Quick Answer / Summary: Size amplifier cable by current, not by reputation: 16 AWG for modest power at short runs into 8 ohms, 14 AWG as the practical default, 12 AWG for high power, 4-ohm loads, or runs over 25 metres. Class D amplifiers reward low-resistance runs for the same reason as linear ones. Bridged outputs double the voltage swing and demand the next gauge up.
The amplifier end of the cable decision is about current: how much the amplifier can deliver, into what impedance, and how much resistance the run may add before damping suffers. Three inputs – power, class, impedance – produce one output: gauge and construction.
Power, Impedance and the Current Math
Current into a speaker is the square root of power divided by impedance: a 100-watt amplifier into 8 ohms delivers about 3.5 amps, and the same amplifier into 4 ohms about 7. The cable’s resistance sits in series with that load, so the lower the impedance and the higher the current, the more the run’s resistance matters. The gauge table follows directly – heavier copper for lower impedance and higher power.
Damping Factor in One Line
Damping factor is the load impedance divided by the total source resistance, including the cable. A long 16 AWG run into 4 ohms can halve the effective figure, and the audible result is looser bass. Shortening the run or stepping the gauge restores what the amplifier was designed to deliver.
Class D and the Output Filter
Class D amplifiers switch at high frequency and filter at the output. Their behaviour into capacitive or unusually reactive loads varies by design, and some specify a maximum cable capacitance. For most domestic Class D designs, ordinary low-resistance copper is ideal; where the manual states a capacitance limit, respect it.
Bridged, Bi-Amped and Multi-Channel Cases
Bridged outputs combine two channels to double voltage, which doubles the current demand for a given power – one gauge step is the safe response. Bi-amping splits the load between two amplifier channels and removes the passive crossover’s insertion losses from the equation, but each run still needs correct gauge for its band and matched length for the pair. Multi-channel installations multiply the arithmetic; the per-channel rule does not change.
Terminal Safety at the Amplifier
Bare wire at the amplifier end is where shorts happen: a stray strand bridging outputs can trigger protection or damage a channel. Twist tightly, insert fully, and check with a torch before powering up. Banana plugs reduce the risk where terminals accept them.
When the Cable Is Not the Problem
A channel that distorts at volume, or an amplifier that trips protection, is more often a load or ventilation issue than a cable one. Measure the speaker’s DC resistance to rule out a failing driver, then check the cable’s continuity and terminations before replacing it.
Matching the Pair, Not Just the Gauge
Left and right runs should share gauge, construction and length. Mismatched runs shift high-frequency behaviour slightly between channels, and the stereo image suffers before any measurement shows it. Cut both from one spool and treat the pair as a single component.
Amplifier-to-Speaker Gauge Table
| Situation | Gauge | Note |
|---|---|---|
| Up to 50 W, 8 ohm, under 10 m | 16 AWG | Typical domestic baseline |
| 50-150 W, 8 ohm, to 25 m | 14 AWG | Practical default |
| High power or 4 ohm loads | 12 AWG | Damping preserved |
| Runs over 25 m | 12 AWG or heavier | Resistance dominates |
| Bridged output | One step heavier | Double voltage swing |
Practical Applications
Matching cable to a new integrated amplifier, planning runs for a 4-ohm floorstander, wiring a bridged subwoofer amplifier, and diagnosing a protection trip that a cable swap will not fix.
Frequently Asked Questions
Does a more powerful amplifier need thicker cable?
Yes – more power means more current, and current is what the cable’s resistance opposes. Step up a gauge for high-power amplifiers, 4-ohm loads, or long runs.
Do Class D amplifiers need special cable?
Generally no – ordinary low-resistance copper is ideal. If the manual states a maximum cable capacitance, respect that figure; otherwise follow the gauge table.
Why does my amplifier trip protection at high volume?
Usually load or thermal, not cable. Check the speaker’s DC resistance, ventilation, and terminal shorts before replacing the wire – a cable fault would typically show as a dead or distorted channel, not a protection event.