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
Informational Note: See Article 424, Part VI, for duct
(A) Ducts for Dust, Loose Stock, or Vapor Removal. No
wiring systems of any type shall be installed in ducts used
to transport dust, loose stock, or flammable vapors. No
wiring system of any type shall be installed in any duct, or
shaft containing only such ducts, used for vapor removal or
for ventilation of commercial-type cooking equipment.
(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.
(C) Other Spaces Used for Environmental Air (Plenums).
This section shall apply to spaces not specifically
fabricated for environmental air-handling purposes but used
for air-handling purposes as a plenum. This section shall
not apply to habitable rooms or areas of buildings, the
prime purpose of which is not air handling.
Informational Note No. 1: The space over a hung ceiling
used for environmental air-handling purposes is an example
of the type of other space to which this section applies.
Informational Note No. 2: The phrase “Other Spaces Used
for Environmental Air (Plenum)” as used in this section
correlates with the use of the term “plenum” in NFPA 90A-
2009, Standard for the Installation of Air-Conditioning and
Ventilating Systems, and other mechanical codes where the
plenum is used for return air purposes, as well as some
other air-handling spaces.
Exception: This section shall not apply to the joist or stud
spaces of dwelling units where the wiring passes through
such spaces perpendicular to the long dimension of such
(1) Wiring Methods. The wiring methods for such other
space shall be limited to totally enclosed, nonventilated,
insulated busway having no provisions for plug-in connections,
Type MI cable, Type MC cable without an overall
nonmetallic covering, Type AC cable, or other factoryassembled
multiconductor control or power cable that is
specifically listed for use within an air-handling space, or
listed prefabricated cable assemblies of metallic manufactured
wiring systems without nonmetallic sheath. Other
types of cables, conductors, and raceways shall be permitted
to be installed in electrical metallic tubing, flexible metallic
tubing, intermediate metal conduit, rigid metal conduit
without an overall nonmetallic covering, flexible metal
conduit, or, where accessible, surface metal raceway or
metal wireway with metal covers.
(2) Cable Tray Systems. The provisions in (a) or (b) shall
apply to the use of metallic cable tray systems in other
spaces used for environmental air (plenums), where accessible,
(a) Metal Cable Tray Systems. Metal cable tray systems
shall be permitted to support the wiring methods in
(b) Solid Side and Bottom Metal Cable Tray Systems.
Solid side and bottom metal cable tray systems with solid
metal covers shall be permitted to enclose wiring methods
and cables, not already covered in 300.22(C)(1), in accordance
with 392.10(A) and (B).
(3) Equipment. Electrical equipment with a metal enclosure,
or electrical equipment with a nonmetallic enclosure
listed for use within an air-handling space and having adequate
fire-resistant and low-smoke-producing characteristics,
and associated wiring material suitable for the ambient
temperature shall be permitted to be installed in such other
space unless prohibited elsewhere in this Code.
Informational Note: One method of defining adequate
fire-resistant and low-smoke producing characteristics for
electrical equipment with a nonmetallic enclosure is in
ANSI/UL 2043-2008, Fire Test for Heat and Visible Smoke
Release for Discrete Products and Their Accessories Installed
in Air-Handling Spaces.
Exception: Integral fan systems shall be permitted where
specifically identified for use within an air-handling space.
(D) Information Technology Equipment. Electrical wiring
in air-handling areas beneath raised floors for information
technology equipment shall be permitted in accordance
with Article 645.