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I’m trying to add a 10/3 circuit, but physical access to the knockout available in my panel is challenging due to the way it was installed. I can get a 3/8” clamp fitting in there with some difficulty, but it has a hole that’s too small for the 10/3 NM to fit through. The hole appears to be made to easily enlarge (see photo):

Two clamps, one with hole enlarged

Unfortunately, I just used pliers to tear out the inner ring, and that has left sharp edges. They’re easily knocked down with a file, and I can put a rigid plastic bushing on the inside of the fitting.

But is this allowed?

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    Have you looked at plastic cable clamps such as the Raco Insider? Jun 22 at 0:05
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    there are markers where to use screwdriver to knock it out (downwards). With players you were pulling in wrong direction. It is relatively brittle material, so knock downwards would have done it. Get another and try.
    – knowitall
    Jun 22 at 0:11
  • @ThreePhaseEel Oh, the Raco Insider might be much easier to use! They’re not quite as secure though, are they? And knowitall, I used a combination of screwdriver and pliers, actually. Sounds like that’s acceptable practice?
    – Rick
    Jun 22 at 0:56
  • @Rick -- I can't really attest to their security, but Arlington also makes a similar clamp (White Button, I believe) that might be worth trying if the Raco ones aren't suitable Jun 22 at 1:00
  • White Button clamps would be nice, as they appear to install from inside the box, but alas seem to unavailable for days without exorbitant shipping fees. Thanks, I’ll try the other options.
    – Rick
    Jun 22 at 1:17

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