I am running 6/3 NM from my panel to an outdoor generator power inlet box, and installing an interlock switch on the panel. I am looking for the correct way to run this wire through the exterior sheathing and into the box. The only knockout on the inside of the box is on the bottom and the box does not sit exactly flush with the siding when lined up with the knockout. The distance through the wall is approx 2.75" inside to outside (sheathing, wood shingles, foam, vinyl shingle). I'm thinking I need to use conduit or fittings to somehow to make a continuous connection from the garage to the inside of the box, but I'm not sure and would like the correct answer here. Thanks in advance!

Hole from inside garage Hole outside

  • It looks like the hole missed the box by about 8 inches. How did that happen? Commented Jul 6, 2021 at 0:58
  • I just took the photo with the box next to the hole so you can see both. Not mounted yet! 😂
    – Vincent
    Commented Jul 6, 2021 at 1:02

1 Answer 1


I would re-center the box so it's actually over the hole. Then I'd come out the back of the box with 3/4” threaded RMC or IMC (since there isn't room for an EMT coupler), through the wall, and into the back of a 4-11/16” junction box. [42 cubic inches, gonna need em maybe).

I'd bring the NM into the 4-11/16” box and ask the AHJ if they are OK continuing the NM through that conduit nipple and into the exterior box. If they are, do that.

Otherwise buy three MAC Block connectors and a tan wire nut, terminate the NM cable inside the 4-11/16 box, and splice it to #6 THHN that is black, black, white, and a #10Cu bare ground abd send that through to the outside box. They sell THHN by the foot. The two hots can be the same color.

  • This inside wall will be finished with Sheetrock so would this junction box need to be exposed inside? If I’m reading correctly you are going from the back of the inlet box, through the wall, and into the back of a junction box, with the assumption that the front of the box would be exposed on the inside?
    – Vincent
    Commented Jul 6, 2021 at 2:16
  • Sorry I thought I responded earlier! It didn't dawn on me you planned to drywall over that. Yes, any junction box would need to be accessible forever. I'd make sure the AHJ is OK with you just going through the hole then, no box, and bring the NM outdoors into that box. (NM isn't technically allowed outdoors). I would still use a conduit nipple to come through the wall though, with a grommet on it to prevent cutting. Commented Jul 6, 2021 at 21:35
  • Thank you, this is helpful. It seems there is a lot of debate online about whether passing NM through exterior sheathing but directly into the back of a box is considered wet location or not. I will check with my building inspector. If it's fine, any reason I can't use PVC conduit with fittings? Having trouble finding a 3/4x4" threaded RMC nipple locally from big box stores when I look online.
    – Vincent
    Commented Jul 6, 2021 at 21:58
  • I was able to use a 1/2” close nipple to connect two 1/2” rigid couplings which makes up the space in the wall. I could use 1/2” chase nipples to connect one side to the inside sheathing and the other side to the inside of the box. Is this an acceptable solution?
    – Vincent
    Commented Jul 7, 2021 at 19:59

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