Am I dreaming, or did I see a special shallow electric outlet box for 1.5″ furred out walls on top of concrete? I am picturing a box that is wider than a standard single outlet box with space in the side for the wires to get the min volume per code out of a 1.5″ deep wall cavity.
Anyone have some sources or ideas?
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They exist, or at least they did about 20 years ago. I used them in a basement with furring strips instead of 2 x 4 studs framing the walls. A real pain to wire them - not much room - and then you have to cram them in the box.
There are 1-3/4" cut-in boxes. You gain the depth of the drywall so they work well. The only real problems to be expected are: The use of old work boxes makes the box mounting dependant on the strength of the drywall. I usually expect any drywall near concrete to be damp and soft.
; Any clamp screws come out the back increasing the working depth of the assembly unless provisions are made. Using spray foam or using plastic anchors through the back of the box makes for a stronger assembly. Cutting the clamp screws, after the clamps have been tightened on the romex, or drilling clearance holes in the wall saves trouble. Don't forget a clearance hole for a ground screw.
Of course there is one other small problem. These boxes are tiny, with a small internal volume. For standard code fill requirements these shallow cut-in boxes are suitable for only a single run of 14/2 romex. Some not even that counting the device as two units and the clamp as one, per code.
Even ignoring code, never a good thing, one of these boxes can be quite a challenge to get everything into. Sometimes your stuck with an existing situation. A GFI and two runs of 12/2 with such a box can test the talent of even an experienced electrician.
The alternative is to use a deeper box and chisel into the concrete. If the wall is block and you luck out by hitting the void it can be done. In old, soft block walls, where there is a lot of moisture coming through the wall or old, hard, concrete chiseling in a hole can be difficult or counter productive. Expect trouble if the wall has been surface coated to prevent water infiltration. In that case use another option to avoid violating the barrier.
If you can' t go back going wider is the alternative. Inch and a half is a standard depth for four square, 4"x4"x1-1/2", boxes. This gives you a lot more room for fill. You can then place a 1/2" deep mud ring on the box, they even make one with the hole on one side with dimples pressed into the face to make plaster stick a little better, and mount the box flat to the wall. Sounds simple.
It is a bit more complicated. You furring strips are probably not a true 1-1/2" probably closer to 1-7/16". The box is usually very close to an exact 1-1/2" but the mud ring adds a good 1/16". Plan for 1/8" and expect that the mounting screws will add a touch. Assuming the drywall can't be bowed out a little expect the assembly to be 3/16" too strong.
Cutting a 4" square hole in the drywall then chisel or grind the wall face back a good 3/16". Try to keep it a flat plane so the box won't rock. A couple of simple plastic anchors in the wall will hold the box but don't forget to get the conduits or cables into the box before mounting it. A shallow clearance hole for a ground screw is a good idea. Mount the box. Mount the mud ring. Patch the drywall, easier if you got the dimpled mud ring. Once you have finished and painted trim out the box normally.