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I am trying to install a metal junction box behind existing drywall. I cut a hole in the drywall next to the stud, and want to screw the left side of the box into the stud, but the holes that are pre-drilled into the box are too far forward and would go into the drywall, not the stud. Pardon the crude drawing.

Ideally, there would be screw holes further back in the box so I could install it into the stud, but I can't find any metal boxes that have that. A few related Diy.SE posts suggest I can just drill my own holes (Can I side-mount metal box with holes in the back?, Why not use a plastic receptacle box for an outdoor outlet?) but doesn't making my own modifications violate the UL listing of the box and thus electric code? Am I allowed to drill my own holes (with some concrete information I can show my inspector to justify that), or if not, is there another solution to mount a metal box behind drywall?

Box in wall diagram

Edit:The purpose of this box is to connect wires that run behind the drywall to conduit that will run on the surface. Thus, following a suggestion from the question in Best way to bring wire from flush-mount panel to surface-mount conduit I was going to use an extension box. If this is an X-Y problem, or if there is a better box or approach to doing this, I would appreciate suggestions.

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  • do not understand your dilemma. There are virtually dozens of stud mount types
    – Traveler
    Mar 13 at 20:25
  • I am trying to mount this behind existing drywall, I guess what could be considered "old work". If I try to use a box with a mounting bracket on the side, I would have to cut a large section of drywall around the box. Ideally, I'm trying to mount the box without removing more drywall. If there is a stud mount type that can be inserted into a 4x4 hole, I would love to hear about it.
    – Daniel
    Mar 13 at 20:28
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    Yes you can drill new holes to mount it so the front is flush with the drywall
    – DJ.
    Mar 13 at 21:32
  • What's your surface solution here—what will be seen once your work is done?
    – Huesmann
    Mar 14 at 12:48
  • You will see a box extender coming out of the wall with conduit running out of the side, and a cover plate (like the one you linked in your other comment) covering that. You can see the hole in the drywall behind the box which I don't like, but I can't figure out how to make that look better.
    – Daniel
    Mar 14 at 13:57

2 Answers 2

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That box should have a mud ring of the correct thickness to account for the drywall and/or other wall finish mounted to the front to mount devices to. At which point the holes it has should land in the stud.

Two-device mud ring

Image source - example not endorsement.

You could also use a box extension ring if you really just want the blank cover that fits the box type rather than a device or a finished cover plate that needs device mounting holes.

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  • The purpose of this box is to take NM wire that was run behind the wall to conduit that will run on the front surface of the wall. Thus, following a suggestion from diy.stackexchange.com/questions/271276/…, I was going to use an extension box. But if I install the box far back enough to use the screw holes in the studs, the extension box will not clear the drywall.
    – Daniel
    Mar 13 at 20:19
  • Also, I have a thick wire coming out the back of the box (NM 6-3). I can't push the box farther back into the wall because the wall is in the way.
    – Daniel
    Mar 13 at 20:23
  • @Daniel notice that your box has no provisions for mounting a cover plate, and you will need to use a diagonally-mounted cover plate in the style of the the mud ring Ecnerwal pictured to cover it anyway. (Flat type linked from HD, but there are raised ones too: homedepot.com/p/…)
    – Huesmann
    Mar 14 at 12:47
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Get one like this one Raco box with bracket from Home Depot. It gives you the space and holes you need to mount it to the studs.

Raco 4" with bracket

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  • While this would be perfect for new construction, I already have drywall up. This would require me to cut a much larger hole to fit the bracket into (and larger still to have space to screw it in)
    – Daniel
    Mar 14 at 2:01

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