We don't want to melt any wires. The first thing you have to do is determine the current you have to carry. For DC circuits, that's relatively easy. Some equipment on a car is rated directly in current draw. Auxiliary fans, fuel pumps and things like that are rated in current draw - Amps.
Some equipment is rated in Watts - mostly the lighting equipment.
The power requirement in Watts will be printed right on the bulb or stamped in the base. To come up with amps use one of the formulas shown. Let's assume you have to run a wire 6 feet from a relay to the lamp and look at the chart on the next page. Now in reality, you have to balance the mathematical results with mechanical reliability and efficiency. For lighting, the rated output is figured at 13.5 volts, not 12 volts.