top of page
Cat6 cables wired to back of patch panel

Plenum vs Riser Cable

Understanding plenum and riser cable requirements is critical for fire safety, code compliance, and inspection approval in commercial structured cabling installations. Cable fire ratings are not optional — they are mandated by the National Electrical Code (NEC) and enforced by local authorities having jurisdiction (AHJs).

​

Using the wrong cable type can result in failed inspections, forced cable replacement, liability exposure, and increased risk to building occupants.

This page explains the differences between plenum and riser cable, where each is required, and how cable ratings affect commercial network installations.

What Is Plenum Cable (CMP)?

Plenum cable, designated as CMP, is designed for installation in plenum spaces — areas used for environmental air circulation.

Plenum spaces commonly include:

  • Drop ceilings used as return air plenums

  • Open ceiling spaces without ducted returns

  • Raised floors used for air handling

Key characteristics of plenum cable:

  • Highest fire resistance rating

  • Low smoke production

  • Low toxic gas emission when burned

  • Required by NEC in air-handling spaces

Plenum cable jackets are typically made from fluoropolymers that resist flame spread and reduce hazardous smoke.

What Is Riser Cable (CMR)?

Riser cable, designated as CMR, is designed for vertical cable runs between floors and in non-plenum spaces.

Riser cable is commonly used in:

  • Vertical shafts

  • Riser closets

  • Wall cavities

  • Non-air-handling ceiling spaces

Key characteristics of riser cable:

  • Flame-resistant for vertical fire spread

  • Lower cost than plenum cable

  • Acceptable in non-plenum areas only

Riser cable does not meet the smoke and toxicity requirements for plenum spaces.

CMP (Plenum) Cable — Key Characteristics

  • Highest fire-resistance rating for communications cable

  • Designed for use in air-handling (plenum) spaces

  • Produces very low smoke when exposed to flame

  • Emits low-toxicity gases compared to other cable types

  • Required by NEC when cabling is installed in plenum environments

  • Typically higher material cost due to specialized jacket compounds

  • Lowest inspection risk when installed correctly

  • Commonly specified to avoid ambiguity with AHJs

CMR (Riser) Cable — Key Characteristics

  • Medium fire-resistance rating intended to limit vertical flame spread

  • Designed for non-plenum spaces only

  • Produces more smoke than plenum-rated cable

  • Higher toxic emissions compared to CMP when burned

  • Not permitted in plenum air spaces

  • Lower material cost than plenum cable

  • Higher inspection risk if space classification is misunderstood

  • Commonly used in wall cavities, riser shafts, and ducted ceiling spaces

CMP vs CMR — Practical Differences

  • CMP is mandatory in any space used for environmental air return

  • CMR is prohibited in plenum spaces regardless of cost savings

  • CMP minimizes risk of inspection failure and forced cable replacement

  • CMR is acceptable only when the space is clearly classified as non-plenum

  • When space classification is unclear, CMP is often selected as a risk-mitigation strategy

Code & Inspection Reality

  • Cable fire ratings are enforced under the National Electrical Code (NEC)

  • Local Authorities Having Jurisdiction (AHJs) determine final approval

  • Installing the wrong cable type can require full removal and replacement

  • Cost savings from riser cable can be eliminated instantly by inspection failure

NEC Code Requirements (Why This Matters)

The National Electrical Code (NEC) governs cable fire ratings in commercial buildings.

Key principles:

  • Plenum spaces must use CMP-rated cable

  • Riser cable cannot be installed in plenum air spaces

  • Local AHJs enforce compliance during inspection

  • Non-compliant cable often must be removed and replaced

Ignorance of cable rating requirements does not protect against inspection failure or liability.

NEC Code Requirements (Why This Matters)

The National Electrical Code (NEC) governs cable fire ratings in commercial buildings.

Key principles:

  • Plenum spaces must use CMP-rated cable

  • Riser cable cannot be installed in plenum air spaces

  • Local AHJs enforce compliance during inspection

  • Non-compliant cable often must be removed and replaced

Ignorance of cable rating requirements does not protect against inspection failure or liability.

Common Plenum Spaces in Commercial Buildings

Many plenum spaces are not obvious.

Common examples include:

  • Drop ceilings used as return air plenums

  • Open ceilings in offices and retail spaces

  • Corridors with shared air return pathways

  • Data centers with overhead air return designs

When in doubt, plenum cable is often specified to avoid compliance risk.

When Riser Cable Is Acceptable

Riser cable may be used when:

  • The space is not part of an air-handling system

  • Ceiling spaces are ducted

  • Cable runs are inside walls or riser shafts

  • Approved by the building design and AHJ

Verification of space classification is critical before cable selection.

Cost vs Compliance Considerations

While plenum cable costs more than riser cable, using riser cable in a plenum space can result in:

  • Failed inspections

  • Project delays

  • Forced cable replacement

  • Increased labor cost

  • Liability exposure

In many commercial projects, plenum cable is selected as a risk-mitigation measure even where riser cable might be technically allowed.

Plenum and Riser Cable in Structured Cabling Systems

Cable fire rating selection must align with:

  • Pathway design

  • IDF and MDF placement

  • Building HVAC architecture

  • Local code interpretation

Proper planning during design prevents costly remediation later.

Plenum vs Riser Cabling by Fiber Infrastructure Solutions

Fiber Infrastructure Solutions installs structured cabling systems that comply with NEC fire-rating requirements and local inspection standards throughout North Carolina.

Cable selection is based on pathway analysis, building design, and AHJ requirements to ensure safe, compliant installations.

Related Structured Cabling Resources

​

​

​

  • Structured Cabling Distance Limits Explained

​

​

bottom of page