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Cable gauge chart
How to determine the best wire size for power and ground
Size matters for current flow
What gauge (thickness) wire to use for power cabling depends on how much electrical current your system will try to consume. To determine the approximate current draw (in amperes) of your amplifier (or amplifiers), you must calculate first the total power of the system (multiply the number of channels by the number of RMS watts per channel). If you have multiple amps, add up the total power figures to arrive at a grand total. Next, double your grand total power figure. Then, divide by 13.8.
Total RMS Wattage x 2 (Inverse of Amp Efficiency) ÷ 13.8 Volts = Current Draw in Amperes (explanation below)
The resulting figure is your system's approximate maximum current draw. Compare this number to the numbers in the "Amperes" column in the chart below. Cross-reference with the cable length (the distance from your battery to the amp mounting location) to determine which gauge of cable you need.
Once you have estimated the total amperage of your system using the described formula, determine the proper power and ground wire gauge from this chart based on the length of power cable your installation requires.
A more detailed explanation
Here is an explanation about the formula we use, in case you want more details. Calculating the amp's total power is straightforward, but the other two parts can be confusing.
No amp is 100% efficient
First of all, why do we double the total power figure? That's because the typical analog amplifier is about 50% efficient. That means about half of the power it generates is turned into audio output while the other half of the power is lost as heat.
So if your amplifier is putting out 400 watts, it's actually drawing about 800 watts of power from its source, and the amp's wiring needs to be big enough to handle that draw. (Digital amps are much more efficient in comparison, but it's still safest to go with the larger wire size.)
Automotive voltage is neither 12 volts nor 14.4 volts
And the 13.8? Yes, vehicles have a 12-volt electrical system, but we're assuming that the vehicle is running, which means its alternator is generating additional current, which typically bumps up the voltage to about 13.8 volts. This is a better real-world representation of the vehicle's electrical supply. Dividing by 12 results in a larger number, which could point to a larger wire gauge, but it's often in the same color range in the chart.
Resistance increases with length
The reason different cable lengths bear different ratings is because the electrical resistance, inherent in all wire, builds up as the cable gets longer, until it forces the voltage down below a useable level. At that point, up-sizing the power cable will restore the voltage to its intended level.
Don't choke the flow of power
Finally, according to our tech support guys, the primary performance limitation in most amplifier installations is in the current delivery — either a weak ground or insufficient wire gauge. Installing too small of a wire gauge results in poor performance, potentially shorter service life of connected components (your amplifier and speakers), and a potential safety hazard.
On the other hand, installing too large a wire gauge doesn't really have a downside, but there is the potential for better performance. Obviously, there's no need to buy 2-gauge wiring when 10-gauge will do. That kind of overkill would be a waste of money. But if the chart could lean either way between two sizes, going with the larger wire size would be the smart choice.