Installing network cables but baffled by terms like CMP, CMR, or CM? Choosing the wrong type isn't just confusing, it's a serious fire safety risk and can violate building codes, leading to costly rework.

Plenum (CMP) cables are required for air-handling spaces (plenums). Riser (CMR) cables are for vertical runs between floors. General Purpose (CM/CMG) cables are for horizontal runs in non-plenum, non-riser areas. Using the correct rating prevents rapid fire/smoke spread and ensures code compliance.

Understanding these fire ratings is crucial. It dictates where you can legally and safely install different types of Ethernet cables within a building's structure. Let's break down what each rating means and where you absolutely need to use them.

What Exactly is Plenum (CMP) Rated Cable?

Running cables through the space above a drop ceiling used for air circulation? Using standard cable here is dangerous because smoke and flames can travel quickly through the ventilation system.

Plenum rated (CMP) cable has the highest fire resistance. It emits minimal smoke and slows flame spread when burned, making it mandatory for installation in air-handling plenum spaces according to the National Electrical Code (NEC).

Let's dive deeper into what makes Plenum cable special. Plenum spaces are parts of a building used for air circulation for heating, ventilation, and air conditioning (HVAC) systems. Common examples include the space above a suspended ceiling or below a raised floor. If a fire occurs, smoke and toxic gases from burning cables in these spaces can be quickly distributed throughout the building via the HVAC system. To prevent this disaster, the NEC mandates the use of cables specifically tested for low flame spread and low smoke production in these areas. Plenum rated cables (marked CMP for Communications Multipurpose Cable, Plenum) undergo the rigorous NFPA 262 (formerly UL 910) or "Steiner Tunnel Test." In this test, the cable must demonstrate very limited flame travel and produce minimal smoke when exposed to intense fire. To achieve this, CMP cables use special jacketing materials like low-smoke PVC formulations, Fluorinated Ethylene Propylene (FEP), or Polyvinylidene Fluoride (PVDF), which are inherently flame-retardant and produce less smoke than standard plastics. This safety comes at a cost, making CMP cables the most expensive type. I always double-check the environment – if air flows through it, it's likely a plenum, and CMP is non-negotiable.

Key Plenum (CMP) Characteristics:

• Highest Fire Rating: Designed for maximum safety in air-handling spaces.
• Low Smoke & Flame Spread: Passes the strict NFPA 262 / UL 910 test.
• Mandatory Use: Required by NEC Article 800 for installation in plenums.
• Jacket Materials: Typically uses FEP, PVDF, or specialized low-smoke PVC.
• Cost: Highest cost among the common communication cable types.

When Should You Use Riser (CMR) Rated Cable?

Need to run network cabling vertically between floors, like through cable shafts or wall cavities? Fire can spread rapidly upwards in these vertical spaces if the wrong cable is used.

Riser rated (CMR) cable is designed to prevent fire from spreading vertically between floors through shafts or risers. It's required by the NEC for vertical runs connecting different floor levels in non-plenum environments.

Riser cables (marked CMR for Communications Multipurpose Cable, Riser) are specifically engineered to limit the vertical propagation of fire. Think about a fire starting on one floor; you don't want the cables themselves acting like a wick, carrying the flames rapidly up to the floors above through walls or utility shafts. The NEC addresses this hazard in Article 800 by requiring CMR-rated cables for these vertical runs. CMR cables undergo the UL 1666 "Standard Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shafts" (often called the Riser Flame Test). This test evaluates how far flames travel upwards along a bundle of cables installed in a simulated vertical shaft. CMR cables must self-extinguish and limit the flame spread to below a specified height. They typically use flame-retardant PVC jackets that are less expensive than plenum materials but offer more fire resistance than general-purpose CM cables. An important rule to remember, which I often rely on during installations, is the substitution hierarchy: You can use Plenum (CMP) rated cable in riser applications (since it has a higher fire rating), but you cannot use Riser (CMR) cable in plenum spaces. CMR is strictly for vertical runs between floors, outside of air-handling plenums.

Key Riser (CMR) Characteristics:

• Vertical Fire Resistance: Designed to stop fire spread between floors.
• Moderate Flame Test: Passes the UL 1666 Riser Flame Test.
• Mandatory Use: Required by NEC Article 800 for vertical runs between floors (outside plenums).
• Jacket Materials: Often uses flame-retardant PVC compounds.
• Cost: Moderate cost, less than CMP but more than CM.
• Substitution: Can be replaced by CMP, but cannot replace CMP. Cannot be used in plenums.

Is General Purpose (CM/CMG) Cable Ever Enough?

Just running Ethernet cables horizontally within walls, conduit, or along baseboards on a single floor, not in air ducts or vertical shafts? Using expensive Plenum or Riser seems unnecessary here.

Yes, General Purpose rated (CM or CMG) cable is sufficient and code-compliant for many horizontal wiring applications on a single floor, as long as it's not in a plenum space or a vertical riser shaft.

General Purpose cables (marked CM for Communications Multipurpose, or sometimes CMG for General purpose) represent the baseline fire rating for communication cables used inside buildings. These cables are tested for basic flame resistance to prevent a fire from spreading along the cable itself under limited conditions. They typically undergo tests like the UL 1685 Vertical Tray Flame Test or the simpler VW-1 flame test, which ensures the cable won't continue burning excessively once the flame source is removed. However, CM/CMG cables do not have the low-smoke characteristics of Plenum cables nor the vertical flame-stopping ability of Riser cables. Therefore, the NEC restricts their use. You can install CM/CMG cables horizontally on a single floor, such as patch cables in the open, runs inside standard walls (that are not plenum air returns), or within conduit in non-plenum/non-riser areas. They are the most common and least expensive type of Ethernet cable jacket. Think of the cables connecting your desktop computer to a wall jack in a typical office or home – that's usually a CM-rated cable. Remember the substitution rules: Both Plenum (CMP) and Riser (CMR) rated cables can be used in place of CM/CMG cables (since they have higher fire ratings). However, CM/CMG cable absolutely cannot be used in plenum spaces or riser shafts. It's crucial for safety and compliance to respect these limitations.

Key General Purpose (CM/CMG) Characteristics:

• Basic Fire Resistance: Passes tests like UL 1685 or VW-1 to limit flame spread along the cable.
• General Use: Suitable for horizontal runs on a single floor in non-plenum/non-riser areas.
• Limitations: Cannot be installed in plenum spaces or vertical riser shafts.
• Jacket Materials: Typically standard PVC.
• Cost: Least expensive cable rating.
• Substitution: Can be replaced by CMP or CMR, but cannot substitute for them.

Quick Substitution Guide:

Cable Rating	Can Be Used In:	Can Be Replaced By:	Cannot Replace:
CMP	Plenum, Riser, General	None	N/A
CMR	Riser, General	CMP	CMP
CM/CMG	General Only	CMP, CMR	CMP, CMR

Conclusion

Selecting the correct fire-rated Ethernet cable – Plenum (CMP), Riser (CMR), or General Purpose (CM/CMG) – is essential. It ensures compliance with building codes and critically enhances fire safety within your installation environment.