Note: I have received numerous faxes and e-mail messages regarding a statement on plenum cabling in the October 1995 "Ask Donna" column (page 49). The most informative of these follow. Thanks to Noel Williams for his expert guidance through the code maze surrounding this issue.
Nothing in Section 300-22 of the 1993 National Electrical Code prohibits the installation of an electrical outlet in a plenum ceiling or in a raised floor used as a plenum. According to 300-22 (c) and the note in that section, plenum ceilings and the raised floors referred to by the questioner are classified as "Other Space Used for Environmental Air." The wiring methods employed in such spaces are limited to the listed cable types and metallic tubing and conduit. There is no mention of a restriction on electrical outlets.
Many types of outlets, including those for typical lighting fixtures, are routinely installed in raised floors and ceilings. Receptacle outlets are not prohibited either, although the lack of an acceptable cord type usually makes receptacle outlets in plenums of little use. (The term "outlet" is defined in Article 100. Receptacle outlets and lighting outlets are two kinds of outlets.)
All of this ignores Section 300-22 (d), which allows Article 645 to govern the wiring installed in raised floors in computer rooms. As indicated in the July 1995 "Ask Donna" column (page 55), these rooms must comply with a number of requirements to apply the special rules of Article 645. If the "ifs" of Article 645 are met, non-plenum-rated cords may be used and connected to receptacles installed in the raised floor. Sections 645-5 (b), 645-5 (d) and 400-7 (a) permit flexible cords for connections from branch circuits to computer equipment in data-processing rooms. Section 645-5 (d) (2) even refers to receptacles located in the raised-floor area.
The questioner`s problem results from misreading 645-5 (d) (5) and Exception No. 3. There is no flexible cord type listed as Type DP, and none of the other acceptable types of cable listed in Exception No. 3 are listed as flexible cords. However, Type DP is not a plenum-rated cable. As explained in the note, the permitted cables are fire-resistant types that may not meet the low-smoke requirement of plenum-rated cables. The additional letter R (riser) or P (plenum) is permitted, but not required. If a local building official chooses to follow the information in the note, a fire-resistant type of flexible cord could be used.
In short, the NEC does not require plenum-rated power cords or cables in raised floors of computer rooms to meet the requirements of Article 645. Many types of power cables are suitable for use in plenums, some of which are listed in Section 300-22 (c). I believe the reason plenum-rated power cords are not available is that there is no provision for their use in the NEC. Section 400-8 prohibits the use of cords in ceiling or floor spaces except as permitted by Section 400-7, and 400-7 permits such use only as permitted in 645-5; 645-5, in turn, does not require plenum-rated wiring methods.
Noel Williams
ESCO Electrical
Salt Lake City, UT