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I understand that NM wire is not permitted to be used in wet or damp locations. That said, it seems generally permissible to run NM wire into the back of an outdoor box (such as a breaker panel). Unfortunately, I have a fixture that doesn't provide any back knockouts (and cutting my own is inadvisable here) so the only solution is to use the bottom entry provided.

To what extent can NM wire be run from inside an exterior wall through conduit outdoors to the intended fixture?

I have drawn some representations of three different cases below:

Wiring Diagrams

Case 1

In this case, I would have a simple elbow running into the bottom entry and into the exterior wall. The NM cable (in orange) would run from the hole in the wall through the elbow and into the fixture. This is the simplest option, but not exactly my preferred option.

Case 2

This is what I would prefer, though I somewhat doubt it is legal. In this case, the NM cable would enter an outdoor junction box (so I can also have another circuit providing a GFCI receptacle here), travel through a minimal conduit segment into the device, and terminate as before.

My guess is that the running on NM cable through the conduit is not legal for any length outdoors so I probably can't do it.

Case 3

I presume this to be the most likely legal solution. In this case, the NM terminates immediately, gets pigtailed into some THHN/THWN (indicated by the colored dots), and the clearly legal individual wires go through the conduit into the fixture.

While I am fairly certain of the legality of this solution, I still don't know what is considered acceptable when NM wire goes outside for short segments (such as to a breaker) and if it's required to be through a back knockout or any knockout.

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  • Why would cutting your own knockout in the back be inadvisable?
    – ThreePhaseEel
    Feb 28, 2017 at 23:12
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    It's an EVSE where the UL listing (and NEMA 3R rating) may be provisioned on the entire housing remaining intact. It's not simply an electrical box.
    – Hari
    Feb 28, 2017 at 23:32
  • You could use a rain tight box these usually have threaded openings on the back and 2 sides to make the transition. Could you describe your outdoor location or provide a photo. Just because it doesn't have 4 walls doesn't make it a damp/wet location many homes have porches that protect the area from rain or is your panel on an exterior wall with no cover eves don't count.
    – Ed Beal
    Mar 1, 2017 at 0:29
  • Could you transition from NM to THWN inside the building? Or do the entire run with THWN in conduit?
    – Tester101
    Mar 1, 2017 at 2:42
  • @EdBeal That's basically the second/third cases. I could add a picture if you'd really like, but it's on a relatively sheltered side of the building (typically downwind) with an eave preventing direct vertical access. That said, I'd consider it relatively damp/wet.
    – Hari
    Mar 1, 2017 at 7:30

1 Answer 1

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Running the NM cable at all outdoors would have me concerned about capillary action wicking moisture upcable and back inside, leading to premature failure of the NM. I'd use a short length of /2 UF from an accessible box on the inside through the wall and then directly into a J-box on the outside for the GFCI. You can then run UF through the nipple that connects the weatherproof J-box to the fixture in question.

Don't forget the "while in use" cover on the GFCI-box, and to make sure that there's a 1/8" drain hole in the bottom of that box. (Elbowing up a conduit directly to the fixture from the inside isn't the greatest idea because then you have raceway arranged to drain...to the inside of your house. Oops!)

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  • Then how do you wire outdoor panelboards? Do you have an internal junction box to switch to UF wire before it goes slightly outside at the panelboard, or do you have the NM go directly to the panelboard?
    – Hari
    Apr 22, 2017 at 16:50
  • @HariGanti -- I suspect most people run outdoor panelboards with NM directly into the back, but that relies critically on the integrity of the panelboard enclosure to keep water away from the NM. Belt-and-suspenders says to run UF into it, but I'm not a big fan of putting all the breakers outside, anyhow :P
    – ThreePhaseEel
    Apr 22, 2017 at 16:52
  • That's what I'd expect as well. I need my panelboard outside for space reasons, but I also plan to build a small roof for it, even though it's pretty well sealed. Also, I went with running THWN from an indoor junction box to the outdoor junction box to avoid needing any splices in the outdoor box (saving a ton of space for devices).
    – Hari
    Apr 22, 2017 at 16:55

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