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I had to replace an entire coax run and noticed that someone who installed my Aunt's TV put a outlet behind the TV and used a low-voltage box! I know why they did this, mostly because there is only 1" clearance to the cement that is behind the drywall (perimeter wall) and they were lazy.

I already "corrected" the problem but I wanted to know if what I did is ok.

I found the smallest Carlon blue box that Home Depot carries and drilled a 1/2" hole in the bottom and put a romex clamp in to hold the romex (for various reasons I couldn't use the side clamps that come with the box). I then secured the box to the cement using a screw through the back of the box and covered the screw with electrical tape (read something about doing that with screws inside a box that aren't electrical in nature).

Is there anything against modifying the box in the way I did?

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As long as the box is rigidly secured, you're good to go regarding the box mounting arrangement

Considering that the concrete wall can be considered to be a structural member, not an exposed building surface, and that the box is mounted independently of the finished building surface, NEC 314.23(B)(1) applies here, which permits the field-drilled mounting hole and screw arrangement used provided it secures the box sufficiently and the AHJ is OK with it upon inspection:

(B) Structural Mounting. An enclosure supported from a structural member or from grade shall be rigidly supported either directly or by using a metal, polymeric, or wood brace.

(1) Nails and Screws. Nails and screws, where used as a fastening means, shall secure boxes by using brackets on the outside of the enclosure, or by using mounting holes in the back or in a single side of the enclosure, or they shall pass through the interior within 6 mm (1∕4 in.) of the back or ends of the enclosure. Screws shall not be permitted to pass through the box unless exposed threads in the box are protected using approved means to avoid abrasion of conductor insulation. Mounting holes made in the field shall be approved.

Field KOs in a box should be OK as well

Making knockouts (whether of standard or nonstandard size) in electrical boxes in the field is a long-standing practice in the electrical industry (in fact, some enclosures ship without knockouts, expecting the user to make their own). As far as I can tell, as long as the cable is adequately secured to the box with a standard cable clamp (as per 314.17(C)), it makes no difference whether the KO is factory or field made.

However, a box that shallow doesn't have enough fill for...much of anything

The shallowest Carlon nonmetallic boxes (at 1.25" deep), while technically deep enough for a receptacle (at only 0.69" for the receptacle + the minimum 1/4" of extra depth required by Code), do not provide enough box fill at 8in3 to accommodate a hot, a neutral, a ground, an allowance for a cable clamp, and the double allowance for the receptacle yoke -- for 14AWG wire, this would require 12in3 of fill. As a result, you'll need to slap a 5/8" deep non-metallic single gang box extender (or construct one using a 1/4" and a 3/8" stacked atop each other) on there to gain that extra couple of cubic inches of fill.

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  • Great to hear about the first few points. The last point I think is interesting; tbh I don't feel concerned about it as I feel nothing was too cramped and made sure everything was properly isolated. Extender would protrude past drywall. With that being said, I checked the other boxes that were installed when the addition was made and they are metal, mounted to the concrete, are daisy chained (2 14/2 goes in) and the same depth. This addition passed inspections so I assume either a) the inspector didn't care to much about it b) metal boxes have different fill requirements?
    – Alex
    Commented Dec 19, 2018 at 5:14
  • I should note: had I done this entire install I probably would have done surface mount. Also the low-voltage outlet box was done after the addition (presumably by some handyman special).
    – Alex
    Commented Dec 19, 2018 at 5:15
  • @Alex -- different box types/dimensions have different volumes, yes, and it's OK for the extenders to protrude past the drywall :)
    – ThreePhaseEel
    Commented Dec 19, 2018 at 12:40
  • I have field modified these boxes and passed inspection, some inspectors question this but I have not been red tagged. As far as box fill I have usually installed a junction box in the top of the wall at the ceiling because these shallow boxes don't have the cubic inches for anything but 1 cable as 3 phase pointed out.+
    – Ed Beal
    Commented Dec 19, 2018 at 14:47

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