Unlocking the code

This article will analyze the NEC for wired systmes, and examine related International Building Code (IBC), NFPA, and standards that may influence the design. The areas of interest will primarily concern the specialty wired systems of data, telephone, and fiber optics that rely on limited amounts of the general code requirements.


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Read the second part in the series of articles from Tim Kuhlman, " Bonding conduit sleeves " that discusses where and how the NEC requires bonding and grounding for conduit sleeves.

This article will analyze the NEC for wired systmes, and examine related International Building Code (IBC), NFPA , and standards that may influence the design. The areas of interest will primarily concern the specialty wired systems of data, telephone, and fiber optics that rely on limited amounts of the general code requirements.


Cabling under a RAF: When not used to transport air

Raised access floor (RAF) systems have become a popular way to route cables in office, labs, call centers, and other areas that have a high density of data and communications cabling. The RAF system may allow for a 2- to 24-in. cavity that can be used to route utilities to the workstation. It allows the architect and building owner the flexibility to change the furniture plan and floor layout without the use of false columns and power/communication poles to feed the cubicles or open floor furniture systems.


In this review, we will consider the appropriate types of cabling to be used under a RAF system when the RAF is not being used to transport air. Throughout this review, this will be referred to as a static air RAF. The cables under the static air RAF will be considered to be exposed to the static air in the space created by the floor system. This review does not apply to applications such as data centers or office-type spaces where the RAF system is also used to transport air to equipment or a habitable space. When the RAF is used to transport air, the NEC is specific for the cabling methods in these types of spaces. On the other hand, the NEC does not specifically address the cabling type when the flooring system is not used for environmental air such as in our static air RAF.


Key issues

A key issue in determining the applicable code sections is to know the type of space the cable is being routed through. For this analysis we are only considering a static air RAF. It is also important to understand if the space below the RAF is considered accessible or a concealed space. RAF systems are assumed to have removable floor panels, which is is true for most manufacturers. The floor panels can be covered with carpet squares and they may be held in place with removable fasteners, such as screws, and still be considered removable. The removable panels allow the space to be treated as a nonconcealed space. Therefore, per the NEC , the cables are considered to be accessible.


Cabling in concealed spaces has additional code restrictions that affect the selection of allowable cable pathways and cable types. The NEC defines a concealed space as a space that is rendered inaccessible by the structure or finish of a building. (See Conditions, page 29) For example, the wall cavity created by framing studs with gypsum board on both sides of the wall studs would be considered inaccessible or a concealed space by the NEC. A door, hatch, or pull box would need to be installed in the wall cavity to allow it to be accessible. In the case of our RAF system, each floor tile could be considered an access way to the cable.


Deciphering the code

The data and telephone cables we are analyzing for placement in the RAF system are covered respectively in NEC Articles 725 , 770 , and 800 (Chapter 8). Each of these articles has paragraphs stating the limits in applying the general requirements of Chapters 1 to 4 of the code. Knowing the limiting requirements of these articles allows us to focus on the specific sections that apply to our cabling without having to take into account additional requirements that may apply to power circuits.


Per NEC 725.3 Other Articles : “Circuits and equipment shall comply with the articles or sections listed in 725.3(A) through (G). Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits.”


Per NEC 770.3 Other Articles: “Circuits and equipment shall comply with 770.3(A) and (B). Only those sections of Chapter 2 and Article 300 referenced in this article shall apply to optical fiber cables and raceways.”


Per NEC 90.3 Code Arrangement: “Chapter 8 covers communications systems and is not subject to the requirements of Chapters 1 through 7 except where the requirements are specifically referenced in Chapter 8.”


In any analysis of data, communications, and fiber optic cabling, these three paragraphs are the key in determining which other articles, sections, and paragraphs beyond the root articles apply. Refer to the NEC Reference Table for a comprehensive list applicable to code articles.


Not a plenum, nor a riser

The NEC defines a plenum as “a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” In our application, the floor is not being used to transport air and therefore is not considered a plenum or an “other space used for environmental air” as defined in NEC paragraphs 300.22 (B) and 300.22(C). The floor system is not a riser, which is described by the code as a vertical run penetrating more than one floor. In our static air RAF, the cable type is not a plenum or riser type. The remaining choice is defined under “Other Cabling/Wiring within a Building” in paragraphs 725.154(E), 800.154(C), and 770.154(C).


As we have noted, our static air RAF system is considered a nonconcealed space. Articles 725 and 800 have paragraphs addressing cabling for nonconcealed spaces. Although these do apply to our application with the static air RAF system, the allowances are not helpful. In the paragraphs for each article, the code allows for a cable with a lower listing classification to be installed for a maximum of 10 ft.


Additional considerations

It is reasonable to consider that the cable will have to enter and exit the static air RAF system. To paraphrase NEC paragraphs 770.26, 800.26, and 300.21 (as referenced by 725.3): Installations in hollow spaces shall be made so that the possible spread of fire or products of combustion will not be substantially increased. Openings around penetrations through fire-resistant-rated floor shall be fire stopped using approved methods to maintain the fire resistance rating.


The RAF system being installed will most likely be made of a fire-resistant product to comply with Chapter 7 of the IBC. The engineer or designer will need to consult with the architect to determine if the RAF system is part of a fire-rated assembly. This is necessary to determine if a cable opening only needs to be grommetted with a RAF-system-manufacturer-approved opening or if a rated fire-stopping assembly is required.


