Basically wire gauge is related to the"load" (amperage) it will carry... If you have something at end of wire and it only draws 1 amp you can use small conductor like 16 gauge,,,,IF you are are connecting battery to fuse box you need a heavier gauge the can carry the sum total of all the device at the end of every fused circuit... So if all the devices total 20 amps then you need a larger gauge like maybe 10 or 12 gauge... There are charts that give give the amperage a conductor can safely carry, determined by wire gauge and distance from power source (fuse panel/battery) to device...
Its also ok to use larger wire gauge for any "run" then the exact match per chart... IN telecommunications industry where I worked for years the rule was "when in doubt run next size larger"....
Also it is ok to use one size wire (gauge) for all runs in TD as long as the size used meets minimum specification for largest device system powers.... Sometimes its simpler to use one size wire to complete all runs than trying to size each run for the power requirements for end device...
This is but one example of many wire gauge/amperage charts...
Many more to be found here...
http://lmgtfy.com/?t=i&q=wire+amperage+chartThis should get you started... Only other thin you really need to know is what the current rating in either amps or watts the device requires that you are powering...
Dale
Lives his life vicariously through his own self.
Any statement made by me are strictly my own opinion.
You are free to ignore anything I say if you do not agree.