Quick answer: Treat the cable as a resistor in series with the speaker. A common budget is five percent of the driver impedance, which is 0.4 ohm for an 8 ohm speaker and 0.2 ohm for a 4 ohm one. Loop resistance counts both conductors, so a ten metre run uses twenty metres of wire. Choose the gauge whose resistance stays under that budget.
Gauge advice usually arrives as a chart with no method behind it, and the next chart disagrees. The disagreement is not dishonest: each one assumes a different speaker impedance and a different acceptable loss. Once you choose the loss and do the arithmetic, the answer stops being a matter of opinion.
Set the budget before choosing a wire
Loss in the cable is a voltage divider between loop resistance and driver impedance. Allow five percent of the impedance and the level change stays small. Allow ten percent and the loss becomes measurable at the bottom end, where the impedance of most speakers dips lowest. Pick the percentage first and the gauge follows.
Loop resistance counts both conductors
Current leaves the amplifier on one conductor and returns on the other, so the resistance that matters is twice the one way length. A ten metre run is a twenty metre loop, and a chart built on single length will recommend a wire one size too small. Double the length, then compare with the budget.
Why a 4 ohm speaker is stricter than an 8 ohm one
The budget is a fraction of the impedance, so halving the impedance halves the resistance you can afford. A cable that costs an 8 ohm speaker four percent of its level will cost a 4 ohm speaker eight percent, which is why the same run needs a heavier gauge on the lower impedance load.
The impedance dip matters more than the nominal figure
Published impedance is an average. Most drivers drop well below their nominal value at some frequency, and that dip is where the cable takes the largest share. A budget built on the nominal number is therefore a floor, not a guarantee, which is another reason to keep the percentage modest.
Where copper clad aluminium changes the answer
Clad aluminium has higher resistance than bare copper at the same size, so it needs one gauge heavier to reach the same budget, and two for a long run. If the reel does not say bare copper, assume clad and size up.
Gauges that meet a five percent budget
| One way run | Loop length | Budget at 8 ohm | Gauge at 8 ohm | Gauge at 4 ohm |
|---|
| 5 m | 10 m | 0.4 ohm | 18 AWG | 16 AWG |
| 10 m | 20 m | 0.4 ohm | 16 AWG | 14 AWG |
| 20 m | 40 m | 0.4 ohm | 14 AWG | 10 AWG |
| 40 m | 80 m | 0.4 ohm | 10 AWG | 8 AWG |
Where the method stops being useful
Three cases fall outside the arithmetic. Short runs to a desk system meet the budget with almost any gauge, so flexibility and terminal fit decide. Installed runs inside a wall are governed by jacket rating, and the local code outranks any table. Runs from an amplifier already near its minimum load need the load fixed first, not the cable.
Practical scenarios
- Bookshelf on a desk: a two metre run stays inside budget with 18 AWG, so buy the flexible cable that fits the spring clip.
- Floorstanding pair across a room: six metre runs on 4 ohm speakers land on 14 AWG once the return conductor is counted.
- Basement to patio: twenty metre outdoor runs push toward 10 AWG, and the jacket rating matters as much as the copper.
- Subwoofer beside the rack: a three metre run at 4 ohm sits comfortably on 16 AWG, so put the budget into the connector instead.
FAQ
Is a heavier gauge ever a problem?
It is harder to bend, may not fit a spring clip, and costs more. Beyond the budget it buys nothing electrical, so there is no reason to oversize a short run.
Do these gauges hold for copper clad aluminium?
No. Clad aluminium has higher resistance at the same size, so move up one gauge, or two when the run is long and the speaker is 4 ohm.
Should both speakers use the same gauge?
Yes. Matching keeps both channels at the same loss and makes a later channel imbalance easier to trace, since the cable is no longer a variable.
Next step
Write your longest one way run and your speaker impedance on a note, then read the row above. The 12 AWG oxygen free wire page covers medium runs on low impedance loads, the 16 gauge 100 ft reel handles a whole apartment of short runs, and the 10 AWG heavy runs page suits long garden or basement feeds. Order the reel length that matches your total, including the returns.
See the matching cables and adapters
