AUD-02

Speaker & Coax Cable Sizing

IEC 60268 · NEC 725 · NEC 820 · NEC 310.15(B) · ANSI/AVIXA A102.01

01 · Circuit type

Cable to size

Awaiting input1 field left: Cable to size

Capacity and layout

The drawing appears when the inputs are complete

01
Size loudspeaker cable from run length, load impedance, acceptable power loss and required damping factor, or check coaxial RF/SDI attenuation against a length and frequency.
02
Tip. On low-impedance runs, damping factor usually demands a heavier conductor than power loss does. Cable resistance sits in series with the loudspeaker, so it degrades the amplifier’s control of the cone long before it wastes an audible amount of power.
03
0.5 dB — Common loss budget
04
70.7 / 100 V — Constant-voltage lines
05
≥ 20 : 1 — Damping factor target
06
5 % — Loss at 0.22 dB

Conductor sizing and signal loss

Cable resistance sits in series with the load, so it both wastes power and degrades the amplifier’s control over the loudspeaker. Two independent criteria therefore apply to every run, and the larger conductor of the two wins. Coaxial runs face a different limit: attenuation that rises with frequency and length.

Loudspeaker cable

Low-impedance runs are usually governed by damping factor rather than by power loss. Constant-voltage lines are transformer-coupled, so only loss applies.

Use cases

0.5 dB — broadcast and critical listening
1 dB — general AV and commercial (AVIXA)
2 dB — background music and paging
70 V / 100 V lines — loss only, damping not meaningful

Formula

Max loop resistance = Z × (10^(dB ÷ 20) − 1)

IEC 60268 · NEC 640/725

Coaxial cable

Attenuation in coax rises with the square root of frequency and linearly with length, so the highest frequency carried sets the maximum run.

Use cases

SDI and RF runs — check at the highest carried frequency
Longer runs need lower-loss cable, not more power
Return loss and impedance match matter as much as length
NEC Article 820 governs coaxial installation

Formula

Loss (dB) = attenuation per 100 m × length ÷ 100

NEC 820

NEC classification depends on the amplifier, not the cable. Wiring is Class 2 only where the amplifier is a listed Class 2 source (≤ 100 VA at ≤ 30 V); most professional amplifiers exceed that, which makes the run Article 640 audio distribution wiring instead. Confirm the amplifier listing before selecting cable to a class.