I believe there has to be a 1/4" air space behind a metal cabinet attached directly to concrete that can accumulate moisture. The wood relieves the installer of that. If I get a minute I will dig up the specifics.
Re: Panel in basement#91510 01/20/0501:02 PM01/20/0501:02 PM
Greg's answer is correct, however you'll find that most panels have little bump outs where the mounting screws go that just happen to protrude about 1/4 of an inch.
If your panel is like this, you should be able to mount it on concrete. However, if the poured wall isn't all that smooth, or the forms weren't well aligned, you may have concrete touching the back of the panel in places. Usually, this is right where you want a screw to go such as for a grounding bar or the main bonding jumper.
I'd suggest using some washers or short slices of 1/2" PCV conduit to make spacers to get the panel well over 1/4" off the concrete unless its a real smooth wall.
Mark Kent, WA
Re: Panel in basement#91511 01/20/0505:49 PM01/20/0505:49 PM
I'm seeing installers shooting the panel on the poured cement walls and not using anchors and screws. I'm sure they don't maintain the .25" standoff that the dimples on the back of the panel provide. Am I getting to picky in my old age?
Re: Panel in basement#91512 01/21/0502:44 AM01/21/0502:44 AM
312.2 requires the 1/4 inch space. If the installer chooses to install the panel directly to the surface (provided it has the 1/4 inch 'dimple', that is acceptable as per code. Going beyond the code is great, but cannot be enforced.
Re: Panel in basement#91516 01/22/0504:45 PM01/22/0504:45 PM
Not directly relevant to the NEC I know, but back in the days when British houses had basements or cellars the normal approach was to mount a sheet of plywood to the walls using porcelain standoffs, leaving a gap of about 3/8 to 1/2-inch.
The standoffs were designed in such a way that they went right through the board, effectively providing not only mechanical separation but also electrical insulation between board and fixing screws.