Plenum vs Riser Cable: California Listing Rules That Get a Job Red Tagged

What actually counts as a plenum?#

The word gets used loosely on job sites, and that is where the money gets lost.

The electrical code defines a plenum as a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system. The rule that catches most commercial cabling is the one covering other spaces used for environmental air: spaces that were not specifically fabricated for air handling but are used for air handling as a plenum. The classic example is the cavity above a suspended ceiling that carries return air back to the air handler. A raised floor plenum in a data center is another.

What matters is how the building moves air, not what the space looks like. Two buildings with identical ceiling grids can be different: if the return air in one is fully ducted back to the unit, the ceiling cavity is not an air handling space and plenum listed cable is not required there. If the other pulls return through the ceiling cavity, it is. The same building can differ floor by floor, and a tenant improvement can convert a space when the mechanical scope changes.

Because of that, the mechanical drawings decide. Ask the mechanical engineer, in writing, which spaces on which floors are used for environmental air. Do not take it from the architectural reflected ceiling plan, and do not take it from the last job you did in that building. If the answer does not arrive before bid, carry plenum. Guessing wrong toward plenum costs money on material. Guessing wrong away from plenum costs a rip out.

One code point explains a recurring argument. The communications chapter stands on its own except where it references the other chapters, so the permission to run plenum listed communications cable in an air handling space comes from the communications articles, not from the general wiring methods article. An electrician quoting one and a cabling contractor quoting the other can both be right and still disagree about the same ceiling.

What do the copper cable listings mean?#

Communications cable listings are graded by fire performance, and the letters describe where the cable is allowed to go.

Listing	Name	Where it belongs
CMP	Communications plenum	Spaces used for environmental air, including ceiling cavities carrying return air and raised floor plenums
CMR	Communications riser	Vertical runs between floors, shafts, and floor penetrations
CM and CMG	Communications, general purpose	Horizontal runs within a floor and general use where a higher listing is not required
CMX	Communications, limited use	Restricted to dwelling units and to short lengths under the code’s specific conditions; not for commercial building work

The plenum test measures flame spread and smoke development in a horizontal air handling space. The riser test measures whether fire propagates up a vertical shaft from floor to floor. General purpose cable is qualified against a general flame test. Each listing answers the fire behavior of a specific geometry.

Substitution runs one direction. CMP may be used where CMR, CM, CMG or CMX is required. CMR may be used where CM, CMG or CMX is required. CM and CMG may be used where CMX is required. Nothing substitutes downward. Put another way, a higher listing is always acceptable and a lower one never is.

One more marking matters on a Power over Ethernet job. Cables listed with an LP suffix, such as CMP-LP or CMR-LP, carry a current rating printed on the jacket for the current per conductor the cable can carry in a bundle without exceeding its temperature rating. On buildings with PoE lighting, high power access points or pan-tilt-zoom cameras, the LP marking satisfies the code’s power delivery provisions without running the bundle count and ampacity table. Our Cat6 and Cat6A article covers the thermal side.

Which fiber and coax listings match which spaces?#

Optical fiber and coaxial cables follow the same plenum, riser, general purpose pattern with different letters.

Optical fiber cable is split first by whether it contains metallic members. Nonconductive cable, marked OFN, has none. Conductive cable, marked OFC, contains metallic strength members, armor or a metallic vapor barrier, which brings bonding and grounding obligations that nonconductive cable avoids. Then the space letter is appended: P for plenum, R for riser, G for general purpose, with plain OFN and OFC as the general purpose types.

Space	Copper communications	Optical fiber	Coaxial (CATV)
Space used for environmental air (return air ceiling cavity, raised floor plenum)	CMP	OFNP or OFCP	CATVP
Vertical shaft, riser, floor to floor penetration	CMR or better	OFNR, OFCR or better	CATVR or better
Horizontal run inside a floor, general commercial use	CM, CMG or better	OFNG, OFCG, OFN, OFC or better	CATV or better
Duct specifically fabricated for environmental air	Not a general cable location; confirm the permission and the AHJ position first	Same	Same
Dwelling unit, limited use conditions only	CMX	Not applicable	CATVX

Coaxial distribution in multifamily and hospitality buildings runs under the community antenna television article, and the listings mirror the copper set: CATVP for plenum, CATVR for riser, CATV for general purpose and CATVX for limited use. In practice a building with a plenum ceiling gets CATVP in the ceiling and CATVR in the riser closet stack. Our CATV and MDU distribution page covers how that gets laid out.

Class 2 and Class 3 control cable has its own set of listings, CL2P through CL3X, following the same pattern. On most commercial jobs the four pair communications cable already in the ceiling is used for Class 2 circuits under the code’s substitution provisions, which is why an access control or building automation drop usually looks identical to a data drop.

