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.
