Amps = Watts/Voltage. When doing this, use 12v as the system voltage. When calculating wattage from amps x volts, use 14v. Real system voltage is 13-14v while the car is running, and some safety margin is called for. A typical headlight is 55/60W (low and high beams), so 4.6 to 5 amps per light, or 10 amps for both high beams on at once.
This obviously depends on which bulbs you're actually running. Some brake light bulbs are 35W each, some are only 20W. You'll need to tote up the current based on the bulbs you end up with. LED "bulbs", btw, only draw 0.1amp or thereabouts.
When deciding on wire gauges, there are a huge number of "standards" used. The most conservative standard would have you using 10g wire for nearly everything, which is pretty silly. There are some online charts that give you some idea of what gauge you should run for what current, but that's only a rough guidline. Length and how much current you'll actually draw make a difference.
Here's an example table:
Amps v. Wire GaugeLonger runs of wire require a thicker gauge of wire to keep the voltage drop across that wire to a safe level. As the current rises, the wire also needs to get thicker. So, long runs of high current wire need to be quite thick (say, rear-mounted battery to headlights). Roughly 10g is used here OEM, and other than the battery to the starter wire (very high current, like 60 amps), this is the thickest wire you'll use. Use 3 or 4g wire for the battery to starter. Use 10g for the front lights, use 14 gauge for other "power" uses (ignition to starter), and use 18g for everything else.
The coil uses about 3 amps, tops. Fuel pumps run 3-4 amps. Blower motors 4-5 amps.
