A topic that seems to cause a lot of confusion in car audio is how to choose the right wire gauge for the amplifiers. Amplifiers require voltage and amperage (remember power is equal to voltage multiplied by current). The voltage is basically set by the vehicle's charging system so the only adjustable variable is the amperage. If you need more power then you'll have to draw more amps (current) because your voltage is basically set.
So why do we need large gauge wire? The answer has to do with voltage drop. Because we can't really adjust the voltage of our charging system (without affecting the rest of the vehicle) then we need to try and minimize the voltage drop. The voltage drops when we try to draw too much current through a wire that has too much resistance. Resistance is greater in smaller wire than it is in larger wire, assuming the same wire material. The more wire you have the easier it is for power to flow through that wire. If your wire is too small then your resistance goes up which causes your voltage to drop.
To calculate the voltage drop we need to use Ohm's Law:
E=I*R (voltage = current multiplied by resistance)
Let's say your amplifier draws fifty amps of current maximum (the actual number should be listed in your owner's manual). Now let's say our power wire has a resistance of 0.01 ohms. So the voltage drop through that wire is:
E = 50*0.01
voltage drop equals 0.5 volts
So is that OK? The amount of voltage drop that is acceptable depends on the owner. If the owner is a sound off competitor that wants every edge than that might be too great of a voltage drop. But for an average stereo owner that might be fine. Keep in mind that voltage loss is at maximum current draw. With only a few amps being drawn from the amplifier (at normal listening levels) the voltage drop would be much less. I generally aim for no more than 0.5 volts of loss through the wire at maximum current draw. If the system is for a competitor I would use a lower voltage loss number.
Now that you know how power wire resistance affects voltage you need to be able to calculate the power wire resistance. Wire has a property known as resistivity. This tells you how many ohms of resistance are in a foot of that wire. The number is seemingly small for the power wire sizes used in car audio but as you have seen even a low number adds up to a large voltage drop at high current draws. Ideally you would like to know the resistivity of the wire exactly but rarely if ever will you find that listed. Instead you will probably have to use a standard wire resistivity table for various AWG gauge wire. Here is an abbreviated table for the most common car audio power wire.
|Wire Gauge||Resistivity (ohms/foot)|
Using those values you would find the resistance of your wire by multiplying the standard resistivity by the length of wire you are using. So for a four gauge wire that is fifteen feet long you would get:
0.0002533 * 15 = 0.0037995 ohms
Now if you have an amplifier that draws fifty amps the voltage drop at maximum current draw would be:
E = 50 *0.0037995
E = 0.189975 volts
Or basically a 0.2 volt drop which is perfectly acceptable.
Final step I promise. You should now have an understanding of why we need larger power wire for larger current draws. You also know how to calculate the numbers you need to determine a voltage drop. But let's do it a different way and determine the wire size we need based on the maximum voltage drop we will accept. Again, let's use a 0.5 voltage drop as the maximum we will accept. You could choose a lower number if you want but this is a good starting point at least. For this example we'll use an amplifier with a maximum current draw of 120 amps.
Now for E = I*R we know E (0.5 volts) and I (120 amps)
So rearranging Ohm's Law we get:
R = E/I
Substituting in our known values for E and I:
R = 0.5/120
R = 0.004167 ohms
So the maximum resistance of our wire is 0.004167 ohms. Now this does not tell us what gauge wire to use because we also need to know our wire length. Let's say for our example we need fifteen feet of wire. We can now calculate our power wire's maximum resistivity.
Resistivity = Resistance/Distance
Resistivity = 0.004167/15
Resistivity = 0.0002778 ohms/foot
Now we use that number and reference our standard resistivity table again.
|Wire Gauge||Resistivity (ohms/foot)|
Note that our resistivity falls between four gauge and eight gauge wire. In fact it is almost exactly in line with four gauge wire. This means we should choose at least four gauge wire in order to keep our voltage drop below 0.5 volts.
In addition to theoretical factors and resistivity there are other things affecting voltage drop. First, the wire resistivity in the table is only a standard resistivity. Actual resistivity may be slightly different though hopefully not too much. Make sure you are using copper wire and not some other material. Also, the wire size printed on the jacket might be different than the standard size for that wire gauge. With the price of copper so high it tempts less-than-scrupulous companies to short change their customers by putting less wire in their products. Another factor that can add resistance are the wiring connections and intermediate items. This includes end of wire connectors and fuse holders as well as the ground wire and its connections. Any additional resistance provided by these connections and components will add to the voltage drop of the system. For these reasons it may be a good idea to choose the next larger wire size if voltage drop is a concern for you. On the other hand this voltage drop only occurs at the maximum current draw so you may not be as concerned if you listen at reasonable volume levels most of the time.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including in depth car audio amplifier installation. Click here.