Some commonly asked questions are "What gauge speaker wire should I use? or "Will using a different brand/gauge wire improve the sound quality of my system?".
These are good questions, because there are a lot of myths involving speaker cables.
Firstly, there are three effects that can have an impact on the sound quality. These are the resistance, capacitance and inductance of the cable. Now the capacitance and inductance of practically all speaker cables used in car audio is very small and will basically have no audible effect on the sound in the real world. The one effect that is important to consider is the resistance, in particular the resistance in relation to the impedance of the speaker. If the resistance is too high, then there will be too much voltage drop across the cable, which will result in loss of volume. Then there is the effect that it has on the output impedance of the amplifier (any series resistance between the amplifier and the speaker will be added on to the output impedance), which can among other things, can have audible effects on the frequency response. The effect that a high resistance speaker cable will have on the frequency response isn't neccsarily bad, but isn't recommended unless this has been accounted for in the design.
Therefore, you should consider the resistance of the cable, not the brand.
So how much resistance is too much? Well the actual amount depends on the actual speaker, amplifier etc, but there is a general guideline which should be fairly safe. The aim is to keep the total speaker wire under this amount of resistance. Here is a table of impedance load vs maximum (recommended) total speaker wire resistance.
Nominal impedance load - Maximum total speaker wire resistance.
8 ohm - 0.3 ohm
4 ohm - 0.15 ohm
2 ohm - 0.075 ohm
1 ohm - 0.0375 ohm
As you can see, with lower impedance loads, the resistance of the speaker wire also needs to be lower.
Also keep in mind, that this is not the individual speaker impedance, but the load that you present the amplifier. For example, if you had two 4 ohm speakers wired in parallel to the amplifier, then there would be a 2 ohm load and you would aim to keep the total speaker wire resistance below 0.05 ohm.
Here is a table of speaker wire gauge (AWG) vs resistance per metre. Keeping in mind, this only accounts for one wire (for example just the wire hooked up to the positive terminal on the speaker). Therefore, you need to double the resistance to account for both wires.
American Wire Gauge - ohms per metre - 2x ohms per metre
24 gauge - 0.0832 ohms - 0.1664 ohms
20 gauge - 0.0330 ohms - 0.0660 ohms
18 gauge - 0.0208 ohms - 0.0416 ohms
16 gauge - 0.0131 ohms - 0.0262 ohms
14 gauge - 0.0083 ohms - 0.0165 ohms
12 gauge - 0.0052 ohms - 0.0104 ohms
10 gauge - 0.0033 ohms - 0.0066 ohms
8 gauge - 0.00206 ohms - 0.00413 ohms
4 gauge - 0.00082 ohms - 0.00163 ohms
So to work out approximately how much total resistance the cable will have, you need to multiply the total length (in metres) by the resistance per metre.
For example, two metres of 16 gauge:
Resistance per metre (2x ohms per metre) = 0.0262 ohms
Length = 2 metres
Total resistance = 0.0262 x 2 = 0.0524 ohms
Secondly, due to the high current demands at high power and particularly at low impedance, it is recommended that you use lower guage (thicker) cables in these cases. These are recommended minimums, but a slightly lower gauge (thicker) cable certainly won't hurt.
14 gauge recommended minimum:
250w at 1 ohm
500w at 2 ohm
1000w at 4 ohm
14 gauge recommended minimum, but 12 gauge preferred:
500w at 1 ohm
1000w at 2 ohm
2000w at 4 ohm
12 gauge recommended minimum:
1000w at 1 ohm
2000w at 2 ohm
4000w at 4 ohm
10 gauge recommended minimum:
1500w at 1 ohm
3000w at 2 ohm
edit -
16 gauge minimum (14 gauge preferred)
125w at 1 ohm
250w at 2 ohm
500w at 4 ohm
16 gauge minimum
125w at 2 ohm
250w at 4 ohm
18 gauge minimum
150w at 4 ohm