Is non-metalic NM cable permitted to run through the cold air return space un-protected, or should it have a metal raceway to cover it? The N.E.C. handbook is non-specific as to what type of cable is permitted, but does show a diagram with the cable installed. No where does it specify Plenum rated cable. This practice seems common in residential wiring. (Inspectors I have delt with in the past never permitted any wiring to pass through cold air returns, unless it was encased in emt.)
yes by exception, 300,22(C) "shall not apply" for residential dwelling units for wiring passing perpendicular through the stud or joist space.
The exception applies to spaces that are created by covering one or two sides of the joist or stud space with sheet metal, wiring entering and exiting through holes in the wood. It does not apply to duct spaces, passing through the sheet metal. If you are passing though the duct, you need a plenum rated method
300.22(C), other space used for environmental air, wiring methods shall be limited to ... intends to limit smoke transmission to other occupied spaces by prohibiting combustible or smoke producing products and methods from that space. It also assumes the plenum or other air handling space is itself built of non combustible products. The residential joist space would itself be built of combustible products, and the exception applies. But a duct in the joist space would fall under 300.22(B) and the exception would not apply.
The romex never passes through a ducted or plenum space, it is allowed by exception to pass through an "other space used for environmental air", compliant with the exception.
