Using Non-Plenum Cable Under a Raised Floor
And, Meeting Code

There are provisions in the National Electric Code that clearly permit the installation of non-plenum cable under a raised floor.  We have done this consistently on many of our projects including multiple 60,000 sq.ft. and 80,000 sq.ft. co-location data centers.  However, on many of our projects, we have been required to use plenum-rated cable.  It doesn't matter whether or not there are sprinklers under the floor.  Even though the National Electric Code permits non-plenum cable under a raised floor under certain circumstances, the final decision on which cable can be used is up to the Fire Marshall signing off on your project.  In some cities and some states, plenum-rated cable is required no matter what you do.  We suggest that either your organization or your General Contractor on behalf of your organization contact the Fire Marshall for your project before completing a cabling RFP and seek written permission to use non-plenum communications cable under the raised floor.  In pursuing this alternative, note three important circumstances:

1.   This article is based on established national standards set forth by the National Fire Protection Association (NFPA) of which the National Electric Code (NFPA 70) is one of many NFPA publications.  
2.   Most cities and municipalities adopt the NFPA codes as their own without revision.  However, some large cities expand upon or modify these national codes.  Make sure you are aware of how your municipality treats these codes.  
3.   In all situations, regardless of code or interpretation of code, the Fire Marshall makes the final decision.    

The remainder of this document describes the code and standards documents that classify communications cable, permit the non-plenum cable under a raised floor system and have overlapping jurisdiction in the raised floor area.  These documents are as follows:  

  1. Underwriters Laboratories 444 - Standard for Safety for Communications Cable
  2. Underwriters Laboratories 910 - Standard for Safety for Test for Flame-Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air
  3. Underwriters Laboratories 1581 - Reference Standard for Electrical Wires, Cables, and Flexible Cords
  4. Underwriters Laboratories 1666 - Standard for Safety for Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shafts
  5. NFPA 13 - Installation of Sprinkler Systems
  6. NFPA 70 - National Electric Code (NEC)  

If you are serious about cable design, we strongly suggest that these publications be included in your technical library.  They can be purchased at the following web sites:  

  1. Underwriters Laboratories - www.comm-2000.com  (Don't forget the hyphen or you'll end up in Italy)  You can also call (800)-704-4050.  
  2. National Fire Protection Association - www.nfpa.org  (Select Online Catalog under Codes and Standards)


The specific document on which we would use to show compliance in using non-plenum (PVC) cable under a raised floor is Article 645-2 of the National Electric Code.  In summary, Article 645-2 describes certain HVAC and disconnect features that must be included in your computer room design in order for the Article to apply.  Usually, a discussion of Article 645-2 or a quick read-through of the Article by the Fire Marshall will yield a "yea" or "nea" decision and no more discussion is needed.  However, if you wish to understand how the codes and standards relate and support Article 645-2 or the Fire Marshall requires additional supporting information, the remainder of this article provides an in-depth discussion of the codes and standards that inter-relate in permitting non-plenum cable to be installed under a raised floor computer room.  Read on.  

Defining the UL Documents 

Despite the long and confusing document titles above, the explanations are simple.  

UL 444 is the defining document by Underwriters Laboratories (UL) for communications cables.  It is in this standard that the categories CM (non-plenum, non-riser), CMR (riser-rated) and CMP (plenum-rated) originate as cable types and cable markings.  These categories are commonly used in many other diversified documents such as Article 800 of the NFPA 70 (NEC) and are printed on communications cables.  Note that other categories were also defined in UL 444.  These include MP (multi-purpose cable), MPR (multi-purpose riser cable), MPP (multi-purpose plenum cable), CMX (communications cable, limited use), CL1 and CL2.  CL1 and CL2 are often associated with signal and alarm cable.  The remainder are not used in normal communications cabling.   

UL 1581 applies to non-plenum, non-riser cable. 
UL 1666 applies to riser-rated cable.  
UL 910 (also known as the modified Steiner Tunnel Test) applies to plenum-rated cable.  

If you look in a communications cable project catalog and look closely at cable specifications, you will see "meets UL 1581", "meets UL 1666" or "meets UL 910".  

Defining the National Electric Code (NFPA 70) in Relation to Communications Cable

The National Electric Code (NFPA 70) is the governing document for the installation of electrical cable in all situations.  Communications cable used for telecommunications applications is, in actuality, low voltage electrical cable.  Thus, the installation of communications cable is governed by the National Electric Code.  

