I've found the knockouts in NM boxes hard to fully open. I've had trouble with both Carlon boxes and Madison Smart Boxes. The plastic molded to make the knockout is very resistant to actually being knocked out.

My current approach is to whack the knockout with a hammer's claw. This generally busts the knockout pieces apart, but it doesn't remove them. They often remain so tight together still that forcing the cable in is very difficult and I worry about damaging the cable sheathing. Grabbing the "arms" of the knockout with pliers and pulling and twisting generally just crushes off a little bit, rather than wrestling out the entire chunk that's in the way.

Sheathing damage was a major problem when trying to push old, brittle rag wiring through the knockout: it was stripping the brittle insulation off the wires rather than admitting them into the box. My solution there was to drill through the knockout arms to get them out of the way, but this was a major pain, and the burning plastic smell generated by the rotating drill bit was not encouraging.

What's the "right way" to fully open these molded knockouts?

  • 2
    "Old, brittle rag wiring"? STOP! As Shirlock says, this approach is not right. Serious danger of further damaging questionable wiring. You really run a risk of short circuits and, given the questionable state of the overall system, maybe inadequate cutouts (breakers or fuses). Seriously consider replacement of the lines at issue, at a minimum.
    – bib
    Jul 28, 2012 at 2:36
  • @bib The rag wiring is getting pulled into the box because it's being made redundant. The new box replaces an old gangable metal box that had fallen apart in the wall, and a new circuit is getting pulled to the box. I can't yank the rag cable out of the finished wall, so I'm pulling the end that was in the old box into the new box and capping the grounded and ungrounded conductors off, rather than leaving a mystery cable hidden in the wall and ceiling with no presence at outlet boxes. Jul 28, 2012 at 5:18
  • I wouldn't pull dead wire into a new box, unless I was going to wire it to ground for some reason. This is assuming I knew for certain that it is dead. Jul 31, 2012 at 1:45
  • @BradGilbert I was concerned about hiding mystery wire in the walls. You're right, though, it is entirely dead, so I guess I need not have gone to the trouble. Jul 31, 2012 at 15:07

2 Answers 2


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.

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    +1 the "flap" also prevents wires from being easily pulled out of the box. Careful when inserting into the box, though, because the sharp edges can damage even new romex sheathing... It's like the nut on metal boxes, though I see the plastic boxes damage the wire more often than the metal nuts.
    – Matthew
    Jul 27, 2012 at 21:02
  • Thank you! So there is no trick, it's just supposed to be that tight a fit. To their credit, the Carlon boxes have been much easier to wrest completely open than the Madison Smart Boxes, but the latter let you stud-anchor boxes even in old work, because the screws are inside the box. Jul 27, 2012 at 21:18

I successfully used a sharp, insulated, slotted screwdriver on a Carlon box. Clean separation.

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