Plenum vs. Riser vs. General Purpose: Understanding Cable Fire Ratings

When selecting networking cable or fiber optic cable, performance specs like bandwidth, distance, and connector type usually get the most attention. However, cable fire ratings are just as important, especially when it comes to building safety and passing inspections.

The National Electrical Code (NEC) establishes strict guidelines for where different types of cable jackets can be installed based on their fire resistance and smoke generation. Choosing the wrong cable rating can lead to failed inspections, costly rework, and potential safety risks.

This guide explains the most common cable fire ratings—including plenum, riser, general purpose, residential, and LSZH—and how to determine which one is appropriate for your installation.

Why Cable Fire Ratings Matter

Cable jackets are made from materials that behave differently when exposed to fire. Some burn easily and release heavy smoke, while others are designed to self-extinguish and minimize toxic fumes.

Because cables are often installed in walls, ceilings, and other concealed areas, the NEC requires specific fire ratings depending on where the cable runs in a building.

For example:

• Cables installed in air handling spaces must produce very little smoke.
• Cables running between floors must prevent flames from traveling vertically.
• Cables used in open office spaces or workstations can have less stringent requirements.

Understanding these classifications helps ensure your installation is safe and code-compliant.

The Main Types of Cable Fire Ratings

Plenum Rated Cable (CMP / OFNP)

Plenum-rated cable is designed for use in plenum spaces, which are areas used for environmental air circulation.

Common plenum spaces include:

• Drop ceilings used for HVAC air return
• Raised floor air distribution systems
• Air ducts or air handling pathways

Because air from these spaces circulates throughout a building, cables installed here must meet the strictest fire and smoke standards.

Plenum cables must:

• Self-extinguish when exposed to flame
• Resist reigniting
• Produce very low smoke levels

These cables comply with NFPA-262 and UL-910 testing, which evaluates both flame spread and smoke generation.

Typical plenum rating codes include:

Cable Type	Plenum Rating
Signaling	CL2P
Optical Fiber	OFNP
Communications	CMP
Coax/Antenna	CATVP

Because plenum-rated cable meets the highest safety standard, it can be substituted for riser or general-purpose cable if needed.

Riser Rated Cable (CMR / OFNR)

Riser-rated cables are designed for vertical cable runs between floors, often through:

• Cable riser shafts
• Utility shafts
• Elevator shafts
• Vertical conduits connecting multiple floors

These installations pose a unique risk: if a cable burns, flames can travel upward like a chimney, spreading fire between floors.

Riser cables are engineered to self-extinguish and prevent flame propagation during vertical burn tests. They comply with UL-1666, which specifically evaluates how fire travels along vertically installed cable. However, riser cables are not tested for smoke generation, meaning they cannot be installed in plenum spaces.

Typical riser rating codes include:

Cable Type	Riser Rating
Signaling	CL2R
Optical Fiber	OFNR
Communications	CMR
Coax/Antenna	CATVR

If needed, plenum cable can replace riser cable, but riser cable cannot replace plenum.

General Purpose Cable (CM / OFN)

General-purpose cables are designed for use in standard commercial building areas where plenum or riser ratings are not required.

Common locations include:

• Open office spaces
• Equipment rooms
• Workstations
• Patch cables

These cables are tested under UL-1581, which evaluates flame resistance but applies less stringent requirements than plenum or riser testing.

General purpose cables:

• May burn under sustained flame exposure
• May partially self-extinguish
• Produce more smoke than plenum-rated cables

Because of their lower fire resistance, they cannot be used in plenum spaces or vertical risers.

Typical general-purpose rating codes include:

Cable Type	General Purpose Rating
Signaling	CL2
Optical Fiber	OFN
Communications	CM
Coax/Antenna	CATV

Both plenum and riser rated cables can substitute for general purpose cable if necessary.

Residential Rated Cable (CMX)

Residential cable ratings apply to installations in homes and residential environments. These cables are typically used for outdoor cable runs, residential networking, or exterior installations.

Residential cables meet lower fire resistance standards than commercial cables but may be permitted in commercial buildings if installed inside conduit.

Typical residential rating codes include:

Cable Type	Residential Rating
Signaling	CL2X
Optical Fiber	n/a
Communications	CMX
Coax/Antenna	CATVX

As with other ratings, higher-rated cables (plenum, riser, general purpose) may be substituted for residential cable if needed.

Low Smoke Zero Halogen (LSZH)

Low Smoke Zero Halogen (LSZH) cables are designed to minimize toxic gas emissions and smoke during combustion. Unlike traditional PVC jackets, LSZH materials do not contain halogens, such as Fluorine, Chlorine, Bromine, or Iodine.

When halogen-based materials burn, they can produce acidic smoke and corrosive gases that can harm people and damage sensitive equipment.

LSZH cables are commonly used in environments where safety and equipment protection are critical, including shipboard installations, data centers, enclosed networking rooms, and areas with limited ventilation.

Key characteristics of LSZH cables:

• Produce very little smoke
• Do not release corrosive halogen gases
• Self-extinguish when exposed to flame

However, it’s important to note that LSZH is not an NEC fire rating. Instead, it describes the chemical composition of the cable jacket.

A cable may be:

• LSZH + Riser rated
• LSZH + General Purpose rated

But LSZH cables alone typically do not meet UL-910 (plenum) or UL-1666 (riser) fire test requirements unless specifically designed to do so.

Cable Rating Substitution Rules

One of the reasons cable fire ratings can be confusing is that the NEC allows certain substitutions between rating levels.

In general, higher-rated cables can replace lower-rated ones:

Cable Rating	Can Substitute for
Plenum	Riser, General Purpose, Residential
Riser	General Purpose, Residential
General Purpose	Residential
Residential	Cannot substitute for other ratings

Because plenum cable meets the most stringent fire standards, it offers the greatest installation flexibility, though it typically comes at a higher cost.

Choosing the Right Cable Rating

Selecting the correct cable rating depends entirely on where the cable will be installed.

A quick rule of thumb:

• Plenum cable – For air handling spaces and HVAC plenum areas

• Riser cable – For vertical runs between floors

• General purpose cable – For standard building wiring areas

• Residential cable – For home installations or outdoor residential use

• LSZH cable – For environments requiring minimal smoke and toxic gas

When in doubt, it's always best to consult building plans, local codes, or an experienced cable supplier before purchasing cable.

Improve Safety and Ensure Compliance with Cables Plus USA

Cable fire ratings are an essential—but often overlooked—part of network infrastructure planning. Choosing the wrong cable can lead to failed inspections, costly replacements, and potential safety hazards.

By understanding the differences between plenum, riser, general purpose, residential, and LSZH cables, you can ensure your installation meets NEC requirements while protecting both people and equipment.

If you're planning a new installation or upgrading an existing network, selecting the correct cable rating from the start can save time, money, and compliance headaches down the road. At Cables Plus, our team of experts is ready to help you find the right cables for your application.

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