This is good information to know when upgrading your headlights or adding on any electronic device to your car. The table will apply to home devices too.


First use this formula:
watts of device divided by volts of source = amps (current being drawn)
W / VDC = I
example: 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 guage 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

 

The quality and length of the wire matters MUCH more then the gauge of the wire/current passed through it.

IE: In my old R/C truck, I used 12 gauge wire to hold surges of up to 120 amps at around 20 volts, 50+ amps sustained, but everything was soldered, short, and high quality. The biggest problem in the F-body electrical system IMO is the connectors, and grounds.

 

This is the chart most professional stereo installers use... http://www.olg.com/ski/ls1/wiregauge.gif

That's over-kill, but is defenetly safe...

And yes, quality of connections and wire is a HUGE factor, but most people can't solder well, or have access to good quality wire.

 

Maybe you can help me understand. I see people using 2 gauge wire all the time. Big enough to run your starter. But the fuse in it is about 50 times smaller. If current flow is enough through the fuse why the large wire?

 

The fuse is made to handle a certain amount of current. If the limit of the fuse is surpassed, the fuse will burn open. Its not a good idea to have the wire and fuse rated at the same current limit because the wire will start to over heat at the same time the fuse does. There is no harm in using over sized wire but you can cause harm by using to big of fuse.


The quality of a connection will not cause current to go up. The resistance will increase of there is a loose connector or a cold solder joint. Current and resistance are inversely proportional. If you put a dead short across a battery (resistance at minimum) there will be MAX current draw.

Everyone knows that having a loose battery terminal connection will cause starting problems. This is due to the resistance at that connection being to high, therefore current is too low to turn properly operate the starter.

 

Very true. At the lengths I usually use, I stick to this table.

guage amps

16..........10

14..........20

12..........30

10..........40

8...........50

I usually use short lengths of wire, and this is what I go by.