Speaker wire is a two-conductor unshielded cable carrying an amplifier's output to a loudspeaker. It sits outside the NEC power wiring rules — it is a Class 2 or Class 3 circuit under Article 725 — but two things about it are governed by code: anything run inside a wall must carry a CL2 or CL3 fire rating, and in a plenum space it must be CL2P or CL3P.
| Property | Value |
|---|---|
| Typical sizes | 18, 16, 14 and 12 AWG |
| Conductor | Oxygen-free copper (OFC) or copper-clad aluminum (CCA) |
| Insulation | PVC, usually clear or with a polarity stripe |
| Shielding | None — speaker level is high enough to be immune to most noise |
| In-wall rating | CL2 or CL3 required (NEC 725.179) |
| Plenum rating | CL2P or CL3P required in air-handling spaces |
| Outdoor / burial | Requires direct-burial rated speaker cable |
| NEC article | Article 725, Class 2 and Class 3 circuits |
| Voltage | Low voltage; CL2 is rated 150 V, CL3 is rated 300 V |
The engineering rule of thumb is to keep the total resistance of the wire run below about 5% of the speaker impedance. That keeps the damping factor intact and the loss inaudible. For an 8-ohm speaker that budget is 0.4 ohms for the round trip; for a 4-ohm speaker it halves to 0.2 ohms, which is why low-impedance speakers need heavier wire over the same distance.
In practice: 16 AWG covers most 8-ohm runs up to about 50 feet. 14 AWG takes you to roughly 80 feet, or 40 feet at 4 ohms. 12 AWG handles long runs, in-wall installations you cannot easily change later, and any 4-ohm speaker past about 60 feet. 18 AWG is fine for short surround or bookshelf runs under 25 feet but is not worth the marginal saving anywhere else.
Beyond that, gauge is the only variable that measurably matters. Exotic conductor geometry, directional marking and cryogenic treatment have no measurable effect on a speaker-level signal, and the difference between a good 12 AWG cable and an expensive one is not audible.
OFC is oxygen-free copper — solid copper strands. CCA is copper-clad aluminum: an aluminum core with a thin copper plating, sold at a fraction of the price and often labelled only in small print.
Aluminum has roughly 61% of the conductivity of copper, so a 16 AWG CCA cable behaves electrically like about 18 AWG copper. If you buy CCA, size up two gauges to compensate. CCA is also more brittle, more prone to breaking at terminations, and corrodes at the copper-aluminum interface over time in humid environments.
For a short run to a bookshelf speaker it makes little practical difference. For an in-wall run you will never see again, buy OFC.
NEC 725.179 requires cable installed inside building walls, floors or ceilings to be listed and marked CL2 or CL3. The difference between them is only voltage rating — 150 V versus 300 V — and both are far beyond what a speaker circuit carries; either is acceptable. The rating is about flame spread and smoke, not about audio.
In a plenum — an air-handling space above a suspended ceiling or under a raised floor — you need CL2P or CL3P, which use low-smoke insulation. For risers between floors, CL2R or CL3R.
Outdoors and buried, use cable specifically listed for direct burial with a UV-stable jacket. Ordinary CL2 speaker cable in a garden trench will fail within a few seasons.