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
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
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:
- Underwriters Laboratories 444 - Standard for Safety
for Communications Cable
- 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
- Underwriters Laboratories 1581 - Reference Standard
for Electrical Wires, Cables, and Flexible Cords
- Underwriters Laboratories 1666 - Standard for
Safety for Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in
- NFPA 13 - Installation of Sprinkler Systems
- 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:
- 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.
- 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
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
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
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
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
(1) Disconnecting means complying with Section 645-10 are
provided. (ABR - We are sub-indenting 645-10 at this point in the discussion
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
(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
(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
(3) Listed electronic computer/data processing equipment is
(4) Occupied only by those personnel needed for the maintenance and
functional operation of the electronic computer/data processing
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
(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
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
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".
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
- 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
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
(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
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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