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What is the proper way to run wire to an AC disconnect box with new construction, knowing that there will eventually be siding on top of a rainscreen?

Mockup

Here is a mockup of my wall assembly. Everything up to the 3/4" Rainscreen Strapping already exists. The siding won't be there until notably later.

Wall mockup with Disconnect The inner wall (sometimes called a "service cavity") is 2x3 and it's where I run all of my Romex/NM plus plumbing and drains and such. It'll have drywall as the finished surface. This is the equivalent to a normal 2x4 wall in most current wood framed homes.

The outer wall is the primary structural and barrier (thermal, air, and water) wall. The sheathing is on the inside and then it's 2x6 stuffed with dense pack cellulose and covered on the outside with 1-1/2" rigid polyiso foam.

There is a 3/4" rainscreen on the outside of the foam made up of strapping. The siding will be attached to the rainscreen strapping.

The AC Disconnect will be attached to a mounting block, suspended between and on top of the rainscreen strapping.

Wiring Possibilities

Given all this, what would be the most proper way to run wire to it?

  • If this was a completely finished wall, then I likely would run conduit down from the attic with THWN wire in it. That's, by far, the most common way I've seen this done online. But visible conduit like that feels like a hack that is done only when it's done after the fact. This is new construction, so I can absolutely hide everything at this stage, since I have access to everything.

  • A direct route would be to just run NM through the wall and into the AC Disconnect with a cable clamp. I see this frequently suggested for "flush-mounted" boxes. That couldn't pass inspection in this case, though, since the NM would be completely exposed for the 3/4" rainscreen gap and the 1/8" gap between the mounting block and the AC Disconnect body (made of the mounting standoffs) and NM isn't rated to get wet even a little bit.

  • Maybe install a rigid pipe nipple into the back of the AC Disconnect and bury it into the wall (well sealed at every point). I could run NM into the wall and through that nipple into the AC Disconnect box? I've seen that done for surface mounted exterior receptacles but never for a disconnect... and I am unsure how the NEC feels about that.

  • Maybe EMT conduit connected to the AC Disconnect and going through the wall to a box in the 2x3 service cavity. NM goes into the inner box and splices to THWN through the EMT to the AC Disconnect. But there's a finished wall at that point inside the house and I don't want the splice box to be accessible there, so the inner box would need to be in the attic and I'd have to run that conduit 90 degrees and up the wall to it? That seems inelegant, at best.

Given all that, what is the method a master electrician would use to wire this box?

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  • do you have housewrap on your polyiso? the horiz blocking between the rainscreen will prevent it from properly working unless the plan there is for a flashing header with end dams finished like a window header flashing? Dec 30, 2021 at 0:17
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    @FreshCodemonger - no housewrap. The outer foil lining on the polyiso is taped and rated as an exterior air barrier. The vertical rainscreen strapping is 3/4" while the horizontal blocking is 1/2", leaving a 1/4" space under it for airflow and water egress. Dec 30, 2021 at 1:08

2 Answers 2

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Technically even the disconnect box itself is a wet location and therefore NM cable should not be brought into it, neither through conduit nor direct entry with a clamp. In practice this appears seldom enforced: we see NM cable in outdoor receptacles etc routinely.

If you prefer conduit and THWN into the disconnect, consider ENT conduit. It's the light blue colored corrugated plastic tubing. THWN is allowed in ENT just as it is in EMT, rigid, etc. The conduit doesn't have to terminate ASAP in the attic -- you can run it as far as you want, even home-run to the breaker panel if you like and avoid the transition to NM cable entirely.

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    Also note that one can use a stub of PVC to transition from the outdoor disconnect box thru the wall sheathing if they (or their inspector) is paranoid about non-outdoor-rated wiring methods run to boxes surface-mounted into exterior walls Dec 29, 2021 at 21:26
  • The docs I can find on ENT ("smurf tube") suggests a bend radius of 6" - I only have 2-1/2" to work with in the inner wall. Is ENT much bendier than the docs suggest, in reality, such that I could make the 90 corner up to the attic or across the wall towards the service panel? Dec 30, 2021 at 1:15
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    @KurtGranroth Yeah, 2-1/2" is probably not enough to make a bend that won't kink. Why not put most of the bend in the 2x6 part of the wall? You can dig out some of the insulation, place the conduit, and restore the insulation around it. The conduit would penetrate the OSB sheating at an acute angle rather than 90 degrees, requiring a slot rather than a small round hole, but I don't see that being a tremendous problem.
    – Greg Hill
    Dec 30, 2021 at 16:23
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I would use UF installed in the manner that you recognized NM would be prohibited. It is a little harder to work with, you could splice from NM to UF in the attic, but the cost difference is minimal. It's just a bit more labor, so likely easier to skip the attic junction.

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