What gauge speaker wire do you need?

What gauge speaker wire do I need?

It depends on three things: the length, the impedance of the speaker and how much loss you accept. A common guideline is that the resistance of the cable, both conductors together, should stay below 5% of the speaker's impedance. For an 8-ohm speaker that is 0.4 ohm. For 50 ft to an 8-ohm speaker, 16 AWG copper has 0.40 ohm, which is 5.0% of the speaker, and 18 AWG has 0.64 ohm, 8.0%. By the 5% rule the thinnest wire for that run is 14 AWG. Wikipedia adds that 16 AWG or heavier copper has no detectable effect in runs of 50 feet or less for a typical 8-ohm speaker. Lower impedance speakers and longer runs need heavier wire.

How much power does speaker wire waste?

The cable is in series with the speaker, so the power divides in proportion to the resistances: the share lost in the cable is cable ÷ (speaker + cable). For 16 AWG over 50 ft to 8 ohms that is 4.8% of the power, which is 0.43 dB of level; for 18 AWG 7.4% and 0.67 dB. These losses are small in decibels. The EAW/Community paper makes the same point: the level loss matters far less than the damping factor when you choose a cable.

What is damping factor and why does the cable matter?

Damping factor (DF) is the speaker's resistance divided by the total resistance the speaker sees looking back toward the amplifier: the amplifier's output impedance plus the cable. A high value means the amplifier holds the speaker cone firmly after a note stops. McGregor's EAW/Community paper says an electrical DF of 20 or more is adequate for sound reinforcement, and that the amplifier's own number matters little over long runs: a DF-100 amplifier has an output impedance of 0.08 ohm, and the cable adds more than that on a long run. His 100 ft run of 14 AWG to a 4-ohm speaker costs only about 1 dB (we get 1.03 dB) but leaves DF at 7.8, under half of 20. The paper notes that the effect is greatest for woofers and subwoofers.

Does the metal or the brand of cable matter?

The resistance of the metal matters, and in speaker cable it is by far the most important property, according to the Wikipedia article on speaker wire. Copper is the standard. Copper-clad aluminum has a somewhat higher resistance, about the same as copper two AWG sizes thinner, so use a thicker gauge. The same source reports that oxygen-free grades are sold as better but have no significant benefit at audio frequencies. This calculator uses annealed copper at 20 °C, 1.7241 × 10⁻⁸ ohm-metres. The wire resistance calculator on this site uses 1.68 × 10⁻⁸ for pure copper, which is 2.6% lower, so its figures differ slightly.

Does it matter if I use one pair for two speakers?

Yes, it changes the load. Two 8-ohm speakers in parallel present 4 ohms, so the 5% rule becomes 0.2 ohm and the damping factor test also gets stricter. Enter 4 ohms to see the run to a pair connected this way. If each speaker has its own cable, enter the impedance of one speaker and the length of one cable.

What length do I enter for a 25-foot run?

Enter 25 ft, the length of the cable from the amplifier to the speaker. The calculator doubles it, because the current goes out on one conductor and returns on the other. This is the same convention as EAW's formula, total cable resistance = 2 × length × resistance per 1,000 ft ÷ 1,000. For 12 AWG that resistance is 1.59 ohms per 1,000 ft and for 18 AWG 6.38 ohms per 1,000 ft.

The three formulas

The cable resistance is twice the length times the resistance per unit length of one conductor, which comes from the cross-section of the wire. AWG sizes follow ASTM B258, where the diameter of wire n is 0.005 inch × 92^((36 − n) ÷ 39); the resistance follows from the area. Next, the level loss in decibels is 20 × log of speaker resistance ÷ (speaker resistance + cable resistance), and the damping factor is speaker resistance ÷ (amplifier output impedance + cable resistance). They are the formulas published by McGregor in the EAW/Community paper, and the results match its table: 50 ft of 14 AWG to an 8-ohm speaker gives a cable resistance of 0.25 ohm and a damping factor of 31 in his table, and the same wire at 100 ft to 4 ohms gives 0.5 ohm, a 1 dB loss and a DF of 8. The speaker's impedance is a nominal figure and the voice-coil DC resistance is usually lower, so the results are approximate, as the paper says.

Worked example: 50 feet to an 8-ohm speaker

With a DF-100 amplifier (0.08 ohm) feeding 8-ohm speakers over 50 ft of 16 AWG, the cable measures 0.402 ohm, which wastes 4.8% of the power and 0.43 dB of level; the damping factor is 17. That sits right on the edge of the 5% guideline (5.0%) and just under the damping factor of 20, so the calculator asks for 14 AWG or thicker; Wikipedia's remark that 16 AWG is fine up to 50 feet at 8 ohms holds if you accept a damping factor in the teens. For 100 ft to a 4-ohm speaker with the same amplifier, the answer changes to 6 AWG: both the lower impedance and the longer run work against you.

Limits of this method

First, the speaker is not a fixed resistor. Its impedance varies with frequency, and the nominal value only approximates it, so a cable that passes the test on paper can still change the tonal balance of a speaker with a passive crossover, as Wikipedia's article notes. Second, the 5% rule is a guideline for home use, not a standard: we found it in Wikipedia and could not trace it to a standard, and the paper from EAW/Community sets its own 20 for the damping factor in live sound. Third, this calculator covers resistance only; capacitance and inductance of ordinary cable are small at audio frequencies and the article says they matter much less. Finally, it assumes copper at 20 °C. A hot cable under a rug or in a wall has a little more resistance. Check the amplifier's manual for its minimum load impedance before you wire speakers in parallel.

Sources: Chuck McGregor, Community Professional Loudspeakers (distributed by EAW), “What is loudspeaker damping and damping factor (DF)?” (September 1999; a vendor paper: the formulas, the DF of 20 for sound reinforcement and the wire table); Wikipedia, “Speaker wire” (the 5% guideline, the 16 AWG at 50 ft remark and copper-clad aluminum, a secondary source; we looked for a Rane Note on this and found none that fits); ASTM B258 (AWG sizes, as summarized by Wikipedia, “American wire gauge”; the standard itself was not opened). Related: wire resistance calculator, wire size calculator, voltage drop calculator and decibel calculator. How we check our calculators.