My electrical panel is mounted in the corner of my garage, it's flush mounted on the outside wall about 2' away from the back wall. I have a heater, water heat, and well pressure tank along that back wall that I would like to box in. I would like to build a wall about 40" off of that back wall, there will be a door to get in and service everything. The only problem I have is the electrical panel. How far away from that panel could I have the wall? How much clearance is needed?

Here is a lame picture, the red line is the wall I want to build, the electrical panel is all the way on the right. My state goes by the 2005 NEC.