| 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). |
CMG
|
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.