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This thread is useful to my needs, thanks everyone. I have a ragged hole in the drywall under by breaker box, where I fed thru the conduit for my transfer switch I installed almost 2 years ago. I'm no drywall wizard, so I've just been staring at it daily, wondering how to fix it so it looks halfway decent (it's in the separate garage, so looks aren't terribly critical).
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I did this, but used a pointed drywall saw to cut a neat square. After installing all of the wiring, I put in backer strips with drywall screws and reinstalled the panel. I can easily spackle it and refinish, but this allows me to get back into it to run another circuit, which I have done a couple times. This is what OP should have done. Cut the hole, don't go all Leroy Jenkins on your drywall.
If you have ever seen a company do a whole house re-pipe, this is exactly how they do it. They also sand the patches with a wet abrasive sponge while the mud is still in the plastic state, so there is no dust.
The holes don't need to be perfect, and that makes it easier to reinstall the panel exactly how it was before.
Carnage: