Code Change Summary: A new exception was added to address wiring methods allowed in ducts specifically fabricated for environmental air-handling purposes.

In NEC^® 300.22(B), wiring methods are only permitted within ductwork (containing environmental air) if necessary for the direct action upon, or sensing of, the air contained inside the duct. Duct detectors (ductwork smoke detectors) are one example.

When installing something that requires wiring inside the ductwork, the NEC^® permits the following metallic wiring methods:

• Type MI (mineral insulated) cable without an overall nonmetallic covering.
• Type MC (metal clad) cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering.
• EMT (electrical metallic tubing).
• Flexible metallic tubing.
• IMC (intermediate metal conduit).
• RMC (rigid metal conduit) without an overall nonmetallic covering.

Using metallic wiring methods and raceways inside a duct ensures that if the wiring melts, the smoke generated from the melting wire is contained within the raceway so as not to be released back into the building air supply system. Too much smoke coming out of the air vents may block emergency lights and exit signs from being seen by building occupants during egress.

In the 2017 NEC^®, a new exception was added to allow wiring methods and cabling systems, listed for use in “other spaces used for environmental air” (plenums). These types of listed, plenum-rated, nonmetallic cables are readily available and have been tested to have low smoke and heat release properties. In order to use the new exception, the wiring must only serve equipment associated with the contained air and the total length of the wiring inside the duct cannot exceed 4 feet.

Before the code change, Chapter 3 did not allow plenum rated cables inside environmental air ductwork but they were allowed in return air plenums which commonly exist above some suspended grid ceilings. This confusion was due to code language in 300.22(B) conflicting with code language for signal circuits in 725.135(B) as well as NFPA 90A Standard for the Installation of Air-Conditioning and Ventilating Systems and NFPA 262 Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces. In the 2017 NEC^®, the new exception was added to get the NEC^® on the same page as the other two NFPA standards mentioned above.

Below is a preview of Article 300. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.