NEC 1996- 370-40(c)
THICKNESS OF METAL.
Metal Boxes over 100Cu In.
Metal boxes over 100 Cu in. in size shall be constructed so as to be of ample strenght and rigidity. If of sheet steel, the metal thickness shall not be less than .053 in. (1.35 mm) uncoated.

I could not find any info for a panel board construction, However, info is given for junstion box construction specs. I would think if you make it .053 to .0625 it should be O.K.

Edward


Thanks
Edward