Re: 4/0 cable into the back of a 3 1/2 inch deep panel
Stew,
I disagree that 312.6(a)does not apply. 408.2 requires panelboards to comply with the provisions of Articles 312 and 314.
314.28(A)(2)requires that the distance between a raceway entry inside the box and the opposite wall of the box equal six times the largest raceway.
The exception to this rule permits a reduction to the distance required by 312.6(A), for one wire per terminal, if the entry is opposite a removable cover like a panel cover.
However, 314.28(A)(3) permits smaller dimensions than those required by 314.28(A)(1) and (A)(2)if the box is listed for and permanently marked with the maximum size of conductors permitted.
Panelboards marked as "Enclosed Panelboards" have been investigated by UL to determine wire bending space. If the panelboard is marked "Use Copper or Aluminum Wire", the wire bending space has been evaluated for aluminum wire.
I would permit the 4/0 aluminum conductors to be installed in this manner if the panelboard is marked to permit the use of aluminum wire.