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I'm rewiring my house, which was built in 1921. I need to bring new cable to a triple-wide switch box (see photos). I cannot push the old knob & tube wire out through the knockouts--it must be stapled near the box. There is an unused knockout, but I haven't been able to knock it out from within the box. I removed the mounting screws, but it is set firmly in the plaster, so I can't just slide the box out.

inside top of box Note the screws coming in from the top. Perhaps another metal plate above holds there together.

triple-wide box

Is there a trick to pushing the old cables out, or removing the knockout from inside the box? Maybe drill a hole in the knockout and then pull it inward? Is there any way less destructive than cutting all around the box to remove it?

  • On the bright side, be happy they used a box in the first place! I've seen more than one situation where the wires were run to a switch which was screwed directly to the plaster! The 3 screw ends you see at the top should be matched by 3 on the bottom - they're what hold the 3 individual boxes together as a gang (and the sides of the box on). It's the flat-head screws at the back center that are most likely the ones holding the box to the wall (after you cut it free from the plaster, if you go that route). – FreeMan Jun 30 at 11:27

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