Electrical – Tips for getting cables through old work box knockouts and into wall

The knockouts on plastic boxes are not designed to be removed completely. Rather, they are kinda like trap doors and some pressure against the NM cable helps keep it from slipping back out of the box. In your case, if you have old fabric covered cable, you really should be using boxes with a 1/2" screw type cable clamp. If you insist on using the plastic boxes, then you will have to use a screwdriver and hammer to knock the square door in first, then grab the flap with a pair of linesmen pliers and twist it until it separates from the box. I guarantee that is going to leave a ragged edge that may still abrade your cable. These plastic boxes were never designed for old degrading cable. BTW, I was the factory rep for Carlon Inc, manufactures of plastic boxes, many years ago, so I have a bit of background.

A horizontal board between two studs is typically fire blocking. The idea of fire blocking is to prevent a cavity in a wall more than 8' high where fire can quickly spread. You'll see this in any room with over 8' ceilings. For it to work properly, you can't have any gaps allowing air to go from the lower cavity to the upper one. This means any gaps need to be caulked with fire rated caulk (that's typically the red or orange caulk). Why the heck someone installed this blocking and electrical box in the same place is bizarre, either one could have been moved up or down a few inches.

To work around this, you have a few options. You can cut a gap into it, and fit your old work box, but from the small opening you'll have a very difficult time cutting out the back. The result will require a load of caulk on the 2x4 that you squeeze the old work box into. And you'll need to install it a few inches to the side so there is some material continuous from side to side.

You could also fix this the right way and open the wall up, and reinstall new blocking a few inches up or down. Then you can remove the old blocking and install a new work box.

And the easiest solution is as DA01 says, install the new switch a few inches higher so it's above the blocking. I'd probably go with this solution myself, unless you feel like doing some drywall repair.