WHAT SIZE WIRE?
A question we get asked here at Streamline Audio on a pretty regular basis is “what size wire do I need for my amplifier?” This is a very important question since using the correct wiring with any amplifier is critical to the amps performance.
Why is wire size so important? Because an amplifier needs the correct size power and ground wiring in order to operate reliably and at its full potential.
How do I know which size wire to use? The size of the wire is determined by the current demand of the amplifier and the length of the wire run itself. To approximate the current draw (amperes) of the amplifier, you need to know its total RMS power. By simply multiplying the number of channels it has by the RMS power per channel you can determine the total RMS power.
Currently there are many types of amplifiers but really two are used the most - Class D and Class AB - and each has a unique formula to calculate current draw;
Class D amplifier - The average Class D amplifier is about 75% efficient. What does that mean? That means in order for a Class D amp to make 100 watts RMS power it actually will draw about 133.33 watts of power from the power source (battery, alternator) and the wiring needs to be able to handle that draw. The other 25% is soaked into the heat sink of the amp which is why they get warm after being used and the harder and longer we use them, the warmer they get. If used hard enough or long enough, they can even get hot to the touch.
Based on this information and assuming an average vehicle charging voltage of 13.8 volts, the current draw of a typical Class D amplifier can be determined as follows;
Total RMS (100 watts) ÷ Amp efficiency (75%) ÷ Source voltage (13.8 volts) = Amperes of current draw or 100 ÷ .75 ÷ 13.8 = 9.7 amps.
Class AB amplifier - On average, a Class AB amplifier is only about 50% efficient. This means only about half of their power is actually turned into audio output and the other half is turned to heat. In this case, a 100 watt amplifier is really drawing about 200 watts of power from the source.
Based on this information and assuming an average vehicle charging voltage of 13.8 volts, the current draw of a typical Class AB amplifier can be determined as follows;
Total RMS (100 watts) ÷ Amp efficiency (50%) ÷ Source voltage (13.8 volts) = Amperes of current draw or 100 ÷ .50 ÷ 13.8 = 14.5 amps.
Below is a chart that shows recommended power wire size based on amperes of draw and length of wire;
Notes - When talking about wire size, the smaller the number the larger the wire. So, an 8-ga wire is actually smaller than a 4-ga wire. Also, this chart is assuming the wire is copper. CCA (copper clad aluminum) wire requirements will be different.
Currently, one of the more popular audio upgrades we perform includes a 500 watt RMS Class D amplifier. Using the formula above and this chart, we can calculate the recommended wire;
500 ÷ .75 ÷ 13.8 = 48.3 amps. If the wire run is 15 feet long we would need an 8-ga power and ground wire.
Something else that has come up in the last few years, is the integration of high power Class D amplifiers in basic radios. Some of these radios are capable of 45 watts RMS per channel which means they have a total RMS power output ability of 180 watts! Now, as impressive as this is, it also means we need to be cautious of how these radios are installed. Consider that many modern vehicles on average have a radio power circuit rated for 10 amps of draw and the size of that wire is usually only about an 18-ga wire. If we use the Class D amp formula (180 ÷ .75 ÷ 13.8) we can calculate a possible current demand of 17.4 amps. It would also be safe to estimate that the wire coming from the battery of the vehicle could be 10 to 15 feet long depending on the location of the battery and the size of the vehicle - not to mention the multiple connection points along the way that can also add to voltage and/or current loss. This would mean that one of these modern high power radios could need a 12-ga or even a 10-ga power and ground wire.
“But my car charges at 14 and a half volts.” We hear this quite a bit as well and while this may be true in some cases, most modern vehicles have smart charging systems that turn down the output power of the alternator or even turn it off to “cycle” the battery as you drive and aid in fuel economy by decreasing the load on the engine. This is why we use an average of 13.8 volts in these formulas. Regardless, if we replace the 13.8 volts in the equation for the high power radio with 14.5 volts, we still get a potential current draw of 16.6 amps.
“What happens if I use the wrong size wire?” A few things can happen if the wire is too small. The amplifier may run warmer and/or produce less power or even go into protection mode prematurely. You may even overheat the wire which could lead to damage to the wire or the fuse holder especially if the wire is not of a good quality.
Using too big of a wire doesn’t really have any negative effects besides the additional cost or in some cases being more difficult to route safely and cleaning through the vehicle.