Where does each listing belong in a building?#

A practical walk through a typical commercial building, floor by floor:

• Ceiling cavity used for return air: plenum listed cable for every system in that cavity, including data, camera, access control, intercom, speaker, clock and coax.
• Ceiling cavity with fully ducted return: general purpose is code compliant. Many owners still specify plenum throughout so that a future mechanical change does not create a compliance problem, and many specifications simply require plenum everywhere for that reason.
• Riser shaft and sleeves between floors: riser listed at minimum. Plenum satisfies it.
• Inside an equipment room, IDF or MDF: general purpose is usually acceptable unless the room itself is in the air handling path or the specification is stricter.
• Under a raised floor in a data center: plenum listed, unless the space is not part of the air handling system.
• Concealed in walls on a single floor: general purpose.
• Exterior, underground or between buildings: outside plant listed cable, subject to the code’s limits on how far unlisted outside plant cable may enter a building before it transitions to a listed cable. Show the entrance point and the transition on the drawings.

Write the estimating point on the takeoff. Plenum cable costs more per foot, is stiffer and pulls differently. Buying plenum where it is not needed burns budget on every drop; buying general purpose where plenum is required means pulling it back out. Neither error is recoverable late, which is why the mechanical answer belongs in the bid file. Our structured cabling cost article puts California planning ranges around the drop.

What do the separation rules require?#

Communications and Class 2 conductors are generally not permitted to occupy the same raceway, cable, cable tray compartment, outlet box, junction box or similar enclosure with electric light and power conductors, except under the specific conditions the code allows, such as a permanent barrier or where the power conductors serve the same equipment. The code also sets a separation distance for communications conductors run in parallel with open power conductors where neither is in a raceway.

The field version of that rule is short: do not share a box, a conduit or a divider-free tray section with line voltage, and keep parallel runs off the power raceway. On a job with an existing tray, look for the barrier before you pull, because a tray that a previous contractor treated as shared is a finding waiting to happen.

A second reason to keep distance is performance. Even where the code permits proximity, running unshielded twisted pair alongside a lighting branch circuit or a drive feeder degrades the link, and separation is cheaper than shielded cable and a bonding scheme.

Why does the firestop system number matter?#

This is the item most often missed in a closeout package and most often cited by an inspector.

Where a cable penetrates a fire rated wall, floor, shaft or corridor, the penetration has to be sealed with a firestop system that restores the rating of the assembly. The critical word is system. A product has no rating on its own. The rating belongs to the complete tested assembly: the specific rated construction, the specific penetrating item, the sleeve if there is one, the annular space, and the firestop material, tested together under UL 1479 or ASTM E814. Those tests produce an F rating, which measures how long the system prevents flame passage, and a T rating, which measures how long the unexposed side stays below a temperature rise limit.

Change any element of the tested condition and you are outside the system. A system tested with steel conduit does not cover PVC. A system tested for a 4 inch sleeve at a given fill does not cover a 6 inch sleeve packed full. A system tested in a concrete floor does not cover a gypsum shaft wall. Listed systems are published by penetration type, and the discipline that keeps a job out of trouble is choosing the system first and then installing to it, rather than sealing the hole and looking for a system that matches.

Three habits make this manageable. Pick the systems during submittals, one per condition, and get them approved. Photograph each penetration before it is sealed, with a tag showing the system number. Deliver the system numbers by location in the closeout package. On DSA and HCAI projects the inspector will ask, and our DSA and HCAI page explains why that package controls certification.

What has to happen to abandoned cable?#

The electrical code defines abandoned cable as cable that is not terminated at equipment and is not identified for future use with a tag, and it requires the accessible portion of abandoned cable to be removed. The same rule appears for communications cable, optical fiber and coaxial cable in their respective articles. Cable in a concealed raceway is not accessible and does not have to come out.

Two consequences follow on renovation work. The tag is the whole difference between abandoned cable and cable identified for future use, and tagging costs almost nothing compared with removal. And the removal obligation belongs to the project, yet it is regularly left out of the bid because nobody counted the legacy plant. On an occupied building with thirty years of cable above the ceiling that is a scope of its own, and it deserves a separate line rather than an assumption. Our cable removal and decommissioning page covers how we scope it.

One California note on citations. The current California Electrical Code, Title 24 Part 3, follows the 2023 National Electrical Code, which added Article 722 for cables used in power limited circuits and fault managed power circuits while leaving optical fiber in Article 770, split Class 1 circuits into their own article, retitled the Class 2 and Class 3 article, and consolidated communications cable listing requirements into the general communications article. A specification written against the previous edition may cite section numbers that no longer point where the writer intended. Check every citation against the edition the AHJ has adopted, and where a specification and the adopted code disagree, ask the engineer for a written clarification rather than picking one.

Next step#

Send us the mechanical narrative or the return air approach, the floor plans and the device schedule, and we will return a takeoff with the listing called out by space so nothing gets bought twice. Start at request a bid. For how we build and certify the plant itself, see the structured cabling and fiber optic cabling pages.