Originally posted by Wizard9056 n2audio is correct. The fuses on the amp are not "AS" important.
In the manual for you amplifier, you will see a max input current (AMPERES) and a recomended fuse size for the input for your amp. That specified fuse size is the one you should use. However, the gauge of wire used and length are not "AS" important either.
Fact of the matter is, I you wish to protect the device used, the fusing should match the specs. If you increase the fuse size larger than the specified size, you risk damaging the component that it is protecting, by allowing it to draw to much current.
I have seen people put a XX Amp fuse on the line by matching what the gage of the wire is. By doing so, and not having the proper fuses on the amp, they ruined their entire system.
The wire Gage and length only plays a part in this equation if you are using a wire to small for the application. If this is the case, you run the risk of a fire hazard due to heating up the wire, so then the fuse doesn't mater anyway.
You do not fuse an application dependant on the gauge of the wire!!!!!
Sure a wire is capable of carrying X amps at a certian voltage, but that doesn't protect the equipment that can only handle less than X amps at a certian voltage.
This, of course, becomes a little more intricate when dealing with multiple devices being powered off the same line. Then the Main fuse, next to the battery, must be large enough to handle the max current draw of the system (X amps + Y amps + Z amps) and/or use a larger Main fuse and smaller individual fuses for the power distribution to each device.
If there are any further questions, or I need to clarify, please feel free to reply!