Fire can move quickly through vertical building spaces. Cable shafts can give flames a direct path between floors if cables burn too easily. UL 1666 addresses this risk by testing electrical and optical-fiber cables installed vertically in shafts.
The standard measures how high flames travel during a controlled large-scale fire test. Cables that meet the applicable requirements can receive riser ratings for suitable building applications. This matters for network installers because choosing the wrong cable can create safety and code problems.
A proper riser-rated cable supports safer vertical installations while helping installers meet the requirements that apply to the project.
Test Objective and Setup of UL 1666
UL 1666 is titled the Standard for Safety for Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shafts. The test focuses on flame propagation in a vertical shaft arrangement. It is a large-scale test rather than a small sample burn.
The setup includes several important conditions that make the test useful for riser applications. The purpose, test environment, flame source, and test duration all affect the final result. Each part helps show how a cable behaves during a severe fire exposure.
Purpose
The main purpose of UL 1666 is to measure the height to which flames travel along vertically installed cables. The test helps determine whether a cable is suitable for riser applications under the applicable code requirements.
UL listed identifies the test specifically for electrical and optical-fiber cables installed vertically in shafts. The test does not simply ask if a cable can catch fire. It looks at how fire behaves along the cable arrangement.
This distinction matters because a cable that burns in a small test may behave differently when many cables are installed together in a vertical shaft. The main points of the test include the following:
- Flame Propagation: The test measures how far flames travel upward along the cable arrangement.
- Large-Scale Exposure: The cables face a substantial controlled fire source during the evaluation.
- Vertical Installation: The test places cables in an arrangement that represents vertical shaft conditions.
- Temperature Measurement: Thermocouples help measure temperatures at specific locations during testing.
- Cable Rating: Successful performance supports the applicable riser classification for the tested cable.
The result gives manufacturers and code users useful information about flame behavior. It also gives installers a clearer basis for selecting cables for vertical pathways.
Environment
UL 1666 uses a vertical shaft test environment because the standard addresses cables intended for installation in vertical shafts. This setup matters because fire behavior changes when cables are positioned vertically. A shaft can provide a path for heat and flames to move upward.
Multiple cables can also add fuel to a fire. The test therefore examines cable behavior under conditions that place significant emphasis on vertical flame travel. The test is different from testing intended for air-handling spaces. Plenum applications use different fire performance requirements.
UL identifies NFPA 262 for plenum applications and UL 1666 for riser applications. Installers should therefore match the cable rating with the actual building space. A riser rating does not automatically make a cable suitable for every location.
Flame Source
The UL 1666 test uses a large controlled flame source to expose the cable arrangement to a severe fire condition. Industry documentation identifies the test fire exposure as a large-scale 527,500 BTU source. The strong flame source helps create a demanding test.
The goal is to see how the cable construction behaves under significant fire exposure. Cable jackets and insulating materials can affect flame travel. Cable construction therefore matters greatly during testing.
The flame source also helps make results meaningful for vertical cable installations. A cable that performs well under this type of exposure has demonstrated resistance to upward flame propagation under the specified test conditions.
Duration
The test uses a controlled exposure period that gives the cable arrangement enough time to show its fire behavior. Industry references describe UL 1666 as a large-scale flame test with a sustained fire exposure. Duration matters because a short exposure may not show how flames develop across a larger cable arrangement.
A sustained exposure places greater demand on the materials and construction. The exact laboratory procedure should always be taken from the current UL 1666 standard rather than from simplified summaries. UL maintains its standards catalog for current editions and official requirements.
Installers do not perform the laboratory test themselves. They select products that have the required listing or rating for the intended installation.
Pass/Fail Criteria
UL 1666 focuses heavily on how far flames propagate and how hot the area becomes at a specified height. These measurements provide the main basis for judging the cable's performance during the test. Smoke and toxicity require separate consideration because UL 1666 does not evaluate every fire characteristic. The main performance points below show what installers should recognize when reading riser cable specifications.
Flame Propagation
Flame propagation height is one of the key measurements in UL 1666. For communications cables carrying a CMR designation, the flame propagation height must remain below the applicable limit.
UL documentation describes CMR cable as cable intended for building use in vertical shafts, with flame propagation below 12 feet when tested under UL 1666.
Industry technical information further identifies the criterion as the flame propagation height not reaching or exceeding 12 feet, or approximately 3.6 meters. This requirement gives the test a clear focus. The fire should not travel excessively high along the vertical cable arrangement.