NEC Article 800

Many codes and specifications refer to Article 800 of the NEC.  Article 800 is entitled "Communication Circuits" and generally covers the types of communications cables and their uses.  Communications cables includes telephone, telegraph (except radio), outside wiring for fire alarm and burglar alarm, CATV and similar central station systems.  The essential classifications and uses for telecom communications cable described in this Article are distilled above under "Defining the UL Documents".  The NEC uses the classifications defined by Underwriters Laboratories.  Our particular interest is with Article 800-53 which is entitled "Applications of Listed Communications Wires and Cables".  Section (a) of Article 800-53 is entitled "Plenum" and we quote from the NEC:  Cables installed in ducts, plenums, and other spaces used for environmental air shall be Type CMP".  This instruction fully supports the intended use described in UL 444.  However, Article 800-53(a) has an "Exception":  "Types CMP, CMR, CMG, CM and CMS and communications wire installed in compliance with Article 300-22".  Our first hint at being able to use non-plenum cable.  We now go there to detail its instructions.  

NEC Article 300-22

Article 300-22 covers cabling in Ducts, Plenums and Other Air-Handling Spaces.  Under Article 300-22(d), it specifically states "Electric wiring in air-handling areas beneath raised floors for data processing systems shall comply with Article 645".  Note:  The word SHALL according to 90-5 of the National Electric Code, means "MANDATORY".  We now go Article 645.  We're getting closer.  

NEC Article 645

Article 645 is entitled "Electronic Computer/Data Processing Equipment".  This Article of the NEC covers the interconnecting wiring for power supply equipment, equipment and the grounding of electronic computer/data processing and systems in an electronic computer/data processing room.  Essentially, this means all computer, server, network, PBX and other electronic equipment in a computer/server/network/PBX room and the peripherals directly connected to these systems.  By extension, this would include all interconnecting voice and data cables within the room.  The following sub-paragraphs are direct quotes from Article 645 of the National Electric Code (1993 Edition).  Our comments are enclosed separately in parentheses.  

Strong Caution
:  We place the NEC quotes here for general information purposes only in support of our explanation for using PVC communications cable under a raised floor.  We do not guarantee typing or interpretation accuracy.  Only the actual National Electric Code should be accepted as the correct version of these writings.  Our comments are our own and may differ from actual circumstances in your community.  Only the Fire Marshall and/or building inspector in your community can advise you if all of the circumstances detailed below apply to your project.  

  • Article 645-2 - Special Requirements for Electronic Computer/Data Processing Equipment Room

Article 645-2 states and we quote from the NEC: "This Article applies provided all the following conditions are met".  

(1)  Disconnecting means complying with Section 645-10 are provided.  (ABR - We are sub-indenting 645-10 at this point in the discussion for convenience).

645-10 - Disconnecting Means.  A means shall be provided to disconnect power to all electronic equipment in the electronic computer/data processing equipment room.  There shall also be a similar means to disconnect the power to all dedicated HVAC systems serving the room and cause all required fire/smoke dampers to close.  The control for these disconnecting means shall be grouped and identified and shall be readily accessible at the principal exit doors.  A single means to control both the electronic equipment and the HVAC systems shall be permitted.  

(ABR - In many communities this means that your fire detection system is set up such that all air units inside the computer room shut down on first alarm.  All smoke dampers on ducts entering the room from other areas close on first alarm.  All air units shut down on first alarm.    

 (2)  A separate heating/ventilating/air-conditioning (HVAC) system is provided that is dedicated for electronic computer/data processing equipment use and is separated from other areas of occupancy.  Any HVAC system that serves other occupancies may also serve the electronic computer/data processing equipment room if fire/smoke dampers are provided at the point of penetration of the room boundary.  Such dampers shall operate on activation of smoke detectors and also by operation of the disconnect means required by Section 645-10.  

(3)  Listed electronic computer/data processing equipment is installed.  

(4)  Occupied only by those personnel needed for the maintenance and functional operation of the electronic computer/data processing equipment.  

NEC Fine Print Note - The computer room is not to be used for the storage of combustibles beyond that necessary for the day-to-day operation of the equipment.  For further information, see Protection of Electronic Computer/Data Processing Equipment, NFPA 75-1992 (ANSI).  

(ABR Note: a "Fine Print Note" in the NEC is not mandatory code.  According to Article 90-5, "Explanatory material is in the form of Fine Print Notes").  

(5)  The room is separated from other occupancies by fire-resistant-rated walls, floors and ceilings with protected openings.  

(6)  The building construction, rooms, or areas and occupancy comply with the applicable building code.  

  • Article 645-5 (d)(5) - Under Raised Floors

Article 645-5 describes the circumstances under which power cables, communications cables, connecting cables, interconnecting cables and receptacles associated with the data processing equipment shall be permitted under a raised floor.  

Subparagraph 645-2(d)(2) describes how to install several types of cables, none of which are telecom communications cables. 

