This article focuses on permitted wiring methods in environmental air spaces. More specifically, areas above suspended/drop ceilings used for environmental air. Mainly because no wiring of any kind is permitted for ducts for dust, loose stock, or flammable vapors. Even wiring for vapor removal in ducts or shafts for ventilation of commercial cooking equipment is prohibited.
In general we want to limit products of combustion and materials that would produce smoke in ducts and other air handling spaces. During a fire this would distribute smoke throughout the building including areas that may not be affected by the fire yet. Environmental air spaces confined to a specific area such as a raised floor in an Information Technology Equipment room does not have these wiring restrictions. See Article 645 of the NEC for specific requirements of ITE Rooms.
Wiring Methods Permitted
Ducts for Environmental Air
Cables – Mineral Insulated (MI) and Metal Clad (MC) Cable of the smooth or corrugated type (not interlocked) without a nonmetallic covering such as PVC jacketed MC cable.
Conduits & Tubing – Electrical Metallic Tubing (EMT), Flexible Metallic Tubing (Not FMC), Intermediate Metal Conduit (IMC), and Rigid Metal Conduit (RMC). Flexible Metal Conduit (FMC) used with proper connectors that close openings in the connection is only permitted for lengths up to 4′ for adjustable equipment. None of these can have an overall nonmetallic cover.
Equipment & Devices – Only that which is necessary to sense or act upon the air contained in the duct.
Luminaires – When necessary for maintenance or repair, only the enclosed gasketed-type is permitted.
Other Spaces Used for Environmental Air (Plenums not Fabricated Ducts)
Cables – MI, MC, and (AC) Armored Cable and cables listed for use in environmental air-handling spaces.
Raceways – EMT, Flexible Metallic Tubing, IMC, RMC and FMC. Where accessible metal raceways and wireways with metal covers.
In dwelling units where stud/joist cavities are used for environmental air wiring is permitted to pass through perpendicularly. Such as inside walls or basements in cavities with panning.
Spaces Not Used for Environmental Air
Areas not used for environmental air-handling such as above suspended/drop ceiling with a fabricated return air duct system, there is no wiring restrictions.
This is not related to the restrictions found in other sections of the Code like NM Cable above drop ceilings in other than dwelling unit occupancies or flexible cords/cables not being permitted to be installed above drop ceilings.