Communications Cables Revised: 08/22/99 18:25
NEC Flame Ratings Reason for Revision: Add header; editorial

Article 800 of the National Electrical Code covers requirements for low-voltage communications cables. Five levels of burn resistance are specified (plus one for undercarpet cable.) These are shown below from most stringent and restrictive to least. The NEC ratings are hierarchical, in other words, from the top down, a cable may be substituted for any cable lower in the table. For example, a CMP rated cable may substitute for a CMR (or any lower rated cable), but a CMR cable may not substitute for a CMP cable.

NEC Designation Common Term Test Comments
CMP Communications Plenum UL 910 Cable must have both resistance to flame spread and reduced smoke generating properties. These cables are approved for placement in air handling ducts and chambers (plenums) without the use of fire-proof conduit. The purpose of the rating is to lessen the transmission of fire and visible smoke to unaffected parts of the building. Toxicity is not measured.
CMR Communications Riser UL 1666 Cable must not transmit flame from one floor to another when placed vertically in a building shaft (called a riser shaft).


Communications General Purpose

CSA C22.2 No. 0.3-M (Vertical Tray)

Cable may not transmit flame for more than 4 ft, 11 in. It shall not penetrate floors or ceilings, i.e, may only be used within a single floor. This designation was added as a part of the harmonization efforts between U.S. and Canadian standards.

CM Communications General Purpose UL 1581 (Vertical Tray) Cable may not transmit flame for more than 4 ft, 11 in. It shall not penetrate floors or ceilings, i.e, may only be used within a single floor.
CMX Communications Limited Use UL 1581 VW-1 (Vertical Wire) Cable must meet the least stringent flame spread requirements* of all ratings. Can only be installed in one and two family (duplex) housing units. Often rated with optional UL requirements for Outdoor use**.

The NEC requires that cables used in Premises, both commercial and residential be "listed for the purpose" by a Nationally Recognized Test Laboratory, or NRTL (pronounced "nurtle"). Underwriters' Laboratories is the most common listing agency to the point where many people believe the NEC requires UL Listing. This isn't true. Other laboratories, such as ETL Testing Laboratories, can provide listing compliance with the NEC. However, UL 444 is the specification used to specify the requirements for the cables and the flame test procedures themselves are UL procedures. Most manufacturers will be UL Listed instead of, or in addition to, another laboratory.

*There are those who advocate using higher rated (and more expensive) cables in the home. They argue that you shouldn't risk your family with "wire which burns". In the first place, a CMX rated cable resists burning. In the second place, compared to everything else in your house that is flammable, e.g. wood studs, paneling, carpet, upholstery, wood siding, etc., how much would the phone wire contribute to the spread of a fire? It's different in a commercial installation where the quantity of wire (potential fuel) is greater and typical structural elements are not flammable. The NFPA is responsible for the safety guidelines of the NEC. Arguably, the U.S. has the best fire codes in the world. Trust the years of experience and data backing up the NFPA. Don't fall for poorly thought out, alarmist statements.

**These "Outdoor" requirements are limited to some cold temperature properties and UV resistance. They do not qualify a cable to be substituted for an Outside Plant Cable. For example, they have no protection against the intrusion of water, which destroys a cable's transmission properties and can physically degrade a cable as well. The "Outdoor" rating purpose is so the cable can be stapled to the side of your house for short runs from the Network Interface Unit until it passes through the wall to the interior.

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