Key Takeaways
- Cable flammability ratings show how a cable behaves in a fire and where it can be installed in a building.
- General purpose (CM, CMG) cables suit runs within a single floor, but not risers or air plenum spaces.
- Riser-rated (CMR) cables are designed for vertical runs between floors and must self-extinguish.
- Plenum-rated (CMP) cables pass the strictest fire test and are the only communications cables permitted in air plenum spaces.
- LSZH cables minimize smoke and eliminate toxic halogen gases, making them ideal for enclosed spaces such as ships, trains and aircraft.
Fires can’t always be prevented, but with a plan in place, fires are more easily contained and people can be kept safer. The same principle applies when choosing cables for your communication network. Some cable materials can pose real threats when ignited, especially if the fire is in an enclosed space where evacuation is not an option. For this reason, cable flammability ratings were developed. Here, we’ll take a look at what these ratings mean and how to use them to keep yourself safe and keep your communications equipment from going up in flames.
Most commercially available cable assemblies have an outer jacket made from polyvinyl chloride (PVC). PVC is durable and flexible, making it a great option for many applications. But for all of its benefits, PVC has some serious shortcomings. In a fire, PVC cables can act as a flame accelerant and emit dangerous, toxic gases. Fortunately, there are cable jacket materials on the market that are less-flammable and much safer. These are especially good options for installations in enclosed spaces such as ships, aircraft, submarines, trains and other vehicles. To see how PVC compares with other common jacket options, read our guide to cable jacket materials: PVC vs. PUR vs. TPE.
That’s where flammability ratings come in. Each rating tells you how a cable performs in a fire and where it can be installed. Here are the most common ones you’ll encounter.
Common Cable Flammability Ratings
General Purpose (CM, CMG, CMx)
These cables pass the UL 1685 vertical tray flame test. They will burn but they partially self-extinguish. These are often used for workstation cables and patch cords, but are not for use between building floors or in air plenum spaces.CMX cables, which are limited to residential and other restricted uses, only need to pass the smaller-scale VW-1 flame test.
When cables need to run between floors, however, a higher rating is required.
Riser-rated (CMR)
Riser-rated cables are UL-1666 compliant and are designated for use in vertical tray applications such as cable runs between floors, through cable risers or in elevator shafts. In order for a cable to be Riser-rated, it must be able to self-extinguish.
For spaces that circulate air through a building, the requirements are even stricter.
Plenum-rated (CMP)
Plenum-rated cables comply with NFPA-262 and UL-910. They are the only cables permitted in spaces identified as air plenums, such as raised flooring systems and air handling ducts. Cables designated as plenum-rated are able to self-extinguish and will not re-ignite.
Where smoke and toxic fumes are the bigger concern, LSZH cables offer a different kind of protection.
Low-Smoke Zero-Halogen (LSZH)
As the name states, these cables produce low-smoke and zero halogen, plus they are self-extinguishing. Because they significantly reduce the amount of smoke and eliminate harmful halogen from being emitted, they are used in enclosed spaces where smoke and fumes can injure people and equipment. LSZH cables are increasingly specified in data centers too; learn more in our article on future-proofing data centers with LSZH cables.
With the main ratings covered, here’s how to put them to work when selecting a cable.
How to Choose the Right Flammability Rating
Start with where the cable will be installed. Runs within a single floor can generally use CM or CMG cables, vertical runs between floors call for riser-rated (CMR) cables, and air plenum spaces require plenum-rated (CMP) cables. For enclosed spaces where smoke and toxic gases are the main danger, such as vehicles, ships and tunnels, consider LSZH.
Ratings also follow a hierarchy. A higher-rated cable can be used in place of a lower-rated one, so a CMP cable can go anywhere a CMR, CM or CMX cable is allowed, and a CMR cable can replace CM or CMX. The reverse is never permitted. Because fire codes vary by location, always confirm requirements with your local building code or authority having jurisdiction.
Still have questions? Here are the ones we hear most often.
FAQs About Cable Flammability Ratings
Q1: What is the difference between plenum and riser cable?
A: Riser (CMR) cables are designed for vertical runs between floors and must stop a fire from spreading up a shaft. Plenum (CMP) cables pass a stricter test for flame spread and smoke, which allows them to be installed in air-handling spaces where smoke could circulate through a building.
Q2: Can I use plenum cable instead of riser cable?
A: Yes. Plenum-rated cable can be used anywhere riser-rated cable is allowed, because it meets a stricter fire standard. Riser cable cannot be used in plenum spaces.
Q3: Is LSZH cable the same as plenum-rated cable?
A: No. LSZH describes a jacket material that produces little smoke and no halogen gases when burned, while plenum is a fire rating based on a specific flame and smoke test. An LSZH cable is not automatically plenum-rated, so check the cable’s listing before installing it in a plenum space.
Q4: Why is PVC cable a concern in a fire?
A: When PVC burns, it releases dense smoke and halogen-based gases such as hydrogen chloride, which are toxic to people and corrosive to electronic equipment. This is why low-smoke, zero-halogen alternatives are preferred in enclosed spaces.
Conclusion
Cable flammability ratings are a simple way to make sure the cables you install won’t become a hazard in a fire. Match the rating to the space, use the hierarchy when in doubt, and choose LSZH where smoke and toxic fumes pose the greatest risk.
L-com offers a broad selection of cable assemblies with a range of jacket types and ratings, with same-day shipping on all qualified, in-stock, online orders received Monday through Friday before 5 p.m. EST.