The Ultimate Guide to Fire Ratings for Communication Cables: CM, CMR, and CMP Explained

In structured cabling and data centers, cables are more than just signal carriers—they also play a role in building safety. Fire risk is often underestimated, but the type of cable jacket you choose can directly impact flame spread, smoke, and toxic gas emissions during a fire.

The Ultimate Guide to Fire Ratings for Communication Cables: CM, CMR, and CMP Explained

The NEC (National Electrical Code) defines three major fire rating categories for communication cables: CM, CMR, and CMP.

But what exactly do these ratings mean? How do CM vs. CMR differ? Can CMR cables be used in plenum spaces? And when should you choose CMP? Let’s break it all down.

What Is a Cable Fire Rating?

A cable fire rating classifies how well a cable resists fire and controls smoke/toxic gas emissions.

• CM (Communication Multipurpose): General-purpose rating.

• CMR (Communication Riser): Designed for vertical riser shafts.

• CMP (Communication Plenum): Highest fire resistance, designed for plenum spaces.

These ratings are mandated by the NEC, and using the wrong cable in the wrong space can lead to safety violations.

Detailed Breakdown: CM vs. CMR vs. CMP

1. CM Cable (Communication Multipurpose)

• Definition: The most basic fire rating.

• Application: Suitable for residential wiring or office spaces where cables run inside walls but do not pass between floors or through plenum spaces.

• Fire Performance: Meets a basic vertical flame test; self-extinguishes but may produce heavy smoke.

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2. CMR Cable (Communication Riser)

• Definition: Also called riser-rated cable, designed for vertical cable runs.

• Application: Used to connect networks between floors in office or commercial buildings.

• Fire Performance: Passes a riser flame test, preventing flames from traveling vertically along a shaft.

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3. CMP Cable (Communication Plenum)

• Definition: Plenum-rated cable, engineered for use in air-handling spaces.

• Application: Required in plenum spaces such as ceilings, raised floors, and HVAC ducts where air circulates.

• Fire Performance: Passes the plenum flame test; offers the highest fire resistance and produces minimal smoke and toxicity when burned.

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CM vs. CMR vs. CMP: Side-by-Side Comparison

Rule of thumb:

• CMP ≥ CMR ≥ CM

• Higher-rated cables can always substitute for lower-rated ones, but not the other way around. For example, a CMP cable can be used in riser or general applications, but a CMR cable cannot be used in plenum spaces.

Real-World Example: Cat6 Cable Ratings

The concept of CM, CMR, and CMP applies directly to Ethernet cables such as Cat6 and Cat6A.

• Cat6 CM Cable: Best for small office or residential use.

• Cat6 CMR Cable: Ideal for vertical risers between floors in commercial buildings.

• Cat6 CMP Cable: Required in data centers and plenum spaces where safety standards are strict.

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Conclusion: Choosing the Right Cable Fire Rating

• For residential or small office use → CM cable is sufficient.

• For riser shafts or multi-floor networks → Choose CMR cable.

• For plenum spaces or mission-critical facilities → Only CMP cable meets the required standard.

⚡ Always remember: Higher fire rating = higher safety (and cost). Never substitute lower-rated cable in high-risk environments.

“Wondering which fire rating to choose for Ethernet? See our full breakdown of Cat6 CMR vs CMP cables.”

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