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The provisions of this section apply to the installation and

uses of electrical wiring and equipment in ducts, plenums,

and other air-handling spaces.

FPN: See Article 424, Part Y1 for Duct Heaters.

(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 systems 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 or Plenums Used 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 metallic conduit

without an overall nonmetallic covering shall be

installed in ducts or plenums specifically fabricated to

transport environmental air. Flexible metal conduit

and liquidtight flexible metal conduit shall be permit-

ted, in lengths not to exceed 1.2 m (4 ft), to connect

physically adjustable equipment and devices permit-

ted to be in these ducts and plenum chambers. The

connectors used with flexible metal conduit shall

effectively close any openings

in the connection. Equipment and devices shall be

permitted within such ducts or plenum chambers

only if necessary for their 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 (fixtures) shall be permitted.

section applies to space used for environmental air-

handling purposes other than ducts and plenums as

specified in 300.22(A) and (B). It does not include

habitable rooms or areas of buildings, the prime

purpose of which is not air handling.

FPN: 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.