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Does MC cable need a raceway, e.g. PVC conduit, when passing from a crawlspace through a mudsill and stucco and terminating in a surface-mount exterior receptacle box?

I am going to install several receptacles surface mounted to the exterior stucco wall of my house at the height of the rim joist. I am using MC cable for this circuit, running it through the crawlspace, the rim joist, the stucco wall, into the back of a surface mounted 1-or-2 gang weatherproof outlet box.

I repurposed a picture below to illustrate. Can the orange "hole" through the rim joist/stucco be just a hole or do I have to provide conduit protection. If I use conduit, the conduit becomes the interface to the back of the box, and how/do I need to secure the MC cable sheath to the box as well? What kind of PVC termination does there need to be at the free end of the conduit inside the crawlspace?

Illustration of MC cable through exterior wall

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Edit: Added Image Source

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  • Is PVC your "go-to" for any and all conduit work, or is there a reason you see PVC as particularly appropriate here?
    – Harper - Reinstate Monica
    Commented Oct 21, 2020 at 16:24
  • I'm not clear why you would want to put MC cable inside a PVC or other type of conduit.
    – jwh20
    Commented Oct 21, 2020 at 16:43
  • MC cable isn't necessarily rated for wet locations. You need to check the 1) cladding; 2) insulation on the wires within the cladding; and 3) the MC/AC threaded connector which you'll screw into the exterior box; to ensure all are wet-rated. It might be easier for you to use PVC & THHN for the penetration and put a junction box or conduit body on the inside; then none of those MC parts has to be wet-rated. If you're doing that, might as well consider PVC instead of MC for the whole run?
    – Jeff Wheeler
    Commented Oct 21, 2020 at 16:54
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    Oh, separately, do not put MC inside of PVC or EMT conduit. This doesn't make it suitable for wet locations. You can't take a non-waterproof wire or cable and run it in a conduit to make it waterproof; that's not allowed. It sounds silly, but the rationale is, conduits will get water in them, therefore, the insulators inside them shouldn't degrade when subjected to wet conditions.
    – Jeff Wheeler
    Commented Oct 21, 2020 at 16:58
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    You can hang PVC conduit under joists perpendicularly similar to the MC. The reason you call that "clotheslined" is the rule 334.15(C) is actually there to prevent occupants from running 12-2 NM-B everywhere and then literally hanging their clothes from those small cables and causing damage. That's why it's okay to do it with a larger-gauge cable or one protected by a conduit. Just support the PVC every 4 feet (every 3 joists if 16" OC.) Or rent a joist drill and drill holes, whatever you prefer. You have a lot of options.
    – Jeff Wheeler
    Commented Oct 21, 2020 at 18:17

2 Answers 2

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The outdoor recep does not need to be a "GFCI Receptacle". It can be a plain recep that is GFCI protected from a GFCI device in the nice dry indoors where it'll last a lot longer.

If able, I would stick another junction box on the interior. Now, they make conduit nuts, and chances are the outside box (if it's watertight) has a threaded hole. It takes standard pipe thread, which is used by IMC and Rigid conduit.

So, I would fit a piece of Rigid conduit there. Get it the right length so it screws into the outside box, and pokes through a knockout on the inside box by about 1/2". Either put the inside box on shims/spacers, or have the pipe cut to the right length and have the hardware store thread it.

On the outside box it screws into the box body. On the inside box, clamp it down with a conduit nut.

This will NOT be a valid grounding path so you need to run a ground wire. Use the inside box to switcheroo from non-outdoor wiring method to several THWN wires. (must be green ground, white or gray neutral, and any other color for hot. So just buy a couple feet of by-the-foot THWN.

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    Basically, er, no, exactly what I did for my wall penetrations this summer. Oh, I added some silicone around the outside of the box against the wall in an attempt to keep water out of the wall (not the wiring). I ran NM-B inside the garage to the box, then out the wall with THHN/THWN or UF-B (two different applications).
    – FreeMan
    Commented Oct 21, 2020 at 17:47
  • Are you referring to the MC armor not being a grounding means, or the RMC into the outdoor box's hub not being a grounding means?
    – ThreePhaseEel
    Commented Oct 21, 2020 at 23:12
  • @ThreePhaseEel I feel the RMC would be an unreliable path. You'd be using the conduit nut in the interior box to compress a huge stack of ??? building materials that could slacken over time, leaving the RMC nut slack, and not getting any tooth to get a reliable ground.
    – Harper - Reinstate Monica
    Commented Oct 22, 2020 at 0:18
  • @Harper-ReinstateMonica -- hm, wouldn't there be two nuts at the indoor junction box though, one on the inside and one on the outside? Or at least, that's my understanding of how it's done
    – ThreePhaseEel
    Commented Oct 22, 2020 at 1:55
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    @ThreePhaseEel That would be better, yes... but it would require getting the Rigid conduit length just right, so there's threads in the places threads need to be for that to work. Ib believe Rigid is pipe threaded, so it's on a taper. (know alot about that from restoring a riDgid machine).
    – Harper - Reinstate Monica
    Commented Oct 22, 2020 at 2:08
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If you run conduit as a sleeve between the cable and the box, the conduit need not connect to the box; the cable however should enter the back of the box on the outside with a clamp / box connector.

If you run conduit as a raceway, connected to the box on the outside, then the simplest thing to do is put another box on the other side of the wall, also properly connected to the inside box. Enter the box with a clamp on the inside, with enough stripped to reach through the wall and terminate on the other side.

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