I am going to run one wire off my battery terminal to an auxillary fuse block under my dash. The fuse block has 8 places for 30amp max fuses and the block is rated for 120amp total. The wire I will be running is about 8'-10' long. What is the formula used to figure what gauge wire I need?
There really isnt a formula for such a short run. You need to determine how much current your device will utilize (radio maybe 10 amps, or a light maybe 5, etc) Once you determine how much current your device will draw, you then determine the proper gauge wire.
You need to size the wire for the maximum current it could carry. If it is directly off the battery and going to a block that is fused for a total of 120 Amps, then you have to assume that the wire could be asked to carry 120 amps before a fuse would blow. According to your chart, that would be a 8 AWG wire (150A for 10 feet). First problem is that 8 * 30A 240A. If you did 8 branch circuits all fused at 30 with no fuse between the block and the battery, you could exceed the rating of the block! The safe way to do this is to add up all the loads you plan to put on this wire, right size the fuses to those loads and then size the wire for that total plus about 10%. You could also put a single fuse right at the battery that would be sightly larger than the total max load current.
