Breakers vs Fuses

It is a function of ohm's law in combination with established values for copper wire gauge vs resistance.

This info in from rec.audio.car FAQ ( http://www.mobileaudio.com/rac-faq/) (originally from IASCA handbook). To determine the correct wire size for your application, you should first determine the maximum current flow through the cable (looking at the amplifier's fuse is a relatively simple and conservative way to do this). Then determine the length of the cable that your will use, and consult the following chart:

                              Length of run (in feet)    Current     0-4  4-7 7-10  10-13  13-16  16-19  19-22  22-28      0-20A     14   12   12     10     10      8      8      8     20-35A     12   10    8      8      6      6      6      4     35-50A     10    8    8      6      6      4      4      4     50-65A      8    8    6      4      4      4      4      2     65-85A      6    6    4      4      2      2      2      0    85-105A      6    6    4      2      2      2      2      0   105-125A      4    4    4      2      2      0      0      0   125-150A      2    2    2      2      0      0      0     00

This is what I often use to consult. It is the accepted competitors chart, and what the safety inspectors at sound off often use. If you would like some background stuff for the math, go here: http://documents.epanorama.net/documents/wiring/wire_resistance.html first. The numbers they have in here are equivilent to the ones given in my electronics text for my college analouge electronics course. From these numbers, you can apply Ohm's law and variants to create your own Table of exact values, if you are so inclined. The values in the chart above are somewhat rounded to be safe. If you need help working it out, post back here and we'll walk through it.