The use of cable tray under the RAF system is the designer's option. Since the RAF system provides the access means required for the use of cable tray, the use is allowed. However, the NEC does not require the use of cable tray or other support methods; the cable can be adequately supported by the underfloor structure. In the “Mechanical Execution of Work” paragraphs in 800.24, 770.24, and 725.24, the code does not identify the requirement to use cable tray under the RAF. For shallow floor heights of 4 in. or less, there may not be space for cable tray distribution, but in deeper floors there would be adequate space. Cable tray, such as basket-style tray, is an effective way to manage the cables under the RAF and to meet the code requirements for separation from other types of circuits. Even though the RAF support structure may be adequate to support the cabling to meet the code requirements, to follow good design practice the design would incorporate a tray system, J-hooks, cable saddles or other means of bundling the cables to manage them under the RAF. Paragraphs 725.139(E), 770.133(B), and 800.133(A) allow for the data and phone cables to share a common cable tray.



In a static air RAF system, when the underfloor space is not being used to transport air, for a typical office environment, a communication (CM)-rated copper cable and an optical fiber nonconductive (OFN) or optical fiber cable (OFC)-rated fiber optic cable would be appropriate. A Class 2 (CL2) cable can be used for Class 2 (Ethernet) circuits. However, with the CM-type cable being an acceptable substitute for CL2, CM-rated cable would be more common. Riser-rated cables such as communication riser (CMR), optical fiber nonconductive riser (OFNR), or optical fiber conductive riser (OFCR) may be more common and similar in cost to the general purpose rated cabling, and also acceptable substitutions (see Cable Types).


Installing plenum-rated cables under the floor, such as communication plenum (CMP), optical fiber nonconductive plenum (OFNP), or optical fiber conductive plenum (OFCP) cables, is an allowable substitution. Plenum-rated cables carry a premium cost. If the code does not require a plenum-rated cable and if the local authority having jurisdiction (AHJ) is not making a separate interpretation requiring a plenum-rated cable, then its use can be avoided.


The inappropriate use of plenum rated cable will increase construction cost and potentially lead to higher moves-adds and change cost through the life of the building space. Often a cabling contractor is hired to install additional cabling in an office space and they may not have access to the mechanical drawings or code review drawings. The cabling contractor will typically install the new cables to match the type of existing cabling. If the initial install had plenum-rated cables, the cables added later will most likely be plenum-rated and be an unnecessary increased cost.


If the cable tray path from the local communications room is through a drop ceiling space used for environmental air, then down a wall chase to the below floor, the cable may have to be plenum rated to transition the ceiling space.



Read the second part in the series of articles from Tim Kuhlman, " Bonding conduit sleeves " that discusses where and how the NEC requires bonding and grounding for conduit sleeves.

NEC reference table


Article 725 Class 2 data (Ethernet) Article 800 communications (telephone) Article 770 fiber optic cable
725.3: Compliance with other articles and sections 300.17 Cable fill 300.21 Spread of fire 300.22 Plenums 392 Cable tray90.3 Compliance with other articles and sections for Ch. 8 800.26 Spread of fire 300.22 Plenums 392 Not referenced770.3: Compliance with other articles and sections 770.26 Spread of fire 300.22 Plenums 392 Not referenced
725.24: Mechanical execution of work 300.4 (D) Cables parallel to framing members800.24 Mechanical execution of work770.24 Mechanical execution of work
300.4 (D) Cables parallel to framing members300.4 (D) Cables parallel to framing members
725.130 (B) Wiring methods for CL2 (Ethernet-data)800 Part V: Wire methods for communications770 Part V: Wiring methods for fiber optics
725.136 Separation from other circuit types800.133 (A) Separation from other circuit types770.133 (B) Separation from other circuit types
725.154 (C) Using cable tray800.154 (D) Using cable tray770.154 (D) Using cable tray
725.154 (E) Other wiring within building800.154 (C) Other wiring within building770.154 (C) Other cabling within building
725. 154 (E) (3) Nonconcealed spaces800.154 (C) (3) Nonconcealed spaces
725.154 (G) Cable substitutions800.154 (3) (E) Cable substitutions.770.154 (E) Cable substitutions
Author Information
Kuhlman has 20 years of experience in the design and construction of telecommunication infrastructure. He is a member of Consulting-Specifying Engineer's editorial advisory board.








%%POINT%% Data cable (Class 2 or Class 3 power limited), telephone cable, and fiber optic cable installed under a RAF.




%%POINT%% The RAF has removable panels.




%%POINT%% A static air RAF where the RAF is not being used to transport air.








CONCEALED: “Rendered inaccessible by the structure or finish of the building. Wires in concealed raceways are considered concealed, even though they may become accessible by withdrawing them.”—NEC 2008 Article 100 Definitions




PLENUM: “A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” —NEC 2008 Article 100




NEC root articles:




DATA CABLING—UTP Category 5e, 6, 6a common for Ethernet LANs: Article 725 for Class 2 & 3 circuits




COMMUNICATIONS CABLING—UTP Category 5e, 6, 6a common for telephone circuits: Article 800




FIBER OPTIC CABLING - Multimode and Singlemode fiber optic cabling: Article 770






Cable types



%%POINT%% Allowable telephone cable types per NEC 800.154(D): CM or communication general (CMG)




%%POINT%% Allowable copper data cable types per NEC 725.154 (E): CL2 or CL3. Per substitution table 725.154(G) cable types CM or CMG are allowed.




%%POINT%% Allowable fiber optic data cable types per NEC 770.154 (C): optical fiber nonconductive general purpose (OFNG or OFN), optical fiber conductive general-purpose (OFCG or OFC).




%%POINT%% Note: CM cable can be substituted for CL2 listed cable, but CL2 cable cannot be substituted for CM cable. Therefore, CM-rated cable commonly is used for both data (Ethernet) and communications (telephony).



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