CMR vs CMP Cable Explained

The jacket print on a spool of Cat6, coax or alarm cable usually carries a short code: CMP, CMR, CM or CMX. On a commercial job that code decides whether the cable can go above a return-air ceiling or up a riser shaft, and an inspector will read it. This is CMR vs CMP cable explained from the code text itself: what each listing means, which fire test sits behind it, where the NEC lets you install it and which cable may stand in for another.

Section numbers below are from the 2023 edition of NFPA 70, the National Electrical Code. Check which edition your jurisdiction has adopted before you rely on a number; more on that near the end.

CMR vs CMP at a glance

The sections below explain each column and cite the code text behind it.

What the CM letters mean

CM stands for communications cable. The letter after it tells you which fire environment the cable is listed for:

• CMP: plenum. Listed for ducts, plenums and other spaces used for environmental air.
• CMR: riser. Listed for vertical runs in a shaft or from floor to floor.
• CM and CMG: general purpose. Listed for general use except risers and plenums.
• CMX: limited use. Listed for use in dwellings and in raceway.

Those definitions come from NEC 800.179(A) through (D). The same section requires communications cables to be listed, rated at not less than 300 volts and rated at not less than 60°C (140°F), and it adds an “-OF” suffix when a cable also contains optical fibers. The product standard the code points to for communications cable is UL 444, Standard for Communications Cables.

The fire tests behind each rating

The NEC describes each listing by what the cable must resist, then names a test in an informational note as one way to show it. UL groups these large-scale tests by application: NFPA 262 for plenum, UL 1666 for riser and UL 1685 for vertical tray.

CMP: NFPA 262

A plenum cable must have “adequate fire-resistant and low-smoke-producing characteristics.” The NEC note cites NFPA 262, under which the cable must show a maximum peak optical density of 0.50 or less, an average optical density of 0.15 or less and a maximum flame spread distance of 1.52 m (5 ft). The NFPA 262 committee states the same pass/fail limits. CMP is the only listing in the family whose NEC definition includes a smoke limit, which is why it is the only one allowed loose in an air-handling space.

CMR: UL 1666

A riser cable must have “fire-resistant characteristics capable of preventing the carrying of fire from floor to floor.” The note cites UL 1666, which UL describes as a test of flame propagation height for cables installed vertically in shafts or in vertical runs that penetrate one or more floors. The NEC riser listing says nothing about smoke, so a riser cable is not a plenum cable.

CM and CMG: vertical tray

General-purpose cable must be “resistant to the spread of fire.” For Type CM the note cites the UL flame exposure in UL 1685, with cable damage (char length) not exceeding 244 cm (8 ft 0 in). For Type CMG it cites the CSA FT4 vertical tray flame test or the FT4 test in UL 1685, with char length not exceeding 1.5 m (4 ft 11 in). In both cases the code says the smoke measurements in the test are not applicable.

CMX: VW-1

Limited-use cable must be “resistant to flame spread.” The note points to the FV-2/VW-1 flame test in UL 2556, a test on a single cable rather than a tray or shaft full of them.

Where each cable may be installed

The permitted uses are spelled out in NEC 800.113 and Table 800.154(a). The short version for the four common types:

Plenums (other spaces used for environmental air)

Only plenum cable may be installed on its own. Riser, general-purpose and limited-use cables are not permitted there unless they are inside a raceway installed in compliance with 300.22(C), or in solid-bottom metal cable tray with solid metal covers. Ducts specifically fabricated for environmental air have tighter rules again, in 800.113(B).

Risers (vertical runs through one or more floors, or in a shaft)

Plenum and riser cables are permitted. General-purpose and limited-use cables are not. There are three exceptions in the code:

• In a metal raceway in a riser with firestops at each floor, plenum, riser, general-purpose and limited-use cables are all permitted.
• In a fireproof riser shaft with firestops at each floor, the same four types are permitted.
• In one- and two-family dwellings, plenum, riser and general-purpose cables are permitted in risers, and so is limited-use cable less than 6 mm (0.25 in) in diameter.

Everywhere else in the building

Plenum, riser and general-purpose cables are permitted in other building locations. Limited-use cable is permitted with a maximum of 3 m (10 ft) of exposed length in nonconcealed spaces, or inside a raceway recognized in Chapter 3. Limited-use cable may not be supported by cable trays or installed in distributing frames and cross-connect arrays.

Penetrations still need firestopping whatever the cable type; the riser rules refer you to 800.26 for floor penetrations.

The substitution hierarchy

Table 805.154 lists the permitted substitutions for communications cable:

• CMR may be replaced by CMP.
• CMG and CM may be replaced by CMP or CMR.
• CMX may be replaced by CMP, CMR, CMG or CM.

CMP has no substitute. Read as a ladder, CMP sits at the top, then CMR, then CM and CMG, then CMX. A cable may always go down the ladder into a less demanding location, never up. A CMP cable is legal in a riser; a CMR cable is not legal loose in a plenum.

CMR vs CMP cable explained on the job

Most of the decision comes down to two questions about the pathway, not the cable.

1. Does any part of the run pass through a space used for environmental air? A ceiling cavity or raised floor that the building uses for return or supply air is a plenum, whatever it looks like. If the run is in one and is not in a qualifying raceway, it needs CMP. Not every drop ceiling is a plenum, so ask the mechanical engineer or the inspector rather than guessing.
2. Does the run go vertically through a floor or up a shaft? In a commercial building that needs CMR at minimum, unless the cable is in a firestopped metal raceway or a fireproof shaft. CMP also satisfies it.

If the answer to both is no, general-purpose CM is permitted, and in a house CMX may be, within its limits. A run that crosses more than one type of space must meet the most demanding one along its length.

Two habits save callbacks. Read the legend printed on the jacket, not the box, because the jacket is what an inspector checks after the box is gone. And if you buy bulk low voltage cable wholesale for mixed commercial work, decide up front whether one plenum-rated spool covering every location is simpler for your crews than carrying several ratings.

Which code edition applies

The section numbers above are from the 2023 NEC. NFPA’s own committee documents for the newer edition show communications circuits included in the title of Article 722, so in a jurisdiction that has adopted the 2026 NEC the same requirements are likely to sit under different numbers. States and cities adopt editions on their own schedule, and many amend them. The authority having jurisdiction decides what is acceptable on your job. This article is a summary of published code text for working installers, not legal or engineering advice.

Frequently asked questions

Where to buy bulk cable through Bursma

Our Bulk Wire category sits inside the Wire, Cables and Connectors division. Check the listing on each product page or on the jacket legend for its CM rating before you order for a specific pathway. Dealers see their own pricing and stock levels in Shop Online after logging in.

Not a Bursma dealer yet? Open a dealer account, or call 616-831-0080 and ask for a sales representative.

Sources

• NFPA 70 (NEC), Code-Making Panel 16 public input report for the 2026 cycle, reproducing the 2023 Chapter 8 text: 800.113, Table 800.154(a), 800.179 and Table 805.154
• NFPA 262 Technical Committee second-revision ballot (pass/fail requirements and NFPA 70 article references)
• UL 1666, Standard for Test for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shafts (scope)
• UL, Wire and Cable Flammability Testing Services (NFPA 262, UL 1666, UL 1685 by application)