300.22 Wiring in Ducts Not Used for Air Handling,
Fabricated Ducts for Environmental Air, and Other
Spaces for Environmental Air (Plenums). The provisions
of this section shall apply to the installation and uses of
electrical wiring and equipment in ducts used for dust,
loose stock, or vapor removal; ducts specifically fabricated
for environmental air; and other spaces used for environmental
air (plenums).
(B) Ducts Specifically Fabricated for Environmental Air.
Only wiring methods consisting of Type MI cable, Type
MC cable employing a smooth or corrugated impervious
metal sheath without an overall nonmetallic covering, electrical
metallic tubing, flexible metallic tubing, intermediate
metal conduit, or rigid metal conduit without an overall
nonmetallic covering shall be installed in ducts specifically
fabricated to transport environmental air. Flexible metal
conduit shall be permitted, in lengths not to exceed 1.2 m
(4 ft), to connect physically adjustable equipment and devices
permitted to be in these fabricated ducts. The connectors
used with flexible metal conduit shall effectively close
any openings in the connection. Equipment and devices
shall be permitted within such ducts only if necessary for
the direct action upon, or sensing of, the contained air.
Where equipment or devices are installed and illumination
is necessary to facilitate maintenance and repair, enclosed
gasketed-type luminaires shall be permitted.