Wire Sizes

Wire Gauge and Ampacity of Automotive & Marine Primary Wire

Taken from Beldens Catalog - As Specified by ASE
As certified by Underwriters Laboratory

Wire Gauge	Stranding	Ampacity	Corrected	Standard Usage
10		19x23		30		n/a		30
12		19x25		25		20		20
14		19x27		18		14.4		15
16		19x29		13		10.4		10
18		not listed	10		 8		 8

Corrected figure is for 4-5 conductors that are cabled, and is %80
of rated current.

Current carrying capacity is for 105c rated Automotive primary wire.

Average draw of common automotive lamps

Instrument/Indicator Lamps

#53(x)				.12 amps (1/8th amp)

Stop/Tail/Turn Signal Lamps

1034				1.8 amps (Stop/Turn)
				 .6 amps (Tail Light)

1157				2.1 amps (Stop/Turn)
				 .6 amps (Tail Light)

As you can see, the average Stop light/Turn Signal draws
approx. 2 amps each.  The average running light about 1/2
amp each.  The average instrument light 1/8 of an amp.
You could wire up most lights/lighting with 18 guage wire,
18 gauge would be practical for light bars and individually
wired running lights but I recommended that you use nothing
less than 16 gauge.(The smaller the number, the bigger the wire)
I seriously doubt that you would ever run a piece of wire that 
was longer than 15 feet,  voltage drop across this length is
not a factor in most applications.

Formula for conversion of Watts to Amps

Watts/Volts = Amps

ie:  60 Watt Headlight
     12 Volt Battery (yes, I know that it's really 13.5V)

60/12 = 5 amps