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?
Best Answer
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:
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.