When you're installing HDMI cables behind walls, above drop ceilings, or through air handling spaces, the cable rating matters just as much as the resolution it supports. Building codes require specific fire safety ratings depending on where the cable is installed — and using the wrong type can fail an inspection, void insurance, or create a genuine safety risk.
Here's what each rating means and when you need it.
What are cable fire ratings?
All commercial and most residential building codes regulate what types of cables can be installed in different spaces. The concern is fire safety: when a cable burns, the jacket material can produce toxic smoke and flames that spread through air ducts and wall cavities. Fire-rated cables use special jacket materials that resist flame propagation and produce less smoke.
The two most common rating systems you'll encounter for HDMI cables are NEC (National Electrical Code) classifications and UL (Underwriters Laboratories) listings.
CL3 rated (Class 3 — In-Wall)
CL3 stands for Class 3, and it means the cable is approved for permanent in-wall installation in residential and commercial buildings. The jacket material is tested to resist flame spread and self-extinguish when the ignition source is removed.
CL3 is the right choice when you're running HDMI through wall cavities between rooms, behind drywall in a home theater setup, through conduit in a commercial space (not in the air plenum), or any permanent installation inside walls or ceilings that are not air return spaces.
CL3 cables are more affordable than plenum-rated cables and are the standard for most residential and commercial in-wall AV installations.
CMP rated (Communications Multipurpose — Plenum)
CMP is the highest fire safety rating for communications cables. "Plenum" refers to the air handling spaces in a building — the open areas above drop ceilings and below raised floors where the HVAC system circulates air. Because fire and smoke in these spaces get distributed throughout the entire building by the air system, cables installed here must meet stricter fire and smoke standards.
CMP-rated cables use special low-smoke, flame-retardant jacket materials (typically fluoropolymers like FEP or LSZH compounds) that produce minimal toxic smoke when exposed to flame.
You need CMP/plenum-rated cables when running through the space above a suspended/drop ceiling that serves as an air return, installing in the space below a raised floor in a data center or server room, running through any air duct or air handling space, or when building codes or the AHJ (Authority Having Jurisdiction) specifically requires plenum rating.
CMR rated (Communications Multipurpose — Riser)
You may also encounter CMR (riser-rated) cables, which are designed for vertical runs between floors. Riser-rated cables prevent fire from traveling up through vertical shafts. CMR falls between CL3 and CMP in terms of fire resistance. You can always use a higher-rated cable in a lower-rated space (CMP cable works everywhere), but not the reverse.
The rating hierarchy
From least to most restrictive: CL3 → CMR → CMP. A CMP-rated cable can be installed anywhere. A CL3-rated cable cannot be used in plenum spaces or riser applications.
Why this matters for HDMI installations
Modern AV installations frequently require long HDMI runs — a conference room projector might be 75 feet from the source, a church sanctuary display might be 100 feet from the media booth, and a home theater projector might be 50 feet from the equipment rack.
For these long runs, Active Optical HDMI cables (AOC) are the go-to solution because they use fiber optic technology to maintain 4K or 8K signal quality over distances that would defeat traditional copper HDMI cables. These AOC cables come in both CL3 and CMP ratings.
The key question is: where is the cable being installed?
If it's going through walls (not plenum spaces), choose CL3. If it's going above a drop ceiling or below a raised floor, choose CMP/plenum. If you're unsure, CMP is always the safe choice since it's approved for all locations.
What about "plenum" vs "CMP"?
These terms are often used interchangeably. "Plenum" describes the space where the cable is installed, while "CMP" is the official UL/NEC fire rating classification. When someone says "plenum-rated HDMI cable," they mean a cable with a CMP rating.
Our HDMI cable ratings
At Kentek, we carry Active Optical HDMI cables in both CMP (plenum) and CL3 (in-wall) ratings, in both HDMI 2.0 (4K@60Hz, 18 Gbps) and HDMI 2.1 (8K@60Hz, 48 Gbps) specifications. All cables are available in lengths from 25 to 150 feet and are UL listed.
Shop CL3 / CMP Plenum / HDMI Cables