Plenum cable
Plenum cable is cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

 that is laid in the plenum spaces of buildings. The plenum is the space that can facilitate air circulation for heating and air conditioning
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 systems, by providing pathways for either heated/conditioned or return airflows. Space between the structural ceiling and the dropped ceiling
Dropped ceiling
A dropped ceiling is a secondary ceiling, hung below the main ceiling. They may also be referred to as a drop ceiling, false ceiling, or suspended ceiling, and are a staple of modern construction and architecture. The area above the dropped ceiling is called the plenum space, as it is sometimes...

 or under a raised floor
Raised floor
A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...

 is typically considered plenum; however, some drop-ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space. The plenum space is typically used to house the communication cables for the building's computer
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

 and telephone network
Telephone network
A telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...

; however, it has been proposed that the growing abandonment of cable in plenum spaces may pose a serious hazard in the event of a fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....

, as once the fire reaches the plenum space the airflow present in the space supplies fresh oxygen to the flame and makes it grow much stronger than it would have otherwise been. Recent testing by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has shown that, while flame spread
Flame spread
Flame spread or surface burning characteristics rating is a ranking derived by laboratory standard test methodology of a material's propensity to burn rapidly and spread flames...

 is limited by accumulated cable bundles, other structural concerns may still exist due to increased load on suspended components. As plenum spaces are restricted from use as areas for storage, the principle behind removal of abandoned cable is that regulated removal prevents the use of plenum spaces as a storage area for abandoned cable. In addition, no high-voltage powered equipment is allowed in the plenum space because presence of fresh air can greatly increase danger of rapid flame spreading should the equipment catch on fire.

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

s used in the construction of plenum cable are regulated under the National Fire Protection Association
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...

 standard NFPA 90A: Standard for the Installation of Air Conditioning and Ventilating Systems. Plenum cable is jacketed with a fire-retardant
Fire retardant
A fire retardant is a substance other than water that reduces flammability of fuels or delays their combustion. This typically refers to chemical retardants but may also include substances that work by physical action, such as cooling the fuels; examples of these include fire-fighting foams and...

 plastic jacket of either a low-smoke polyvinyl chloride
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

 (PVC) {patented 1987} or a fluorinated ethylene polymer (FEP
Fluorinated ethylene propylene
Fluorinated ethylene propylene or FEP is a copolymer of hexafluoropropylene and tetrafluoroethylene. It differs from the PTFE resins in that it is melt-processible using conventional injection molding and screw extrusion techniques. Fluorinated ethylene propylene was invented by DuPont and is sold...

). Polyolefin
A polyolefin is a polymer produced from a simple olefin as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An equivalent term is polyalkene; this is a more modern term, although polyolefin is still used in the petrochemical industry...

 formulations, specifically based on polyethylene compounding had been developed by at least two companies in the early to mid-1990s; however, these were never commercialized and development efforts continue in these yet-untapped product potentials. Development efforts on a non-halogen plenum compound were announced in 2007 citing new flame-retardant synergist packages that may provide an answer for an yet-under-developed plenum cable market outside the United States. All materials intended for use on wire and cables to be placed in plenum spaces are designed to meet rigorous fire safety
Fire safety
Fire safety refers to precautions that are taken to prevent or reduce the likelihood of a fire that may result in death, injury, or property damage, alert those in a structure to the presence of a fire in the event one occurs, better enable those threatened by a fire to survive, or to reduce the...

 test standards in accordance with NFPA 262 and outlined in NFPA 90A.

Note that diligence is required to make sure that a non-plenum airspace stay that way. A non-plenum airspace can become a plenum airspace by accident if the ductwork is disconnected and not properly repaired and resealed. Ductwork disconnection can occur due to building damage such as earthquakes, aging, or adverse environment causing the metal to corrode and fall apart, or simply negligence on the part of building contractors that leave work unfinished. In all such cases, discovery, and repair of such problems to eliminate unintended plenums is difficult due to the hidden nature, limited space, and difficult access of most installed drop ceilings. For highest fire safety, it is best to assume all drop-ceiling airspaces are plenums, whether or not they are officially designated as one.

Cable that is to be run between floors in non-plenum areas is rated as riser
Riser may refer to:* Riser , a reservoir in a manufacturing mold* Stair riser, the vertical elements in a set of stairs* Drilling riser, a device used on a ship or offshore drilling rig...

 cable. The fire requirements on riser cable are not as strict. Thus, plenum cable can always replace riser cable, but riser cable cannot replace plenum cable in plenum spaces. Cables like twisted-pair, coaxial
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

High-Definition Multimedia Interface
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...

, and DVI
Digital Visual Interface
The Digital Visual Interface is a video interface standard covering the transmission of video between a source device and a display device. The DVI standard has achieved widespread acceptance in the PC industry, both in desktop PCs and monitors...

 are available in plenum and riser versions.

In 2006, significant concern developed over the potential toxicity of FEP and related fluorochemicals including the process aid perfluorooctanoic acid
Perfluorooctanoic acid
Perfluorooctanoic acid , also known as C8 and perfluorooctanoate, is a synthetic, stable perfluorinated carboxylic acid and fluorosurfactant. One industrial application is as a surfactant in the emulsion polymerization of fluoropolymers. It has been used in the manufacture of such prominent...

 (PFOA) or C8 such that California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 has proposed some of these materials as potential human carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

s. The NFPA Technical Committee on Air Conditioning, in response to public comment, has referred the issue of toxicity of cabling materials to the NFPA Committee on Toxicity for review before 2008.

In 2007, a development program specifically targeting the production of a non-halogen plenum cable compound was announced to specifically address lingering toxicity concerns presented by halogenated compounds for use in European and other global markets.

Cable stiffness

Plenum-rated cables are restricted to only allow certain chemicals for manufacture of the wire insulation and cable sheath. Typically this results in reduced flexibility of plastic cables, making it stiff and hard to bend. The bend radius
Bend radius
Bend radius, which is measured to the inside curvature, is the minimum radius one can bend a pipe, tube, sheet, cable or hose without kinking it, damaging it, or shortening its life. The smaller the bend radius, the greater is the material flexibility...

may also be increased, and tight bends can potentially crack or tear the insulation and sheathing.

External links

The videos here, while presenting a comparative viewpoint, do not show the fire tests used to determine the relative acceptability of cables for use in plenum spaces. The fire tests shown in the referenced link are small scale (bench) demonstrations of relative burning characteristics under one condition that is not sanctioned by or used by any safety code or regulation in determining suitability for use in plenum or other applications.
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