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):
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?