When installing Ethernet cable in walls or ceilings, you may encounter terms like CM, CMR, CMP, or some others like PVC and LSZH. Do you know what these terms really mean and which one does your project actually need? Among all these terms, CM, CMR and CMP are the most common Ethernet cable ratings specified by the National Electric Code (NEC). They’re named by the ability of resisting to fire to ensure safety in case that you didn’t use the right cable rated for its intended use. This post will give an explicit analysis on CM, CMR and CMP Ethernet cable jacket ratings and the situations where they are suitable for.
CM stands for “Communications Multipurpose”. It is a cable jacket fire resistance rating defined in Article 800 of the NEC. In the case of CM, the standard outlines general use, or use in cases where the fire code does not place any restrictions on cable type. The CM cables are generally the minimum jacket ratings suitable for in-wall installation of Ethernet cables, and are appropriate for installation inside a residence or a single-story commercial building. As with any in-wall rated cable, the goal is to prevent fire from traveling along a cable from one part of a building to another. The Ethernet cables that are labeled "Type CM" must pass a standardized flammability test before the cables are put into use.
CMR cable stands for "Communications Multipurpose Cable, Riser". It is a cable jacket fire resistance rating defined in Article 800 of the NEC. CMR cable is also referred to as “riser-rated cable” and designed to prevent fires from spreading between floors through risers or vertical shafts. The goal of CMR cable is to be flame-retardant enough to prevent the spread of fire from one floor to another. The fire resistance requirements for CMR is much stricter than that of CM, which means that CMR cable can be used as a substitute in any area where CM cable would be required. CMR cable is more flame-retardant and consequently more expensive than CM cable.
CMP cable stands for "Communications Multipurpose Cable, Plenum" It is a cable jacket fire resistance rating defined in Article 800 of the NEC. CMP cable is also referred to as “plenum-rated cable” and is constructed to restrict flame propagation to no more than five feet and limit the amount of smoke emitted during a fire. Since air travels throughout a building via plenum spaces, it is critical that cables installed in such spaces use material in the construction of the jacket which don’t give off toxic fumes if burned. Thus, the CMP or plenum-rated cables are designed using materials that burn more cleanly and self-extinguish more easily. CMP cable is suitable for installation into air plenum spaces. An example of the most common types of plenum spaces would be a dropped ceiling or raised floor. As the flammability requirements for CMP cables are stricter than CM and CMR cables, CMP cables can be used as a substitute in any area where CM and CMR would be required.
CM is the most basic cable jacket rated for general use. CMP is higher rated than CMR. The most significant difference is that they are suitable for different situations with various type of airspace. Both CM and CMR cable jacket are made from the relatively inexpensive polyvinyl chloride (PVC). Price and flexibility are the upside of PVC, but the PVC material can release thick smoke and dangerous gases like hydrogen chloride in a fire. While CMP cable jacket is made from either a low-smoke polyvinyl chloride (PVC) or a fluorinated ethylene polymer (FEP). Since the jacket material requirement for CMP is stricter than that of CM and CMR, their fire resistance and application ranges are different. CM cable for general use can not replace CMR and CMP cables but both CMP and CMR Ethernet cables can be institutes of CM Ethernet cables. Ethernet cables with CMP jacket can always replace cables with CMR jacket, but CMR cable cannot replace CMP cable in plenum spaces.
As mentioned above, PVC is the material that makes up the Ethernet cable jackets with CM, CMR and CMP ratings. That’s why we can often see Ethernet cables are labeled PVC CM, PVC CMR or PVC CMP on the jackets. Due to various applications, Ethernet cables are divided into Riser, Plenum and LSZH type to choose in practice. Among them, the LSZH (Low Smoke Zero Halogen) cables are built with a jacket material free from halogen. The LSZH cable is slightly inferior to Plenum type and commercialized mostly in Europe. Other related terms you may feel confused include OFNR (Optical Fiber Non-conductive Riser) with a jacket for riser airspace and OFNP (Optical Fiber Non-conductive Plenum) with a jacket suitable for plenum airspace. Compared with the copper-made CMR and CMP Ethernet cables, OFNR and OFNP are more superior and flame-retardant since they’re fiber optic cables constructed of materials that do not conduct electricity. More information about OFNR and OFNP, please read this article Understanding Fiber Optic Cable Jacket & Fire Rating.
To maximize cabling performance, it’s important to ensure that the type of cable selected meets the standards outlined for your particular project. Generally speaking, CMP cable is more commonly recommended, since it can be used for any type of cable needs. The more safer CMP cable can allow peace of mind that if a house fire were to occur, and there would be less risk of toxic fumes being inhaled. However, CMR cable can also be a great choice if you are not required to run cables through air ducts. Since many homeowners are able to run cables without the use of air ducts, CMR cable is already enough for most home runs.
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