Although the article about conduit and innerduct (see "Conduit and innerduct increase cable system capacity," March 1997, page 6) was informative, the use of plenum-rated innerduct and associated cable-rating requirements of the National Electrical Code (nec) should be clarified.
Plenum-rated innerduct is not a replacement for metallic conduit in plenum areas. According to the nec, cable installed in ducts, plenums, and other spaces used for environmental air shall be rated type cmp (communications plenum cable) for copper cable, and types ofnp (optical-fiber nonconducting plenum) or ofcp (optical-fiber conducting plenum) for optical fiber. These cable ratings are required even if the cables are in a plenum-rated innerduct.
The nec allows exceptions to these requirements if the cables are installed in compliance with Section 300-22(c). The code states, "Other types of cables and conductors shall be installed in electrical metallic tubing, intermediate metallic conduit, rigid-metal conduit, flexible-metal conduit, or, where accessible, surface-metal raceway or wireway with metal covers or solid-bottom-metal cable tray with solid-metal covers." Because plenum-rated innerduct does not meet these requirements, it cannot be used as a substitute for metallic conduit or enclosed wireways.
Steven C. Goldfarb, rcdd
Kling Lindquist
Philadelphia, PA