Subparagraph 645-5(d)(5) states, and we quote from the NEC: "Cables, other than those covered in (2) above shall be listed as Type DP cable having adequate fire-resistance characteristics suitable for use under raised floor of a computer room.  This listing requirement shall become effective July 1, 1994".  (ABR Note:  The classification DP is not in UL 444.  DP cable is much less robust than today's telecom communications cable).  

Subparagraph 645-2(d)(5) has an exception which again permits the use of non-plenum cable (as well as riser rated and plenum cable).  .  

  • Exception No. 3 under the same subparagraph states that "Other cable type designations that satisfy the above requirement are Type TC (Article 340); Types CL2, CL3 and PLTC (Article 725); Types OFC & OFN (Artcle 770); Types CM and MP (Article 800); Type CATV (Article 820).  These designations shall be permitted to have an additional letter P or R". 

  • Summary

    It is clear from the explanations above that non-plenum cable is permitted under a raised floor if certain conditions are met.  And, these conditions are usually met on most computer room projects.  This code compliance only refers to communications cable inside the computer room.  This code compliance does not apply to communications cable entering the computer room from areas outside the computer.  They can be either non-plenum or plenum depending on circumstances outside the computer/server room.  And, don't forget to firestop any cable or cable conduit penetrations coming into the computer/server room.  We recommend firestopping any telecom penetrations into the computer room regardless of the fire rating of the wall.  

    That's it.  That's how to do it.  It may be a bit difficult to read and comprehend here but if you are in possession of all of the documents listed above, it will be much more clear and understandable.  We strongly recommend the acquisition of these documents.  The web sites where you can purchase the codes and standards are listed above just before the discussion.  

    Wrapping Up the Discussion 

    The discussion above normally will resolve all issues involved in identifying those pertinent areas of code that permit the use and installation of non-plenum cable under a raised floor in a computer room.  However, there are two additional circumstances that rarely cause a problem and they are explained below.  

    • Defining NFPA 13 in Relation to a Raised Floor Computer Room 

      Normally, NFPA 13 is not involved in this issue.  However, on occasion, we have difficulties in getting non-plenum cable approved as the Fire Marshall or inspectors will refer to NFPA 13 and that the underfloor of a raised floor system is a concealed space or that the underfloor is a plenum.  So, we deal with it here.  

      In reality, both definitions are true.  The underfloor of a raised floor system is both a concealed space and a plenum.  However, the National Electric Code approves non-plenum cable in precisely this area if certain conditions are met.  These conditions were covered earlier in the discussion portion of this document.  For now, our opinion about NFPA 13 and the National Electric Code.  

      NFPA 13 describes the installation of sprinkler systems.  Aside from the signal cable associated with the sprinkler system itself, NFPA 13 does not discuss the National Electric Code (NFPA 70) nor does it mention communications cabling anywhere in the document.  Likewise, there is no reference to NFPA 13 in the National Electric Code in the areas that deal with communications cable.  We have not seen any linkage between the two standards for the installation of communications cable nor have we seen either standard contain subordinating language to the other with regards to communications cable. 

      Our copy of NFPA 13 does not contain a definition of "concealed space" although it does use the term in Section 4-13.1.1 where the following is stated:  "All concealed spaces enclosed wholly or partly by exposed combustible construction shall be protected by sprinklers".  We define this paragraph as defining whether or not you need sprinklers and not having anything to do whatever with communications cables.  Further, a typical raised floor system is not constructed with exposed combustible materials.  The only material that comes close is gypsum walls that border the computer room and that appear below the raised floor and, that wall is usually fire-rated.  
    • Using UL-910 to Require Plenum-Rated Cable

      On even rarer occasions, UL-910 is referred to as the controlling document for what kind of cable is to be installed under a raised floor of a computer room and that plenum-rated cable is always installed in a plenum.  This is generally true.  However, not in the case of placing cable under the raised floor of a computer room.  We are strongly supported here by two circumstances:  

      (1) The Underwriters Laboratories is a testing agency and only tests the cable for fire and safety; it has no controlling role in final cable installation.  UL must defer to the National Electric Code which is the national standard for the installation of electrical conductors and cables.  

      (2) The opening words of UL-910 are: "Cables that are intended for installation in accordance with Section 800-53(a) of the National Electric Code...".  The sentence goes on to describe the requirement to meet UL 910, but the operative words are "in accordance with Section 800-53(a) of the National Electric Code.  UL-910 does indeed defer to the National Electric Code.  Under "NEC Article 800" above, we clearly list Section 800-53(a) as our beginning point.  

    Summary: We have gone full circle through the UL applicable standards and the National Electric Code and we still end up at Section 800-53(a) and Article 645-2.

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