300.22(C)(1) Other Spaces Used for Environmental Air (Plenums).
Code Change Summary: Clarifications were made regarding the use of mineral insulated type MI cable and plastic cable ties in an environmental air plenum.
Only certain wiring methods are permitted in a plenum. Plenums can exist in the space above a suspended ceiling where an air conditioner may be present. Here are two rules to determine if the space above a grid ceiling is a plenum:
- If the above grid AC unit pulls return air from the office space below, through a dummy grill, instead of return air ductwork, then the return air circulates above ceiling where wiring methods also exist. This is a plenum.
- If the above grid AC unit pulls return air from the office space below, through a ducted return air grill, then the return air does not circulate among the wiring methods above ceiling. This is NOT a plenum.
Wiring methods in a plenum are generally limited to metal cables, metal boxes, metal raceways and certain nonmetallic jacketed plenum rated cables with low smoke and heat release properties. Testing and listing of plenum rated wiring methods and equipment has nothing to do with the toxicity of the fumes released when something melts but has everything to do with visible smoke release and optical density levels. The idea is to make sure that if an above ceiling cable melts, the smoke released back into the return air, and recirculated back into the building supply air system isn't so dense that it blocks emergency lights and exit signs from being seen by building occupants during egress.
MI cables are now only allowed to be in a plenum if the MI cable does not have a nonmetallic covering. Before this change, any type of MI cable (including plastic sheathed) could be used in a plenum.
Also, cable ties used for cable support in a plenum are now required to be listed as having low smoke and heat release properties.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
300.22(C)(1) Wiring Methods. The wiring methods for such other space shall be limited to totally enclosed, nonventilated, insulated busway having no provisions for plug-in connections, Type MI cable, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory-assembled multiconductor control or power cable that is specifically listed for use within an air-handling space, or listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath. Other types of cables, conductors, and raceways shall be permitted to be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit without an overall nonmetallic covering, flexible metal conduit, or, where accessible, surface metal raceway or metal wireway with metal covers.
2014 Code Language:
300.22(C)(1) Wiring Methods. The wiring methods for such other space shall be limited to totally enclosed, nonventilated, insulated busway having no provisions for plug-in connections, Type MI cable without an overall nonmetallic covering, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory-assembled multiconductor control or power cable that is specifically listed for use within an airhandling space, or listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath. Other types of cables, conductors, and raceways shall be permitted to be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit without an overall nonmetallic covering, flexible metal conduit, or, where accessible, surface metal raceway or metal wireway with metal covers.
Nonmetallic cable ties and other nonmetallic cable accessories used to secure and support cables shall be listed as having low smoke and heat release properties.