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I am trying to install a simple surface mounted junction box, in terms of the Code it seems pretty straightforward, but have run into several practical problems. Thought I might as well ask more experienced people here how they would solve the issue.

I have 3 8AWG solid wires and a bare ground in a recessed 1-gang metal box at a covered patio stucco wall (see Pic 1). Those are connected to a double 60 Amp breaker at the main. Now, I would like to splice those to extend them just a few feet to a subpanel (which cannot be mounted in the current low position, behind an AC unit).

I got some huge wire nuts (see pic 2) and because of the number of large wires and connectors I will need a deep box (e.g pic 3). Ideally I would have a deep extension box, but cannot find those. Also a minimum 3/4in (likely Liquidtight) conduit going to the subpanel. Here are the problems:

  1. the existing box is a bit proud of the wall, which makes the standard surface box difficult to mount and insulate. Sigma boxes I referred to above come with two tiny flat mounts. I could add some spacers, and cut foam for makeshift insulation?
  2. The wires are quite short, adding a box with a single hole in the back leaves me with only about 1.5" of wire to work with. ideally I could use the existing wall box and the new addition as a single unit

So, any other types of box that would mount better over the existing setup? Should I create a small wooden frame for mounting and insulation? Are there any code challenges?

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  • What's on the inside of this wall? If it's accessible, I would move the splice indoors, patch the existing hole in the wall, then make a new penetration for a conduit to go through the wall and into a conduit body which can then direct the wires up to your new subpanel.
    – Jeff Wheeler
    Oct 20, 2020 at 14:54
  • Thanks Jeff, but there is a very nice dining room wall inside, so I'd rather not mess with it, lest I provoke the ire of my better half! :)
    – Misha0001
    Oct 20, 2020 at 16:56

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The problem I see is your wire fill with 4 ea #8 wires counted at 3 cubic inches each you are at 12 without a clamp. I would probably be looking at a double gang bell box. One thing that regularly has to be done in industrial facilities is modifications to boxes or gutters. I would verify your local AHJ will allow this on residential. I would cut the back of a bell box out to slide over the existing device box and mount it directly to the wall this will look better and provide the additional space to splice the wires. Since the existing box is proud of the wall a bead of calking across the top or even around that box prior to sliding it over the existing box would make it waterproof. I have done this with both boxes and gutters coming out of concrete walls and steel plate. Your stucco would be the same as a concrete wall.

Trying to splice heavy wires in two tight a space is how wires get the insulation damaged and end up shorting out on the box or they vibrate through at a later date.

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  • I can find very nice 4" square extension boxes, they could sit over the existing box nicely, but nothing weatherproof so far... It's also easy to find good 2-gang weatherproof boxes, but not extension-type, just standard small round opening... I guess I could cut the back, but not sure if that's allowed, that's why I was asking about the Code... will check! Even a single gang DEEP box seems large enough in combined volume with the wall box to splice everything nicely, if only the back is removed.
    – Misha0001
    Oct 20, 2020 at 17:31
  • Regarding the mounting system, is it allowed to modify the flat brackets as needed? I could use larger plates... (I am pretty good when it comes to cutting and modifying things, as well as sealing them, in standard construction, but when it comes to electrical, I am always concerned that it would be disallowed on one or more counts :))
    – Misha0001
    Oct 20, 2020 at 17:40
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    The tabs are the minimum in some areas I make a plate and use 4 screws then attach the plate to the surface.
    – Ed Beal
    Oct 20, 2020 at 17:47
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Honestly, if it were me, I'd make full use of the proudness to solve the rough surface problem. I'd get a 2-gang outdoor rated extension box - what makes an extension box special is it has an large opening at the back that is 1-gang sized, that basically makes it a box extension not another box. I say "choose the box" so you know how big its footprint is.

Then, for that footprint + maybe 1/4" margin... I'd either mill away the stucco or use epoxy filler and shims to bring the surface true flush with the existing metal box. That's all "body work". My go-to would be West System epoxy, since it lets you "dial-a-bondo" to the thickness you desire, and also use an adhesive filler (or none at all) for gluing or sealing. Also the very slow cure time is nice for epoxy novices. And mix it with no filler and you get a paintable sealant.

Then mount the extension box using the 2 original threaded holes in the metal box, caulk it on, and let the caulk cure.

From there, conduit system of your choice onward to the new subpanel, using THWN wire as Ed Beal discusses. Remember you complete assembly of the conduit before you pull the wires, so no plumbing elbows lol. Or if conduit isn't your cuppa, run cable out a weatherproof fitting - but remember you need #6 cable not #8.

Note that your #8 wire cannot carry 60A. I see no markings on the wire, making it either NM-B or UF-B type cable. Both of those are confined to 40A ampacity. The breaker must be changed to a 40A breaker. If that's not enough for your loads, you will need to run a new #6 cable. Sorry.

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  • Thanks, I will definitely change the breakers once the project is good - this is an old house with many dead end circuits and strange combinations ;)
    – Misha0001
    Oct 20, 2020 at 17:27
  • The subpanel will have one circuit for 50A welding machine (different ampacity calculations, though, 10AWG specified), and one 20A circuit for some motion-detection lights and a computer outlet. So, this will all be protected by subpanel breakers... I was planning to change the Main breakers to 50A double, I know it's too high for 8AWG, but given the expected load at the subpanel, should be protected enough in practice? There is no way of pulling the wire again through internal walls :(
    – Misha0001
    Oct 20, 2020 at 17:38
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    No, I would not ever intentionally overload a wire, but that's probably moot because based on your loads, 40A will surely suffice, as you know welders are allowed a considerable favorable derate based on duty cycle. Unless your PC has quad 980s in it and you're gaming while welding, I doubt it + the welder will trip a 40A breaker. Even then I would not run a PC and welder on the same sub at the same time, given how spiky welders are, they work by creating arc faults lol...
    – Harper - Reinstate Monica
    Oct 20, 2020 at 18:35

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