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I'm installing a new receptacle to power a wall-mounted TV, it is feeding from an existing receptacle on the other side of the wall. The existing box has 2 cables coming in from the top, and I believe another cable from the bottom. I drilled a 1/2 inch hole on the back of the box to insert the new NM cable feeding the new receptacle.

New cable coming out of back of existing box Bigger picture

But I am running into some problems/questions:

  1. Is it acceptable to insert metallic flex conduit into a non-metallic box, assuming the conduit is bonded on the other end to a grounded metallic box? Is it acceptable to use a bare NM cable without any conduit?
  2. The flex metallic conduit coming out from the back of the existing box does not appear to be bendable enough to avoid touching the drywall. Is it acceptable to patch over this, or should I put a blank face plate as a way to protect the conduit from nails or drilling?
  3. The electrical box appears to have 3 existing cables coming in, not including the new cable. The cables in the box feel crammed. Could this lead to a code violation or safety hazard? Should I attempt to replace with a larger box, or what other options do I have?
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  • Where in the world are you? Does your local code require conduit? It's likely so, as most people don't use conduit in a residence just for fun. NYC and Chicago are two notable cities that require conduit, so in those cases, NO you may not use bare NM-B without conduit.
    – FreeMan
    Apr 15 at 11:14

2 Answers 2

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You cannot just start drilling holes in outlet boxes and shoving FMC into the hole. You'll need the proper FMC to box connectors and a metal box. You can run the NM cable from the top of the existing box up to your new outlet and just patch the lower opening with drywall. Without doing a load fill, I think four NM cables and an outlet will exceed the box capacity. Look inside the box, if possible, to see if you can read any of the numbers in there about cubic inch capacity. If the cables feel crammed, they probably are. Plus, you'll have to plug the 1/2" hole you drilled into the box so just replace it. I've use boxes similar to the one pictured below. They are much more secure than the old work boxes that just use the tabs to secure them in place since they screw right into the stud.

enter image description here

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  • The plastic box is attached on the side with a metal plate to a stud, I think the screws are hard or impossible to access. Do you have any suggestion for how to replace it, just cut the metal? Or cut open the wall and patch it? Is it OK to replace it with an old-work box?
    – user84207
    Apr 14 at 23:52
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    @user84207 I've always used a rotary tool with a cutoff wheel or just a single handle hacksaw to remove the old box. Old work boxes will work for this. See update on my answer for a picture of a great box to use.
    – JACK
    Apr 15 at 0:59
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I think now you will need a larger box requiring more extensive drywall removal and patching. A square box will likely be needed, a single gang mudring on a 4" x 4" square box would allow you to keep a single device appearance with a wider box behind it.

  1. NEC 314.3 prohibits metal raceways with plastic boxes unless bonding means is provided. NM cable can be used without conduit, but not individual THWN type conductors.

  2. FMC can touch drywall, but NEC 348.24 say minimum bending radius for FMC is per Chapter 9 Table 2, which says 4" for 1/2" raceways. 348.42 prohibits concealing angle connectors.

  3. Boxes have a maximum fill calculation, 2.0 in³ for each #14 circuit conductor, 2.25 in³ for each #12, four grounds count as 1, a device adds a count of 2 of largest wire, additional internal fittings and clamps in the box also also add to the count.

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