To determine the approximate current draw (in amperes) of your amplifier (or amplifiers), you must calculate first the total power of each amp (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.

The resulting figure is your system's approximate 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.