Cable Jacket Ratings Explained: CMP, CMR, CMG, CM, CMX, LSZH and Direct Burial, Where Each Is Allowed by NEC 800.113 and Article 725
Sep 10, 2026
Summary: Communications cable jackets are rated by how they burn. CMP (plenum) passes the NFPA 262 flame and smoke test and is required in air-handling spaces above suspended ceilings and under raised floors. CMR (riser) passes the UL 1666 vertical shaft test and is required in shafts and runs between floors. CM and CMG (general purpose) pass UL 1685 and are for horizontal runs within one floor. CMX is for limited use, residential dwellings and exposed lengths under 10 ft, and is the rating on most outdoor-jacketed cable. Any higher rating substitutes for a lower one: CMP anywhere, CMR where CM is required, never the reverse. LSZH is a European low-smoke, halogen-free jacket with no NEC rating of its own; in the US it must also carry a CM rating to be installed. Direct-burial and outdoor cables are typically CMX or unrated and must transition to a rated cable within 50 ft of entering the building (NEC 800.48).
The ratings and their tests
| Marking | Name | Flame test | What it proves | Required in |
|---|---|---|---|---|
| CMP | Communications plenum | NFPA 262 (Steiner tunnel): flame spread under 5 ft, peak smoke under 0.5 optical density | Low flame spread and low smoke in a moving-air space | Plenums: above suspended ceilings and below raised floors used for environmental air, NEC 800.113(C) |
| CMR | Communications riser | UL 1666: vertical shaft test, flame must not reach 12 ft | Will not carry fire floor to floor in a shaft | Vertical runs penetrating more than one floor, 800.113(D) |
| CMG | General purpose | UL 1685 vertical tray, FT4 in Canada | Limited flame spread in a horizontal run | Anywhere not plenum or riser, 800.113(E) |
| CM | General purpose | UL 1685 vertical tray | Same class as CMG | Same as CMG |
| CMX | Limited use | UL 1581 VW-1 vertical wire | Self-extinguishing single cable | One- and two-family dwellings, or runs under 10 ft exposed in other buildings, 800.113(H); most outdoor and direct-burial jackets |
| CMUC | Under carpet | UL 1685 | Flat cable under carpet squares | Under carpet only |
| OFNP / OFNR / OFN | Optical fiber nonconductive | Same tests as CMP / CMR / CM | Fiber equivalents | Same spaces as the CM equivalents, Article 770 |
| CL2P / CL2R / CL2, CL3P / CL3R / CL3 | Class 2 and Class 3 power-limited | Same tests as CMP / CMR / CM | Thermostat, speaker, security, HDMI in-wall | Same spaces, Article 725; CM may substitute for CL3 and CL2 |
| LSZH / LSOH | Low smoke zero halogen | IEC 60754, IEC 61034 | Low toxic gas and smoke; a material description, not an NEC rating | Not recognized alone; must also carry CM, CMR or CMP to be installed under the NEC |
Substitution hierarchy
NEC 800.154 and Table 800.154(a) allow a cable of a higher rating to be used wherever a lower rating is permitted: CMP replaces CMR, CMG, CM and CMX; CMR replaces CMG, CM and CMX; CMG and CM replace CMX. The reverse is never allowed. In practice, stocking CMR for everything but plenum spaces covers a job with two SKUs.
Where each rating goes
| Location | Minimum rating | Common choice |
|---|---|---|
| Above a drop ceiling that is a return-air plenum (most commercial offices) | CMP | CMP Cat6, CMP RG6 |
| Above a drop ceiling with ducted return (not a plenum) | CM | CMR, because the crew cannot always tell |
| Under a raised data-center floor used for cooling air | CMP | CMP |
| Vertical shaft or chase between floors | CMR | CMR |
| Inside walls, single floor, commercial | CM | CMR |
| Inside walls of a one- or two-family home | CMX | CMR (cheap insurance and inspectors like it) |
| Exposed run under 10 ft entering a room | CMX | CM |
| Outdoor aerial or on a wall | CMX outdoor, UV jacket | Outdoor CMX, transition within 50 ft of entry |
| Direct burial | Direct-burial rated (gel or flooded), usually CMX or unrated | Gel-filled Cat6 DB, RG6 flooded; transition at entry |
| Conduit outdoors or underground | Wet-location rated conductor; CMX outdoor or DB | Outdoor CMX in PVC |
| Thermostat and HVAC control | CL2 (CL2P in plenum) | CL2R thermostat cable |
| Speaker and audio in-wall | CL2 or CL3 (P in plenum) | CL3R |
| HDMI in-wall | CL2 or CL3 (P in plenum) | CL3-rated HDMI |
Reading a jacket
A legend like CAT 6 UTP 23 AWG 4PR (UL) CMR C(UL) FT4 550 MHz tells you: category, shielding, conductor size, pair count, the UL rating (CMR), the Canadian rating (FT4 equals CMG/CMR class), and bandwidth. A jacket that reads only CMX Outdoor or CMX-Outdoor UV is for exterior use and the 50 ft rule applies inside. A jacket with no CM marking at all is not a listed communications cable and cannot be installed in a building under the NEC, regardless of what the box says.
CCA and the jacket
Copper-clad aluminum Cat5e and Cat6 is sold with CM and even CMR markings that are frequently not backed by a UL listing; genuine CMR requires the UL holographic label and the file number on the box. CCA fails the DC resistance limits of the TIA category standard and the ampacity assumptions of PoE. Conversions Tech stocks bare-copper category cable only; every reel carries the UL file number and the rating for the space it is going into.
Quick answers
Can I use CMR in a plenum? No. Plenum spaces require CMP. CMR may be used anywhere CM or CMX is permitted.
Is a drop ceiling always a plenum? No. It is a plenum only when the space above it is used to move environmental air, typically as a return. If it is, CMP is required for every cable in it.
Is LSZH the same as plenum? No. LSZH describes the jacket material's smoke and halogen behaviour under IEC tests; it has no NEC rating and must also be marked CMP to be used in a plenum in the US.
How far can outdoor cable run inside a building? 50 ft from the point of entrance to the point where it terminates or transitions to a rated cable, per NEC 800.48.
Conversions Tech stocks Cat6 and Cat6A in CMP, CMR and outdoor CMX, Cat5e CMP and CMR, RG6 in CMR, CMP, dual and quad shield and direct burial, and CL2R thermostat cable. See Cat6 vs Cat6A vs Cat8 for the category choice.
