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.
Thanks!
Best Answer
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:
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.