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I'm planning a 220v circuit from a subpanel in my garage to an outdoor A/C disconnect for a mini split install. The cable would up run from the panel into the attic, across the garage along a running board, and down into the wall cavity on the appropriate wall. At some point the cable needs to exit the wall cavity into the disconnect.

But I'm unsure of what the common practice is for getting this wire from inside to outside.

Ideally, I wouldn't have to transition to conduit at all, and could just feed the cable in through a cable clamp on one of the knockouts on the back side of the A/C disconnect. However, given the box will be mounted on siding, I'm not sure how easy it will be to get the box flush enough that water intrusion isn't of at least some concern. And, even in this case, I'm not sure what the right way to go into that box is.

So maybe it would make more sense to transition from the wall cavity into flexible water tight conduit and then into the disconnect. Or maybe the solution is to run the entire circuit in conduit from panel to outside?

To try to figure out how this might be handled in a professional install, I checked out my existing A/C unit. There is a 10-2 (w/ground) UF cable which runs from my main panel in the basement, through the joists in the crawlspace, and exits the house here:

Exit from the house

Here's the outdoors side, where it connects to the disconnect (which is right out of view at the top of the picture):

A/C disconnect outdoors

It doesn't look like whoever did this install did anything particularly special when exiting the house - there's just a hole, and although I can't see the inside end of that flex conduit, I'd bet that the cable is just "stuck in there".

In general - what's the best practice for making the transition from cable in the stud cavity to that outdoor A/C disconnect box? Should I transition into conduit somehow? Should I feed the cable into the back of the box and caulk it well? Or should I stop overthinking it and just shove the wire into some flex conduit like the people who built my house apparently did? This work will eventually have to pass an inspection as it requires a permit in my jurisdiction.

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  • It looks like there may have been some caulk around the wiring/plumbing at some point, but that most of it has now come off. I agree - kinda shoddy. – FreeMan Jan 21 at 13:06
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    This arrangement ensures that water will run down the flex conduit to help rot out the wall. – Ecnerwal Jan 21 at 20:20
  • Is the interior space opposite where the new disconnect is landing finished or unfinished? – ThreePhaseEel Jan 22 at 0:31
  • The new disconnect will probably be pretty close to where the current one is - so unfinished. – epiccoleman Jan 23 at 4:32
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I'd use an indoor JB directly opposite the disconnect with a nipple connecting the two

Rather than trying to run the cable out exposed and then into the bottom of the box, or try to clamp it when it's going into the back, I'd put an ordinary 2½" deep, "5S" (really just under 4¾") square metal junction box on the inside back-to-back with the disconnect, then use a nipple with sealing locknuts to connect the JB to the disconnect. The cable for the AC comes into the indoor box and transitions to THHN wires there; these wires then run off to the disconnect via the nipple.

You can also have a general lighting/receptacle indoor branch circuit land in the indoor box and transition to THHN as well to take it outside for the serviceperson's receptacle. This is handy if your air conditioner disconnect provides that feature, and also allows you to "camouflage" the junction for the air conditioner behind an ordinary outlet. Note that you may need that "5S" box to do this, as smaller boxes may not provide enough space for large A/C feeds.

You'll want to caulk and flash around the nipple penetration just like it was any other plumbing penetration, too, since the sealing locknuts help keep water out of the electrical boxes but don't do anything for keeping water out of the wall cavity. The good news is that being a piece of pipe, it's much easier to flash and seal around the nipple than it would be if you were trying to run a flat-ish UF cable thru the sheathing!

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Caulking should never be the "first line of defense" against water intrusion. You'll want to at least do a proper flashing. When I mount outdoor disconnects I always put them on a backer board of some sort when mounting on beveled siding. So in your case, I'd carefully cut the siding and install a backer board. Make it about 1" larger in both dimensions than the disconnect. Paint or stain it to match the siding and caulk the top and sides. I prefer cedar because it's rot resistant. You should install some Z metal flashing on the top of the backer board.

I also always run the power thru the back of the disconnect via a very short length of conduit, just to get inside the stud wall.

Regarding the refrigerant lines, darn. It looks like a pretty sloppy install but I don't know what best practice is for them to enter the home. One thing I've seen many times is they run up the side of the house into the attic and go from there. And they are covered with a protective channel. But I'll leave it up to the HVAC experts here to chime in.

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The disconnect is outside - so it's a wet (or at least damp) location, so you need wet-rated wire or cable to connect to it. UF is rather expensive for use inside.

Having done this recently, I ended the inside cable or conduit at a junction box, ran a short section of rigid conduit through the wall into the knockout on the rear of the disconnect, and used THWN to connect from the junction box to the disconnect. Don't forget to pack around the wires with duct seal (electrical putty - keeps from having air and bugs moving through the conduit.)

There are other ways, that's just one.

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  • Running UF is slightly more expensive but when you consider a box and access or the additional chance of a bad connection it can be well worth it. The other way is to inset the box inside the siding at that point it is inside the wall and NM can be run from the panel to the disconnect. With the box only partly in the wall there is plenty of room to put the flex in the box and have a tap for the service receptacle if there is not one available. – Ed Beal Jan 21 at 20:17

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