We fish Romex (type NM nonmetallic cable) through hvac duct chases all the time without any objections from our electrical inspector. As long as there are no sharp edges in the chase that might physically damage the cable, there’s no code violation if Romex touches metal hvac supply or return ducts. Romex can also come in incidental contact with a masonry chimney, so it’s okay to fish it through a masonry chimney chase too, but if an access to the chase is ever provided in the future, all of the wires would then need to be secured. Metal chimneys and other combustion vents are a different matter, however, since they have necessary clearances that must be maintained. We leave these chases alone and find other routes for our Romex.
Romex isn’t normally run through the ductwork itself; if something like a smoke detector or damper needs to be wired inside a duct, a metal wiring system as specified in NEC 300.22 is required. On the other hand, it’s common practice to run Romex perpendicularly across a panned stud cavity or joist bay used as a cold air return duct. In such a case, we drill into the framing on both sides of the panned return, pull the cable through the holes, then seal the holes around the Romex with caulk.
Posted with permission from Hanley Wood, LLC. Originally posted on JLConline.com by Harlen Madsen on August 1, 2010 at http://www.jlconline.com/electrical-codes/q-a–duct-chases-and-electrical-wiring.aspx