Don, I think this is as close as you get to a fill rule for short sections of raceway
Chapter 9 Table note
(4) Where conduit or tubing nipples having a maximum length not to exceed 600 mm (24 in.) are installed between boxes, cabinets, and similar enclosures, the nipples shall be permitted to be filled to 60 percent of their total cross-sectional area, and 310.15(B)(2)(a) adjustment factors need not apply to this condition.
If you can't use that I don't see any relief from the normal 53% single conductor rule. (31% for 2, 40% for 3 and up)
[This message has been edited by gfretwell (edited 03-17-2006).]
