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?
Best Answer
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.