I'm looking to wire up the second floor of my house and the cold air return provides a convenient, straight run from the office to the utility room. Is it safe and within code to run ethernet cable in the air return?
The air returns are not metal ducts, they are just open cavities in the studs.
Scope of this question is purely limited to running network cable through an air return, decisions on wired/wireless and the various wired options have already been made.
Looking in 2011 National Electric Code Changes of Interest, they reference NFPA 90A-2009 :
Section 4.3.4 of NFPA 90A-2009 states that “Wiring shall not be installed in air ducts” unless it is “directly associated with the air distribution system and does not exceed 1.22m (4 ft).”
I don't have access to the full code, but just in the notes on the revisions to NEC 2011, they seem to allow CMP (plenum rated cable) in air ducts or "other spaces used for environmental air" based on the table on page 15, 'Applications of Communications Wires and Cables'. All others must be in a raceway ... other than CMUC (under carpet) or cables w/ power, which are both banned completely.
I'd personally ask a local inspector -- they're the ones who are going to know what the local interpretation of the codes are. If you're running something that's not required to be inspected, I'd only run plenum rated cable -- the jacket won't give off toxic fumes if burned, so you'll have a chance at escaping a fire without being poisoned first.
National Electrical Code 2008
300.22 Wiring in Ducts, Plenums, and Other Air-Handling Spaces.
(B) Ducts or Plenums Used for Environmental Air. Only wiring methods consisting of Type MI cable, Type MC cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering shall be installed in ducts or plenums specifically fabricated to transport environmental air. Flexible metal conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to connect physically adjustable equipment and devices permitted to be in these ducts and plenum chambers. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for their direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type luminaires shall be permitted.
Which means, NO. The only wiring allowed in ducts, is wiring that is "necessary for their direct action upon, or sensing of, the contained air.".
Mike Holt explains it well in this YouTube video.
If you read article 725, you'll see that it references back to section 300.22.
Yes, it is within code to run a low voltage wire inside an air return (aka plenum) in the US. You'll even find cable that is "plenum rated" -- which means it has a fire-resistant jacket on it -- that is specifically meant for use in plenums where, any fire could quickly become life-threatening with toxic fumes, and it's less likely to encounter sharp edges or be overly stretched during pulls. (I'd still use normal cable, though, because pulling plenum cable, even through a plenum, is difficult unless you pull it in a large bundle with proper knowledge.)
Source: NEC 2008: Chapter 7, Article 725
