My house has mostly AC/BX wiring. I have added a light to an existing lighting circuit and pulled #12 Romex into an existing metal box. Problem is, this box has no 1/2" or standard size knockouts. It has only the smaller AC knockouts. I need a snap-in cable clamp to properly ensure my non-metallic insulation doesn't get cut on the metla box opening.

I feel like this would be a nornal request but the folks at home Depot looked at me like I was speaking a different language. I did find some 3/8" push-in cable clamps but they still looked to big.


  • I looked again. There is definitely a 1/2" knockout in that box, but I chose to pull the cable through the BX knockout, don't ask me why. Anyways, does such a cable clamp/gland exist to properly do this?
    – DrTarr
    Dec 31, 2018 at 21:53
  • So the Romex is already in and you want to fit a clamp without disconnecting and removing the wire? You might be able to enlarge the hole with tin snips and then fit in a two-part clamp, but I think you would find it easier to disconnect the Romex inside the box, pull it out, enlarge the hole enough to fit a cable clamp, then re-install the Romex. Dec 31, 2018 at 21:56
  • Actually, I can disconnect the wire. Problem is I can't remove the box without tearing out lots of drywall, so it has to be a snap in and not a two part clamp.
    – DrTarr
    Jan 1, 2019 at 13:44
  • Old thread I know but this may help someone else. Look at this box at Home Depot. RACO 2-Gang Electrical Square Box with NMSC Clamps and Bracket
    – Bob
    Feb 1, 2023 at 21:27

1 Answer 1


After looking at the comments, the real problem isn't the knockout, it's a clamp that will work with access only to the inside of the box.

Luckily, this is a solved problem. There are plastic push-in NM connectors made to install inside the box, such as the Raco Insider:

Raco Insider

This will install in a standard 1/2" KO.

This is a very common problem, so connectors that will work with cables fished into boxes are available almost everywhere electrical supplies are sold.

Big box stores are basically self service, I am sure there are exceptions, but I wouldn't expect ANY technical expertise from the staff. I've even seen egregious code violations in DIY videos on their official Youtube channels.

  • 2
    My local electrical supply house manager confessed to me that he regularly trolls big-box looking for anyone competent, and hires them away. Clerks who remain at Home Depot are the "rejects". Home Depot is good at pre-employment screening, so has done that job for him! Apparently that's SOP in the supply house game. HD only really wants stock boys. Jan 1, 2019 at 18:08
  • 1
    Thanks. I'm aware of 1/2" knockout plastic and metal snap ins. What I was specifically looking for was this exact thing for an AC/BX knockout, which is much smaller. I am thinking they just don't exist.
    – DrTarr
    Jan 1, 2019 at 18:49

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