ECN Forum Forums Code Related Discussion NEC & other Code issues Cold air returns

Article 300 section 300.22(C) Exception talks about the wiring in cold air returns as being exempt from complying with the requirement of metallic raceway (short version). I wonder if I understand this correctly in that if NM cable passes through the joist space perpendicular to the air flow, it's code compliant and doesn't need any other consideration. Now, if we decide to install a switch box or a receptacle opening in the CA, we would need a metallic wiring method.

Yep
That applies to Space used for Environmental Air(300.22c) only. Not Ducts or Plenums(300.22b) as you stated.2002 Handbook NFPA shows a picture example.

By careful though, even if our code does allow it, the local building or fire code may not. I would check with the local AHJ on this one. You would hat to have all the work rough-in and then have to change it later on.

Recently, I was working alongside an HVAC guy, and I saw an interesting practice. To make a duct, he simply sheathed the open face of a rafter bay with sheet metal, and sealed the edges. This way, the subfloor above, and the two rafters, formed three sides of the duct.
I believe that this is what the code writers had in mind whan the exception was written into the code- as opposed to, say, someone actually punching holes in a metal duct in order to run their wires.
In other words, I suspect the exception is there for the convenience of the HVAC guy, rather than the electrician- though it also recognises that such a duct would be impossible to "go around."

And I agree with you Reno, Where it get's controversial is when the installer installs outlet boxes or switch boxes in these spaces. I've asked them about that and they offer to install a barrier around the box and thus divorcing the box from the cold air return. Now the mechanical contractor gets upset because the electrical restricts the air flow. You can't win sometimes.