post is acceptable, but the JB will need to be "permanently accessible", which means you can get to it without removing sheetrock, insulation or any other destructive method. It can't be hidden. That is a contrast to "readily accessible" which is for panel boards, switches, outlets, etc.

Bottom line: There is no need for the transition to THHN/THWN, just run a large enough conduit and run the NM thru it.

EDIT: Thinking more about this...you could mount the JB on the ceiling of the garage, run the NM into it, then conduit to the outlet. If you wanted to "pretty it up" WireMold makes surface mount conduit that's pretty decent looking and would accommodate 3 wires easily. WireMold has a variety of products that could meet your needs if you really wanted to transition from NM to THHN.

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    If the conduit is only serving as a protective sleeve, it does not need to meet fill rules, beyond the "it fits" rule, and no junction box is required (but a clamp or bushing to protect the cable as it enters the conduit is required.)
    – Ecnerwal
    Commented Nov 30, 2022 at 16:34
  • FWIW, I have a "permanently accessible" junction box in one of my walls, covered with a blank plate painted to match the wall. It isn't all that noticable. And since it's high in the wall it will probably wind up behind a picture frame eventually. "Blind maniac." ( Out of sight, out of mind.)
    – keshlam
    Commented Nov 30, 2022 at 17:46
  • @Ecnerwal Agreed! but I think others have said differently, not sure. My only guidance was to make sure it was large enough for an easy pull. Hard to pull 12/2 NM thru a 1/2" conduit. , Commented Nov 30, 2022 at 19:21
  • The JB would be readily accessible, in terms of just needing to unscrew an access panel to enter the attic (which I assume meets the intent of "readily accessible").
    – Huesmann
    Commented Dec 1, 2022 at 14:16
  • @Huesmann Im not sure, but having to unscrew a cover would mean it's "permanently accessible". "Readily accessible " applies to outlets, switches, main panels, sub-panels, etc. No big deal though, either would work. Commented Dec 1, 2022 at 15:15
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The junction box needs to be accessible, not buried. Which I guess might be the case if the unheated attic is open. I'd move it up a few inches to make it more obvious if insulation is ever installed, but you don't have to.

You do need GFCI protection, and should already have it on the GDO circuit unless that's hardwired (no receptacle,) with a reset in an accessible location (not a GFCI receptacle for the GDO on the ceiling. Reset should not require a ladder. Perhaps the breaker is a GFCI breaker? An accessible deadfront GFCI is another way.)

Otherwise it's fine, but I will say that drywall repair, particularly in a garage space, is not that big of a hurdle. Cut some neat holes and save what you cut, use them to access the cavity, drill holes in blocking and/or studs, run the cable, and firestop any holes through blocking and/or studs. Then put the cutouts back in the holes and patch, easy-peasy.

I'd choose EMT over PVC for this job, but either is OK by code (though the parts below either 8 feet or 6.5 feet will have to be schedule 80 PVC depending how your LAHJ defines "exposed to damage," if PVC.) Wiremold is a third option.

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    GDO might not have GFCI if it (or rather, the receptacle) is old enough - a quick search shows overhead receptacles did not require GFCI until NEC 2008, and it could be a few years after that before locally adopted) but the new receptacle will require it in any case, either at that receptacle, at the breaker or at another easily accessible receptacle - not at the GDO overhead receptacle. Commented Nov 30, 2022 at 17:00
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    Yes, depending on age and code applicable at time of construction , it might not be.
    – Ecnerwal
    Commented Nov 30, 2022 at 17:36
  • House is a 2002 build, in case it matters. I don't believe the GDO outlet is on a GFCI circuit. But making the new outlet GFCI is certainly facile.
    – Huesmann
    Commented Nov 30, 2022 at 17:52
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So far as the "ideal world" musings are concerned:

  • You can use a stud finder, or even just knock on the wall and listen, to determine whether there is any blocking midway up the wall.
  • If there's enough headroom to drill down from attic through top plate with even a little spade bit, then you can tie string to a weight (smooth chain, stack of washers, bolt, etc) and drop that down the hole. Once it gets to one side of the insulation it should fall down the wall cavity fairly easily. You might need to use a length of wire, rod, dowel, etc to pry the insulation aside and/or guide the weighted string to get it started.
  • You can use a flex/auger/bellhanger drill working upward, rather than downward. Cut the hole in the drywall where the outlet is to go and insert the drill there. After it penetrates into the attic space, the drill can be used to pull a string or even the NM cable itself back down the wall.
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  • I highly doubt the plumb-bob method would be useful with a wall full of insulation. I'm actually reconsidering the drywall holes cutting option. It may not be so bad.
    – Huesmann
    Commented Dec 1, 2022 at 14:17
  • @Huesmann Doubting is fine. But I can tell you from experience that if the wall is insulated with fiberglass batts, it works fantastically once you get the weight in between the batt and the drywall. If it's insulated with something firm, like a blown-in cellulose, the method will absolutely fail.
    – Greg Hill
    Commented Dec 2, 2022 at 5:00
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Well, to close this out, the part of the wall I wanted to put the outlet in turned out to be uninsulated—probably because it jutted out from the front of the house by a few feet, it wasn't required to be insulated since there was no habitable space on either side. So I just drilled a hole in the top plate from the attic ('twas tight fitting myself close to the eave) and dropped a cable straight down. No horizontal blocking, was able to grab the end through the hole I'd cut for the outlet. No junction box or conduit or drywall work needed.

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