312.8 Switch and Overcurrent Device Enclosures. The wiring
space within enclosures for switches and overcurrent devices
shall be permitted for other wiring and equipment subject to
limitations for specific equipment as provided in (A) and (B).
(A) Splices, Taps, and Feed-Through Conductors. The wiring
space of enclosures for switches or overcurrent devices shall be
permitted for conductors feeding through, spliced, or tapping
off to other enclosures, switches, or overcurrent devices where
all of the following conditions are met:
(1) The total of all conductors installed at any cross section of
the wiring space does not exceed 40 percent of the crosssectional
area of that space.
(2) The total area of all conductors, splices, and taps installed
at any cross section of the wiring space does not exceed
75 percent of the cross-sectional area of that space.
(3) A warning label complying with 110.21(B) is applied to
the enclosure that identifies the closest disconnecting
means for any feed-through conductors.