My main electrical panel is mounted on the outside wall of my garage. It is stucco on the outside and the framing is 2x4's. The garage is mostly unfinished. The section around the back of the electrical panel has drywall. It's there to protect all the NM cable (romex) that comes out of the panel. There is a drywalled raceway at the top of the wall that the NM cable runs through to get into rest of the house.
The drywall panel that covers the back of the electrical panel is quite damaged and broken up. I would like to replace it. The back of the panel is flush with the inside drywall. Do I have to replace it with another piece of drywall, or can I use a sheet of plywood? I would much rather use plywood to cover the panel and the romex. I like the extra strength, so the damage doesn't happen again. Is it code that the panel must have drywall behind it?