This is good information to know when upgrading your headlights or adding on any electric device to your car. The table will apply to home devices too.
I have done some searching and found the following for current being used and what gauge wire to use.

First use this formula:
watts of device divided by volts of source = amps (current being drawn) W / VDC = I
exp: 55watt bulb divided by 12volts (battery) = 4.6 amps
Most people say add 20% for safety......4.6 + 20% = 5.5 amps

Then use this chart to determine wire gauge size
(from Reference Data For Radio Engineers)

Guage of wire---------Max Current
20---------------------7.5
18---------------------10
16---------------------13
14---------------------17
12---------------------23
10---------------------33
8-----------------------46
6-----------------------60
4-----------------------80
2-----------------------100
1-----------------------125
0-----------------------150