All right, here is another chart which can be useful. Start by deciding how much current you will need, and how long the wire needs to be.

Using the chart data, and the formula E=IR, where E=Voltage, I=current, and R=resistance you can calculate the voltage drop for any wire of choice.

For example a 10 foot long piece of #6 wire would have a resistance of 0.0444 ohms and if it were carrying 30 amps of current, it would develop 1.332 Volts.

There will be some other small voltage losses in the connectors/lugs etc, but for a 12 Volt system, the loss will leave you about 10.668 Volts or less.

Now if you were to use a 10 foot piece of number 0 wire, the loss would be about .02946 Volt, leaving you with about 11.97 Volts or less.

PLEASE NOTE that the electrical resistance in the first chart is for 1000 feet but for only 100 feet in the second chart!