What is plenum cable?
A plenum is a space that is part of a building's air distribution system, most often the gap above a drop ceiling or below a raised floor that serves as an air return path. Air in a plenum circulates through the building, so a cable fire there can carry flames and toxic smoke into occupied rooms quickly. Fire and building codes therefore require cable run through plenum spaces to be plenum-rated.
How plenum cable differs from standard cable
- Materials. The jacket and insulation use flame-retardant, low-smoke materials such as fluorinated ethylene propylene (FEP) or low-smoke PVC compounds.
- Standard cable uses an ordinary PVC jacket. PVC burns more readily and gives off dense, toxic smoke, including hydrogen chloride gas.
- Handling and cost. Plenum cable is stiffer and costs noticeably more than the PVC version of the same cable.
- Performance is the same. A plenum Cat 6 cable meets the same Cat 6 specifications as non-plenum Cat 6. The rating describes how the cable behaves in a fire, not its speed, category, or shielding.
NEC fire ratings
In the United States, the National Electrical Code (NEC) assigns fire ratings that are printed on the cable jacket. A higher-rated cable can replace a lower-rated one, never the reverse.
| Rating | Name | Where it can go |
|---|---|---|
| CMP | Communications plenum | Plenum spaces, and anywhere riser or general-purpose cable is allowed |
| CMR | Communications riser | Vertical runs between floors through shafts and risers, but not plenums |
| CM or CMG | Communications general purpose | Ordinary horizontal runs in walls and non-plenum ceilings |
| CMX | Communications limited use | Residential and other limited, low-exposure runs |
Fiber optic cable uses a parallel scheme: OFNP is nonconductive optical fiber rated for plenums, and OFNR is rated for risers. Coaxial and other low-voltage cables come in plenum-rated versions as well. Other countries use their own rating systems, so local code is what matters.
Where plenum cable is required
- Above drop ceilings and below raised floors that are used as air returns or supply spaces.
- In other air-handling spaces that local code defines as plenums.
- A drop ceiling that is not part of the air path, such as one where return air travels through sealed ducts, may not require plenum cable. Many organizations buy plenum cable for every ceiling run anyway, so no one has to work out which ceilings count.
The local fire code and building inspector have the final say. Running PVC cable through a plenum can fail inspection and force the cable to be pulled out and replaced, which costs far more than the price difference in cable.
Plenum vs. other cable ratings in practice
- Plenum (CMP): air-handling spaces, fire and smoke safety.
- Riser (CMR): vertical runs between floors, limits fire spread up a shaft.
- Direct burial: underground outdoor runs, resists water and soil. This is about environment, not fire.
- Outdoor-rated: resists sunlight and moisture on exterior walls or aerial runs.
A single cable run can need more than one property. Cable that enters a building from underground and then runs through a plenum ceiling, for example, must meet both requirements, or be spliced or terminated where it enters the building.
Plenum cable on the CompTIA A+ exam
Core 1 objective 3.2 lists plenum-rated under network cables, alongside copper categories, shielded and unshielded twisted pair, direct burial, and optical fiber. Know that plenum-rated means a fire-resistant, low-smoke jacket for air-handling spaces, that PVC is the cheaper non-plenum jacket and gives off toxic smoke, and that the rating does not change bandwidth or category. If a question describes running Ethernet above a drop ceiling used for return air, the answer is plenum-rated (CMP) cable. If the run goes underground between buildings, think direct burial instead.