NEC cabling installation requirements are straightforward, but markings and new offerings may create confusion.


You're not alone if markings and listings are making your plenum cabling selection a confusing task. Fear not, help is out there. The requirements of Art. 800 in the NEC may provide guidance in the selection and proper installation of communications circuits, but what about new plenum cabling product offerings? Can you trust manufacturer claims of performance?

Learning the communications cable marking game.

NEC Table 800.50 (Cable Markings) lists the various types of communications cables, along with their respective cable markings and NEC section reference (Table 1). Of these cables, only the top two are listed as suitable for use in ducts, plenums, and other environmental air-handling spaces.

This table lists cable types in descending order of fire resistance rating. In other words, multipurpose plenum cable (MPP) and communications plenum cable (CMP) have the highest fire resistance rating while communications cable and limited-use (CMX) and undercarpet communications wire and cable (CMUC) have the lowest.

The Fine Print Note in 800.51(A) describes one method manufacturers can use to establish the performance of their low-smoke producing cables, citing NFPA 262-1999, “Standard Method of Test for Flame Travel and Smoke of Wire and Cables for Use in Air-Handling Spaces.”

Table 800.50 lists multipurpose cables above communications cables because you can substitute multipurpose cables for communications cables. NEC Table 800.53 lists communications cable types and their respective permitted substitution (Table 2). You'll also find an easy-to-use substitution hierarchy in NEC Figure 800.53 (Fig. 1).

You may wonder why, except under conditions noted in NEC Sec. 800-50, communications wires and cables don't have voltage-rating markings on their jackets. The restriction is in place to prevent any installers from mistakenly using these cables in Class 1, electric light and power applications.

What about optical fiber cabling? The requirements of 770.50 state that you must use optical fiber cables listed as suitable for their respective purpose. Table 770.50 expands this requirement by providing specific markings for each cable type (Table 3 at right).

As with the guide for communications cabling, this table lists optical fiber cable types in descending order of fire resistance rating. That is, nonconductive optical fiber plenum cable (OFNP) and conductive optical fiber plenum cable (OFCP) have the highest fire resistance rating, while nonconductive optical fiber general-purpose cable (OFN) and conductive optical fiber general-purpose cable (OFC) have the lowest.

Again, the NEC cites NFPA 262-1999 as one method manufacturers can use to establish the performance of their low-smoke producing optical fiber cables.

Note that Table 770.50 lists nonconductive optical fiber cables above conductive cables; you can substitute the former cables for the latter. NEC Table 770.53 lists optical fiber cable types and their respective permitted substitution (Table 4 above). You'll also find an easy-to-use substitution hierarchy in Figure 770.53 (Fig. 2 below).

What cable can you use, and where can you use it? These are common questions for many datacom system designers and installers. The NEC notes the various types of communications and optical fiber cables and provides guidance in cable selections for specific applications. You need to read it very carefully, however, and try to match the cable callouts with the stated approvals.

To help you get through this quagmire, Tables 5 and 6 above provide an installation approval matrix for communications cables and optical fiber cables.

Air-handling spaces and plenums

The difference between these two systems' components can be difficult to spot. The requirements of Art. 100 state a plenum is “a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” Sec. 300.22 expands this definition by clarifying that a plenum is an air-handling space that is used only for environmental air. This section also points out the three types of air-handling spaces:

  • Ducts used for dust, loose stock, or vapor removal.

  • Ducts or plenums used for environmental air.

  • Other space used for environmental air.

As an example of the last type of air-handling space, Sec. 300.22 cites the space above a hung ceiling that is used as a return air plenum.

Although the NEC defines the first example as an air-handling space, it's not suitable for supporting any type of wiring system.

You do have alternatives for use in the second classification of air-handling space: MPP and CMP communications cables, MI cable, MC cable with smooth or corrugated impervious metal sheath but without an overall nonmetallic covering, EMT, flex, intermediate metal conduit, or rigid metallic conduit are all acceptable in plenums used for environmental air.

The NEC also permits AC cable and other factory assembled multiconductor control or power cable specifically listed for the use. In the third example of air-handling space, you can also use listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath.

What about new products?

Limited-combustible plenum cables, which are relatively new to the industry, are listed by UL as CMP-50. These cables generate as much as 1,700% less smoke than conventional CMP combustible plenum cables, according to a paper titled, “Fire Sale — New ‘Limited Combustible’ Plenum Cables” by Jim Hoover of DuPont Specialty Polymers. The paper also states that these new cables have a fire load (potential heat) that can be 300% to 400% lower than conventional CMP cables.

The reported performance characteristics are noteworthy because the industry has seen the annual growth of new datacom cable installations often exceed 25% per year. With this growth comes a continuing accumulation of combustible cables in plenums and other environmental air-handling spaces. More importantly, the industry has noted a number of devastating fires involving combustibles in concealed spaces in buildings, which have resulted in an increased concern among inspectors about cable installations in plenums. It's also the reason the NEC added numerous references to the 2002 version of the Code on the removal of abandoned communications cables.

NFPA 90A, “Standard for the Installation of Air Conditioning and Ventilating Systems,” sets mandatory performance requirements for construction products used to build plenum spaces. They include flame spread, smoke, fire load, environmental aging, and additional fire testing after aging and slitting. According to Hoover's paper, CMP-50 cables must meet all of these performance requirements, unlike CMP cables. Table 7 above notes the difference in performance requirements between both types of plenum cables.

The recent proliferation of cabling installations in data centers has placed a new emphasis on the importance of finding safe enclosed spaces for communications cabling. Air-handling spaces and plenums are the obvious choice, but the NEC is specific about which types are acceptable and the location of specific types of cabling. Pay close attention to Art. 800 before tackling such an installation.