Quick answer: Use CL3 rated speaker wire in residential walls, CL2 in commercial walls, and a plenum-rated cable in air-handling spaces. Choose 16 or 14 AWG for most runs, protect the cable with conduit or a plate, and leave a service loop at both ends.
In Wall Speaker Wire: CL2, CL3 and Plenum Rules
In wall speaker wire is judged by two standards at once: whether it carries the signal and whether it is legal to put inside a wall. The electrical part is the same as any speaker cable. The legal part depends on the jacket marking. A CL3 cable is tested for flame spread in a residential wall, CL2 for commercial, and a plenum cable for the air-handling cavity above a ceiling. Installing the wrong jacket can fail an inspection and may void insurance, even if the sound is perfect.
What the CL Ratings Mean
CL2 and CL3 are cable listings for communication and low-voltage circuits in walls. The number refers to the cable class and the flame test it has passed; CL3 is tested to a higher flame spread requirement than CL2. Neither rating describes sound quality. A cable without a CL marking is not listed for in-wall use, even if it is thick and well insulated. The marking must be printed on the jacket, because an inspector will look at the cable, not the packaging.
CL2 and CL3
CL2 is common in commercial low-voltage work and CL3 in residential. Many cables carry both listings. Local code decides which is acceptable in a given wall, so check before installation rather than after inspection.
Plenum and Riser
A plenum space is a ceiling or floor cavity used for air return. Cable there must pass the NFPA 262 Steiner tunnel test and is marked CMP or plenum. A riser between floors may require a riser-rated cable. Standard CL3 is not a substitute.
Gauge and Route Length
In-wall runs are often longer than they look because the cable follows studs, joists and plates. A run that is 8 m in a straight line can be 14 m through the wall. Use 16 AWG for 8 ohm speakers up to about 20 m and 14 AWG beyond that. For 4 ohm speakers, use 14 AWG and keep the run under about 15 m. The gauge must also fit the wall plate and the speaker terminal.
Measure the Route
Measure along the path, not through the air. Add 1 m at each end for a service loop. A cable that is 50 cm short cannot be stretched, and a splice inside a wall is not acceptable.
Gauge and Termination
12 or 14 AWG is harder to fit into a wall plate and a spring clip. Check the plate rating before buying. A plate rated for 14 AWG will not accept 10 AWG without trimming.
Conduit, Plates and Protection
Run the cable through conduit or a smurf tube where possible, even if the jacket is rated. Conduit protects the cable from nails and allows a future upgrade without opening the wall. Use a wall plate or an outlet box at each end so the cable is not left dangling through a hole. Keep the cable away from mains wiring, and cross at right angles where paths intersect. Label both ends with the room and channel.
Conduit and Bushings
Use a bushing at every box entry to protect the jacket. Fill conduit to no more than about 40 percent of its area. Leave a pull line for the next cable.
Wall Plates and Boxes
A wall plate gives a finished connection and protects the cable end. Use a plate with binding posts or spring terminals rated for the gauge. Label the plate with the channel and polarity.
Inspection and Testing
Before closing the wall, photograph the route and check the jacket marking. Test continuity and polarity at both ends, and measure loop resistance against the expected value. A reading that is much higher than expected suggests a damaged conductor or a poor termination. After the wall is closed, test again at the plate. Keep the photographs and the cable specification for the inspection record. If the inspector cannot see the jacket print, the photograph is the evidence that the cable was rated for the location.
Photograph the Route
A photo of the cable before drywall shows where it runs and proves the jacket rating. It also helps the next installer avoid a nail through the cable.
Test at the Plate
Measure from the amplifier end to the plate, then from the plate to the speaker. A fault between the plate and the speaker is easier to fix than one inside the wall.
In-wall cable ratings and use
| Rating | Wall type | Plenum | Typical jacket | Note |
|---|---|---|---|---|
| CL2 | Commercial | No | PVC | Low-voltage class 2 |
| CL3 | Residential | No | PVC | Higher flame test |
| CMP | Any | Yes | Low-smoke | Air-handling spaces |
| CL3P | Residential | Yes | Low-smoke | Combined rating |
| Unrated | None | No | Zip cord | Not legal in wall |
Real-World Application Scenarios
A home theater pre-wire uses 14/2 CL3 cable in smurf tube for the surrounds, with a wall plate at each end. An office installs 16/2 CL2 cable in a commercial partition and uses a plenum-rated cable in the ceiling return air space. A renovation reuses an existing conduit by pulling a new 12/2 CL3 cable with a pull line left in place. A landlord wiring a rental unit photographs every wall cavity before drywall, so the next installer can see the route and the jacket marking.
Related Guides on This Site
For boxes and plates, read the in-wall speaker outlet box guide and the speaker wire wall plate guide. The wall plate installation checklist is in speaker cable wall plate, and gauge selection in the speaker wire gauge guide.
Frequently Asked Questions (FAQ)
Q: What does CL3 mean on speaker wire?
A: It is a cable listing for low-voltage wiring in walls, tested for flame spread. CL3 is common in residential walls. It does not describe sound quality or gauge.
Q: Can I use ordinary speaker wire in a wall?
A: No. Unrated zip cord is not listed for in-wall use and can fail inspection. Use CL2 or CL3, and a plenum rating where the cavity handles air.
Q: Do I need conduit for in-wall speaker wire?
A: Code may not require it, but conduit protects the cable from nails and allows a future upgrade. It is worth the small extra cost, especially in a finished wall.
Q: What gauge for in-wall speakers?
A: 16 AWG for 8 ohm runs up to about 20 m and 14 AWG beyond that or for 4 ohm speakers. Check that the wall plate and speaker terminal accept the gauge.
Q: Can I splice speaker wire inside a wall?
A: No. A splice must be accessible in a box. If the cable is short, run a new length or add an accessible junction box. A hidden splice is a fire and reliability risk.