Cabling below a raised floor

Q: When you are installing communications cabling below a raised floor in a room that doesn`t meet the requirements of an Information Technology Processing Room (as defined in Article 645 [nfpa-70] of the National Electrical Code [nec]), does the cable have to be type cmp or mpp, and does it have to be enclosed in conduit as outlined in Section 300-22(b)? My confusion stems from Section 800-53(a), which states: "Cable installed in ducts, plenums, and other spaces used for environmental air shall

Q: When you are installing communications cabling below a raised floor in a room that doesn`t meet the requirements of an Information Technology Processing Room (as defined in Article 645 [nfpa-70] of the National Electrical Code [nec]), does the cable have to be type cmp or mpp, and does it have to be enclosed in conduit as outlined in Section 300-22(b)? My confusion stems from Section 800-53(a), which states: "Cable installed in ducts, plenums, and other spaces used for environmental air shall be type cmp." However, the exception lists several types of cables allowed in plenums that must be installed in compliance with Section 300-22, and type cmp is listed as one of those types of cables. Also, Figure 800-2 depicts type mpp and cmp cable in a plenum, but not in conduit. Is type cmp cable in a plenum required to be installed in accordance with Section 300-22?

Jesse Tolliver

asaf Air Intelligence Agency/QA

Kelly afb, San Antonio, TX

A: No, nfpa-70 does not require that cmp cable be installed in conduit in a plenum space, but it does not prohibit installation of cmp cable in conduit in a plenum space either. That is why cmp appears in both the requirement and the exception. The following citations should clear up any confusion.

Section 800-50 of the nec, "Listing, Marking, and Installation of Communications Wires and Cables," states: "Communications wires and cables installed as wiring within buildings shall be listed as being suitable for the purpose."

Section 800-53(a), "Applications of Listed Communications Wires and Cables, (a) Plenum," states: "Cables installed in ducts, plenums, and other spaces used for environmental air shall be Type cmp. Exception: Types cmp, cmr, cmg, CM, and cmx and communications wire installed in compliance with Section 300-22."

Section 300-22(c), "Other Space Used for Environmental Air," states: "Other type cables and conductors shall be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit, flexible metal conduit, or where accessible, surface metal raceway or metal wireway with metal covers or solid bottom metal cable tray with solid metal covers."

Note that the listing of type cmp in the exception to 800-53(a) is not a typographical error. There are parts of the country that require all telecommunications cable to be installed in conduit. This wording allows for those folks to use cmp or mpp cable in those conduits.

Donna Ballast is a communications analyst at the University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: ballast@utexas.edu.

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