I am installing/replacing a sub-panel in the basement of a split-level. The new panel is on an unfinished wall (drywall on opposite side, towards living-room, but no drywall on the basement side), but the studs at the point of installation are too close for me to install between them. So, since I had the pieces, I ran a pair of wood cross-beams (morally, 2X4's, but in reality cut from old stairs) perpendicular to the studs, and screwed the sub-panel onto them, and then installed wood, exterior of the panel, level with the cross-beams, so that I can be secure the wires going into the box. Therefore, there is a void behind the box, except where I have the cross-beams.
My question is: have I committed a NEC/fire-safety violation? As far as I can tell, no, but…
[In effect, it's as if I had screwed a large piece of plywood to the wall, with a square cut out in the middle, which I hid with the panel.]
Best Answer
You may notice that the panel has knockout holes all along both sides. Mounting them in the void between joists is not a requirement, and those side knockouts are for when you surface mount the box.
Generally in that situation, you mount the service panel on a sheet of plywood somewhat larger than the panel. Leave yourself room for externally mounted things like the electrician's outlet, whole house surge suppressors, power monitors, datacomm, etc.
I'm not sure about mounting it to a couple sticks of 2x4. Certainly if you finish the basement later, it's a lot easier to mate up drywall to a sheet of plywood to make it presentable, rather than a couple 2x4s.
Since this is a "best practice" done almost universally, needless to say, I recommend doing it.