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I'm putting a duplex outlet on the outside of my enclosed cargo trailer and need to run wire to it from the inside using PVC conduit.

I found this for the outside cover, which I think is pretty nice:

Leviton While-in-Use Cover

The wall of the trailer is just shy of 2.0"/50mm thick (aluminium skin, 1.5"/38mm of XPS foam, 0.375"/10mm of plywood.

On the inside, I need to run conduit from the box up the wall to a breaker panel knockout. Probably makes sense to use ¾" conduit since I may need to use the larger knockout on the panel.

Questions that I have:

  1. The outside cover needs something to pull it against the wall, for it to seal, what is holding that?
  2. The box won't protrude into the inside space to be able to run conduit to it on the side, so it either needs to be much deeper, which I haven't found, or I need to run the conduit to the back of the box. If to the back, I don't see how to have it stay close to the wall and not end up with an elbow or conduit body sticking into the space a few inches. Is there a better way?

Here is a sketch of what I think I'd want, showing how the box is through the wall and visible on both the outside and inside:

sketch

I purchased one of these boxes as a starting point, as that was the only box with a rear hole my local Home Despot had:

1-gang PVC box

There must be a smarter approach to my problem, please help!

Thanks,

A.

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  • You have 2 criteria that seem to be contradictory. You want minimal thermal bridging to keep all the warmth possible inside the trailer, yet you also want an electrical box to fully penetrate the insulation, leaving a box sized hole in it. I'm not certain you can have it both ways. Maybe someone else knows of something that will work for you...
    – FreeMan
    Oct 7 '21 at 15:49
  • Out of curiosity, where will the electricity come from? Will the trailer have a generator or inverter onboard? Oct 7 '21 at 16:10
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    Also how do you feel about the entire box being on the outside and only conduit coming through the insulation jacket? Oct 7 '21 at 17:14
  • You can use a plastic weatherproof box for less thermal bridging, and a weatherproof box extension ring for more depth. But plastic conduit into a completely surface-mounted box will be even less thermal bridging, if the box is not in a location where a fully external box makes the trailer exceed its maximum towing width (assuming it's a trailer that actually moves, anymore, as opposed to a house that was towed to its final resting place, once.)
    – Ecnerwal
    Oct 7 '21 at 18:35
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Question 1

Outlet covers like the weatherproof one you've chosen (and all others) are mounted to the box. They all have holes on standard spacing so they'll all match up.

Question 2

The box you've chosen is designed to be surface mounted. That's what the 4 little ears sticking off the sides are for. It's not designed to be mounted within the wall which is what your sketch shows you're after.

As for a box that would work, I'd suggest a metal box similar to this:

2-gang metal box
Images courtesy of Lowes.com. No recommendation of vendor or brand implied or intended.

With an extension ring (maybe 2 if necessary for the depth of your wall) like this:

2-gang metal extension ring

The metal box has small holes on the back that can be used to attach it directly to your interior plywood. You might need to use nuts, bolts and washers to get a solid contact.

Note: There's no need to use 2-gang boxes (though they do provide plenty of room for wiring), those were just the first images I found to describe what I was after.


I noted in a comment on another answer that you're hoping to reduce thermal bridging because of extremely cold temps. In that case, maybe you do want the surface mount box you've chosen.

You'd need to use long bolts to mount it so they go all the way through the insulation and ply on the inside and get nuts/washers on the inside. You may want to consider carriage bolts (which would require squaring off the holes in the mounting ears of the box) so they can't be removed by vandals/bored teens from the outside.

Then, you'd need to thread conduit into the hole in the back of the box, run it through the insulation and ply on the inside and attach it to an LB for the tightest corner possible. You can then run the conduit from the LB up the wall and into the panel.

You'll probably want to use Schedule 80 conduit (it may be required, I don't know code for wiring inside a trailer) because it is much more impact resistant than Sch 40 conduit. I'm suggesting PVC conduit because you've picked a PVC box. You could use RMC, an appropriate LB, and the appropriate fittings, then use the conduit itself as a ground. However, there's a TON of room inside the 3/4" conduit you mentioned for 3 #12 wires (hot, neutral, ground), so saving room isn't really an issue. IMHO, PVC is easier, but I'm just a DIYer, not an electrician - I'm sure some/many/all pro electricians would disagree with me.

Using the surface mount box would limit your insulation penetrations to 4 bolts of the appropriate size and one hole slightly larger than 3/4" for the conduit. I'm pretty certain that you're not allowed to fill the conduit itself with insulation.

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  • To clarify my sketch: the box is through the wall, since the box is 2" and so is the wall. Thus it is visible on both sides. The problem with using an LB is there is still a nearly 2", plus the LB itself, protrusion to the inside. I'm concerned it will get whacked too often. I thought I could maybe just cut off the LB's conduit adapter and glue it straight to the back of the box, but it gets unpleasant. Oct 7 '21 at 15:32
  • From your sketch, it appears that the panel is surface mounted inside the trailer. Consider adjusting the outlet height upward to minimize the amount of conduit between the box and the panel. That way, that which would hit the conduit would also likely hit the panel, which would offer it some additional protection. Or, you could create a metal or wood protector to go around the LB/conduit. I'm not sure how, exactly, code applies to mobile installations, but it does not consider modified pieces to be "installed according to code", which could be an issue if there's an insurance claim.
    – FreeMan
    Oct 7 '21 at 15:37
  • Additionally, the LB could be installed flush to the wall, though it would stick out some - it's just the nature of the beast. You could use smaller diameter conduit - I believe the LBs are smaller, too. However, you still have to meet up with the knockout in the panel, so there's a minimum depth you're going to be able to achieve. You may just have to live with it, even if it's not your ideal.
    – FreeMan
    Oct 7 '21 at 15:39
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Metal box - lots of sizes available and knockouts all sides - and metal conduit. May cost a little more but not much for a small job. Plus no ground wire needed.

You also almost definitely need GFCI protection - certainly highly recommended. That can be with the breaker (more expensive) or with the receptacle (more concern about weather-related damage over time).

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  • But that won't help with my depth problem. Also, since it gets cold as f$$k here, I am trying to avoid thermal bridges. Oct 7 '21 at 13:59
  • Can't speak to the thermal issue. But depth is easy with metal extensions. Oct 7 '21 at 14:04
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    This was my answer, just in a lot less words... :)
    – FreeMan
    Oct 7 '21 at 14:16
  • @FreeMan That's what happens when I post an answer from my phone. Oct 7 '21 at 14:18

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