Q. When installing wiring methods in ducts or plenums used for environmental air, all but which of the following wiring methods are permitted?

  1. Flexible metal conduit no greater than 4 feet in length
  2. EMT
  3. IMC
  4. MC cable with overall nonmetallic covering

Answer: D

Per 300.22(B), outer nonmetallic coverings are not permitted in this type of installation. This correlates with the requirements of 300.21, which focuses on the topic of Spread of Fire or Products of Combustion.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.