I'm installing a 40-amp subpanel on the outside of a cinderblock basement wall. I'll run 8-3 NM-B (aka. Romex) from the main load center into the basement.

What is the best way to get the NM-B cable through the cinderblock into the subpanel ?

  1. The belt-and-suspenders way would be to run PVC conduit from the knockout in the rear of the subpanel, through the cinderblock, and into a junction box on the inside. Transition from romex to THWN in the j-box. This seems like overkill.

  2. I could drill a hole through the cinderblock and pass the cable through that into a PVC nipple in the knockout in the rear of the subpanel. But that leaves the potential for water to get into the subpanel - if it seeps down between the cinderblocks and the rear of the panel - although I could caulk around the rear of the subpanel. But the cable in not secured.

  3. My inclination is to run PVC conduit from the knockout, though the cinderblock and into the basement. I'd secure the cable where it enters the conduit (using a Romex clamp screwed into a PVC female adapter). I'd probably strip the romex sheath to just after it passes through the clamp, and run the individual conductors the rest of the way. I believe this meets code if the conductors are considered to be within the subpanel once they enter the conduit. And it's water-tight.

  • I don't know if Romex passing through the cinderblock is considered "outdoor", but here in North Carolina, a small amount of Romex (I want to say 5ft) is allowed in conduit outdoors. People frequently run multiple Romex cables through a large PVC 'L' into an outdoor-mounted main load center. Jun 29, 2022 at 0:48
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    I can't imagine that stripping off the insulation from NB cable for a few feet and running that in conduit to outside would be OK. The insulation is not rated for individual conductor use. Another alternative would be to run the entire length with UF cable. In my home store the UF is just about the same price as the NB.
    – JACK
    Jun 29, 2022 at 1:08
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    Or run #6AL of any type for 50A instead of 40A. Romex has a low current rating because of its poor insulation. If you have to say it's watertight, Romex is the wrong stuff. I know everyone want to "use what they know" but Romex actually does not work outdoors. The wire insulation degrades in damp environments and you get GFCI trips. Jun 29, 2022 at 2:47
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    Having, unfortunately, wired my deck outlets with UF-B, I can say with a certain amount of authority, that it's not only big (requiring larger conduit), but it's also a major pain to work with because it doesn't strip easily like NM-B does.
    – FreeMan
    Jun 29, 2022 at 13:30
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    @jack it would be a code violation in 2 ways to strip the jacket off of NMB , #1 the insulation must enter the box 1/4” and NMB wires are not labeled with the size only the covering is so there are 2 violations.
    – Ed Beal
    Jun 30, 2022 at 1:30

2 Answers 2


For exterior panels Where I see a difference is when a panel or disconnect is surface mounted or flush mounted.

The flush mounted is considered within the building envelope and NMB is allowed.

Push the exact same panel to the surface mount and now it is outside the building envelope and this is where NMB is not allowed.

Yes I have seen exterior panels pass inspection with NMB but I have seen them fail also.

In my jurisdiction there is a state wide exception(alternate wiring method) that allows a vertical section (up to 8’) of raceway to contain NMB outside.

As this should be a permitted job I would just call the inspector and ask believe it or not I do this as a pro.

if I think I might be on the edge that may be flagged I actually text him providing the question and my code reference that I question.

In this case I would mount a box inside a nipple to the outside box and just go ahead and run the NMB straight to the terminations, if it gets flagged it is 5 minutes to cut it off at the box inside and run dual rated THHN out to the terminations. It doesn’t look like you are trying to save a buck or 10. This is where I have had to make the change but not always the cinderblock would get a nipple, a wood wall would not in my opinion and how I would proceed without calling.


An answer, of sorts, is given by this lengthy discussion at Mike Holt's forum:


Note again that I'm in North Carolina, apparently unique in its tolerance for small run of NM-B in a wet location.

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    It's my understanding that the issue with NM-B in wet locations is that the paper around the EGC gets wet and then causes the insulation on the conductors to deteriorate. Hence my thinking in stripping the sheath back. Jun 29, 2022 at 22:57
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    Rusty it is not degrading to the insulation the paper actually will wick moisture from outside to inside.
    – Ed Beal
    Jun 30, 2022 at 1:32

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