I bought a junction box in order to splice three Romex together along the 2x4's in the ceiling of my (unfinished) basement. The box looks like this:

junction box

When I bought it, I assumed that I could just screw/nail it to the side of the joist using those holes in the back. However, I can't do that because the screws for the cable clamp stick out the back by about 3/4"!

So, what is the correct way to mount this box to a 2x4? Or did I just buy the wrong thing?

  • Also, you may want to consider the volume requirements of this box. The largest octagonal box I can find on sale at Home Depot is 15.5 cubic inches. If the wires are 12-2, you will require 18 cubic inches to splice three wires with ground wire, clamps, and no attached devices per section 314.16. For 14-2 and the same situation, the requirement is 16 cubic inches. Please ensure you have sufficient volume in your box for what you plan to do. Here is a website that walks through the calculations: code.necplus.org/sample/…
    – user14416
    Sep 3 '13 at 22:09
  • @statueuphemism: I believe the octagonal box I bought is 22 cubic inches. I made sure it's large enough. Sep 3 '13 at 22:10
  • @statueuphemism Nice catch. You'll need either a 4 x 2 1/8" octagonal box, or any square box. Typically when I'm not attaching a fixture, I use a square box.
    – Tester101
    Sep 4 '13 at 16:25

Method 1

  1. Place the box where you want it and screw it lightly to the stud.
  2. Run the clamp screws in until they bite into the wood.
  3. Remove the box.
  4. At the marks left by the clamp screws, drill 1/4 inch relief holes about 1 inch into the stud to make room for the ends of the screws
  5. Reinstall the box.

Method 2

  1. Remove the existing clamps
  2. Knock out the inner discs on the sides (not the ones near the original clamps) by pushing one side with screw driver, then wiggling back and forth until it breaks off
  3. Install bolt on cable clamps like these (screw section goes on the outside) cable clamp
  • I just remembered that I do have some of those other clamps to spare. However, I already knocked out the popouts for the existing clamps, and those bigger clamps use a different hole. Is it allowed to use a junction box with unused holes popped out? Or is there some way I can fill them back in (superglue the knockouts back? :P) Sep 4 '13 at 15:18
  • 2
    @BlueRaja-DannyPflughoeft You'll have to fill the unused knockouts with knockout seals, they come in both 3/4" and 1/2". NEC 110.12(A)
    – Tester101
    Sep 4 '13 at 15:39
  • @Tester101: But the knockouts next to those tiny NM-clamps are only 1/4"... I couldn't find anyplace that sells knockout seals that small. Sep 6 '13 at 20:15

Bottom of the Joist

You could attach the box to the bottom of the joist, but you'd have to rotate it in such a way that the joist was perpendicular to clamps.

Joist location

Bottom of Joist

The problem with this is that if you finish the ceiling with drywall, the box will stick out quite a bit (~2").

Bottom of Joist with Drywall

Between the Joists

The standard approach, is to attach the box to a 2x4 that is installed between the joists. The box will be attached to the 2x4, the same way as if you attached it to the bottom of the joist.


You'll set the 2x4 back from the joist edge about 1" (assuming 1 1/2" deep box).

Between Joists

Then if you install 1/2" drywall, the box will basically be flush with the drywall face.

Between Joist with Drywall

A Different Box

Alternatively, you could simply purchase a different box. They tend to be fairly cheap, so purchasing a box with an external mounting bracket may be an option.

enter image description here


Since no one else has mentioned it as an option yet, rather than drilling two additional holes in your joist, you could also add cut a small piece of wood that is narrow enough to sit between the two NM clamp screws to serve as a 3/4" (or more) riser and use two longer screws to secure the junction box (going through the junction box holes, through the riser, and into the joist). Then, you could mount the box to the side of the joist if you like. You could also use a wide riser if you are worried about stability and drill the holes for the NM clamp screws into it. I propose these slightly different options to avoid putting extra non-functional holes in the joists of your home.

If I had to use this box and did not want to mount it on the end of the joist, one of these methods would be my personal choice. That said, I would probably opt for another box and add NM clamps as already suggested in another answer.


Use the smaller holes in the "bottom" to screw it to board.

  • 2
    Er, did you read the question? I can't do that because the cable-clamp screws stick out the back (or "bottom" if you prefer) Sep 3 '13 at 20:35
  • @BlueRaja-DannyPflughoeft This solution would probably work on the 2" edge of the 2x4.
    – user14416
    Sep 3 '13 at 20:42
  • @statueuphemism: I know you're not allowed to run cable along the bottom edge of a joist; are you allowed to secure a junction box there? I didn't think it would be, since that would mean you could run cable along the bottom edge by simply securing a junction box to every joist :) Sep 3 '13 at 20:48
  • @BlueRaja-DannyPflughoeft You say "simply" as though it wouldn't be easier to drill holes and run a continuous wire rather than screwing in a junction box on each joist and splicing cable every 16" or so. Also, wire has to be secured within every 12" of a junction box (per the 2011 NEC and possibly earlier) and I am not positive whether the NM cable clamps as part of the fixture would count as securing the cabling. This solution might be ugly (just as attaching a length of 2x4 perpendicular to the ceiling and attaching the wire to it is ugly), but I am not aware of any code violations.
    – user14416
    Sep 3 '13 at 21:00
  • Use a drill to bore out small dimples for the screws.
    – DMoore
    Sep 4 '13 at 0:50

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