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Cable Rating FAQ
What are all these cable ratings?
Building authorities (usually county or city) in your locality adopt standards and codes to which construction must conform for the overall good and safety of the community. Remember that regardless of national codes and standards, the local building authorities have the last word on what is considered acceptable building and wiring practice in your area.
For both high voltage and low voltage electrical wiring, all building authorities adopt standards from the National Electrical Code, or NEC. The NEC is a collection of requirements for electrical wiring and appliances that safeguard against electrical fire and electrocution. |
Cable Ratings
All signal cable used for computer networks, telephone, video, audio, and control applications of less than 50 volts is considered low voltage cabling. Low voltage cabling is categorized into the following five basic groups within the National Electrical Code (NEC):
| Cable Type: | Use: |
| CM | Communications |
| CL2, CL3 | Class 2, Class 3
remote-control lighting, signaling, and power-limited cables |
| FPL | Power-limited fire protective signaling cables |
| MP | Multipurpose cable |
| PLTC | Power-limited tray cable |
Fire safety ratings under the NEC are conducted according to a common group of flame retardancy tests, which makes the cable markings similar across all of these designations. Video, audio, and low voltage control cables fall into Class 2 typically due to the available power limits set in the NEC. All computer network and telecommunications cabling falls into the CM class.
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What do these cable ratings mean?
Plenum-rated cables (suffix "P") are at the top of the cable safety food chain because they are constructed of materials having very low "fire load." Fire load is the term used to describe how much fuel a given material provides a fire. A lower fire load rating means that the material is more fire resistant and produces less smoke, which accounts for most fire-related deaths. Cables obtain the plenum rating upon successfully passing UL 910, Test for Flame-Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Air. Plenum is a commonly used term today in the construction and system installation industries because, in most cases, plenum-rated cables may be installed in air handling systems (air plenums) without expensive metallic conduit. Plenum cable can cut installation costs dramatically.
Riser (suffix "R") describes cables having a lesser degree of flame retardancy than plenum, but may be used to convey signals vertically in shafts without requirement for metallic conduit. The compliant cable has a flame propagation of less than 12 feet and has a temperature of 850 degrees Fahrenheit or less at a height of 12 feet per UL 1666.
General-purpose (no suffix) cables may be used in conduit, behind walls, or other enclosed locations where the cable is protected and not in an air plenum. Commercial installations, at a minimum, must use general-purpose cables (the typical CL2 designation for coaxial video cables, for example). This type of cable must comply with UL 1581, the Vertical-Tray Flame Test. For CSA (Canadian Standards Association), the vertical flame test differs in loading (more cable in bundles), burner angle, and failure criterion.
CL2X and CL3X are the lowest rated cable and must comply with UL VW-1 Vertical-Specimen Flame Test. The cable is not marked VW-1. This rating may be used in residential dwellings.
PLTC (power-limited tray cable) complies with a 70,000 BTU/hr vertical-tray flame test. Cables of this type are marked PLTC with ink or marker tape.
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What does Plenum mean?
What makes a cable into a plenum rated cable? The materials making up the insulation on the wires and jacket on the cable must be capable of withstanding a specified amount of heat for a specified amount of time without combustion or contributing significantly to the sustenance of a fire. The ideal cable will not burn at all.
The most common insulation and jacketing material used on wire and cable is Polyvinyl Chloride, PVC. PVC has many attributes that make it a great material for general-purpose wire. Unfortunately, PVC is very flammable. When PVC burns, a key byproduct is hydrochloric acid. The smoke and residues are very corrosive. While there are several versions of PVC with varying characteristics, none are able to pass the plenum test. Some versions of PVC and another group of polymers from the family of plastics called Polyolefin may attain plenum capability when combined with certain other polymers that are more fire resistant. However, maintaining the safety margins against the plenum flame test is sometimes difficult. Construction must be highly controlled and, in some instances, cable designs that pass the test one time may not pass on another trial.
The best insulation for fire resistance to date is also one of the best dielectric materials for lower loss cables…Teflon® FEP (fluorinated ethylene propylene). Teflon FEP is a registered trademark of Dupont. But, Teflon FEP is much more expensive to manufacture which explains the higher cost of plenum rated cables. The material is tougher and more difficult to extrude. This is why plenum cables are not as flexible as PVC.
To obtain a plenum rating, the cable must pass the Steiner Tunnel Test within UL 910. The Steiner Tunnel is a specially constructed fire chamber that positions a group of cables of the same type and about 24 feet in length into a horizontal frame within an air handling plenum. Air rushes into one end of the plenum. Gas burners supply a specific level flame under the cable bundle about 4.5 feet from the end near the air inlet. While the flame is applied for a specified period of time, the length of flame travel along the cable is monitored as well as the amount of smoke produced. At the opposite end of the tunnel, a vent shaft funnels the air and smoke past photoelectric sensors. Criteria under which the test results must comply are:
- Smoke Peak Optical Density: less than 0.50
- Smoke Average Optical Density: less than 0.15
- Maximum Flame Propagation: less than 5.0 feet from point of application
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What does AWG mean?
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AWG stands for American Wire Gauge, and it is a standard used for denoting wire conductor diameter. The system is counter-intuitive, so the lower the AWG, the thicker the conductor and the lower the resistance per unit of length. Thicker cables are mainly used in longer lengths of cabling to ensure quality signal transfer. A lower AWG cable for short distances is not needed as it would not offer many benefits over a higher AWG of the same type of cable.
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