Enter the run length and your speaker impedance. We work out the cable resistance, how much level you lose, what it does to damping factor, and the thinnest gauge that is still safe to use.
One speaker. Measure the actual cable path, not the straight line.
These are Amazon search links and we earn a small commission if you buy through them, at no cost to you. The gauge above is worked out from the arithmetic on this page, not from what pays best — and we do not link the sizes we have just told you are too thin.
| Gauge | Area | Round-trip resistance | % of impedance | Level lost | Damping factor |
|---|
Damping factor is what the speaker actually sees: impedance divided by everything in series with it, cable included. An amplifier claiming 500 is delivering well under 100 by the time the cable is in circuit.
Three formulas, no fudge factors:
R = 2 × ρ × L / A — the round-trip resistance of the run. ρ is
1.724×10-8 Ω·m for copper at 20 °C. The 2
matters: current travels to the speaker and back, so a 5 m run is 10 m of conductor. Plenty of
tables on the web forget it and tell you a gauge that is one size too thin.
d(mm) = 0.127 × 92^((36 - n) / 39) — this is the definition of American Wire
Gauge, not an approximation of it.
loss(dB) = 20 × log10( Z / (Z + R) ) — the cable and the speaker form a voltage
divider, so the cable keeps a share of the signal in proportion to its resistance.
Only in one place: the rule that cable resistance should stay under 5% of the speaker's impedance. That is the long-standing consensus figure, and it lands at roughly 0.4 dB of loss — below the point anyone can reasonably claim to hear. The 2% option is the cautious version. Everything else on this page is arithmetic.