Temperature Limit
Temperature is another important measurement during the UL 1666 test. Industry references identify a temperature limit at the 12-foot measurement height. The temperature of any thermocouple at that height should not exceed 850°F (454°C) under the referenced test criteria.
This measurement adds another layer to the flame propagation assessment. The test therefore considers both the height of flame travel and the temperature measured near the upper test boundary.
Installers should avoid treating these numbers as general operating-temperature limits. They are part of a specific fire test procedure and should not be confused with the normal operating temperature rating printed on a cable datasheet.
Smoke and Toxicity
UL 1666 focuses on flame propagation height and related temperature measurements. It does not investigate toxicity, combustion, or decomposition. That distinction is important for cable selection. A UL 1666 riser rating does not mean the cable has passed every possible smoke or toxicity test.
Plenum cables face different requirements because they may be installed in spaces used for environmental air. NFPA 262 addresses flame travel and smoke characteristics for wires and cables used in air-handling spaces.
Installers should therefore review the required cable classification for each building space instead of relying on one fire rating for every application.
Where Riser Cables Are Used
Riser-rated cables are intended for specific building pathways where vertical cable installation creates a potential route for upward fire movement. The common examples involve shafts and vertical spaces between floors. Let’s look at where riser-rated cables are commonly appropriate and where another cable classification may be required.
Areas Where Riser Cables Are Allowed
Riser-rated cables have a defined role in building communications systems. They are generally selected for vertical pathways that are not classified as environmental air-handling spaces. The following examples show common locations where riser cables may be used under the applicable code requirements:
- Vertical Shafts: Riser cables are commonly installed in vertical shafts connecting different building floors.
- Interfloor Pathways: They can serve communications runs that travel vertically between designated telecommunications spaces.
- Riser Closets: Vertical cable routes between telecommunications rooms can use properly rated riser cables.
- Building Backbones: Riser-rated communications cables may support indoor backbone runs through suitable vertical pathways.
- Indoor Vertical Routes: They can serve vertical indoor pathways where the applicable code permits CMR-rated cable.
The actual installation must still follow the local adopted code. A product label alone should never replace a review of the installation space.
Areas Where Riser Cables Are Not Allowed
Riser-rated cable has limits. A CMR rating does not make a cable suitable for every building pathway or environmental condition. The following locations may require a different cable classification or a different product altogether:
- Air-Handling Spaces: Plenum spaces generally require cable meeting the applicable plenum requirements.
- Outdoor Locations: Standard indoor riser cable is not automatically suitable for outdoor exposure.
- Direct Burial: Riser-rated communications cable should not be treated as a burial-rated product.
- Wet Locations: Cable designed for indoor riser use may lack the construction required for wet environments.
- Hazardous Locations: Special locations can require additional cable and installation requirements beyond UL 1666.
Product specifications should always be checked before installation. For example, current manufacturer listings can separately identify riser suitability, outdoor suitability, sunlight resistance, and burial limitations.
The same approach applies to fiber. A fiber cable may carry an OFNR classification based on UL 1666, but its environmental ratings still need separate review. CommScope, for example, lists UL 1666 as the flame test method for an indoor riser fiber product.
Build Safer Riser Networks with TS Cables
UL 1666 gives cable manufacturers and installers a clear way to evaluate flame propagation in vertical shaft applications. Choosing properly rated riser cable supports safer installations and helps match the cable to its intended building space.
TS Cables provides networking cable solutions for structured cabling projects where dependable performance and proper ratings matter.
FAQs
What does UL 1666 test?
UL 1666 tests electrical and optical-fiber cables installed vertically in shafts to measure flame propagation height and related temperature performance.
What does CMR mean on network cable?
CMR means Communications Riser. It identifies cable intended for permitted indoor riser applications and is associated with UL 1666 flame testing.
Is UL 1666 the same as NFPA 262?
No. UL 1666 addresses riser applications, while NFPA 262 addresses flame travel and smoke performance for cables used in air-handling spaces.
Does UL 1666 measure toxic smoke?
No. UL 1666 does not investigate toxicity, combustion, or decomposition. Separate testing and requirements address those fire characteristics.
Can CMR cable be installed in a plenum?
CMR cable should not be used simply because it has a riser rating. Plenum spaces generally require cable meeting the applicable plenum requirements.
Further Reading
Explore our other TS Cables networking articles for practical guidance on cable ratings, fire standards, structured cabling, Ethernet systems, and professional installation practices.
