By the way, before I saw your post, I was doing some heavy Googling, and found this forum post that was really helpful to understand the fuse: http://www.the12volt.com/installbay/...ID~122533~PN~1
Here's the post (in case link dies):
There are three ways to determine proper fuse size that are generally acceptable:
1. Look at the chart in the bottom right hand corner of this page for your wire size and use the fuse/breaker listed: http://www.the12volt.com/info/recwirsz.asp
2. Add up all the fuses in the equipment you are adding and use a fuse equal to that number AS LONG AS THE FUSE IS NOT GREATER THEN THE NUMBER LISTED IN THE ABOVE LINK FOR YOUR WIRE SIZE.
3. Actually calculate the expected load and use that size fuse/breaker.
So whats the difference in the different methods? The #1 method is the easiest and requires the least work, but may lead to damage to your vehicle when you start getting in to bigger power wires. For anything 8awg or smaller it is probably fine. For anything bigger then that I would use a more appropriately sized fuse.
The second method is pretty straight forward. If you have 3 amps and one has an 80A fuse, one has a 30A fuse and one has a 25A fuse you know that the max current load can never be greater then 80+30+25=135A so you wouldn't need a fuse/breaker bigger then that. This will get you much closer to the actual load required on the wire.
The third method is the most difficult, but the "best" way. It may require some trial and error and you may blow a few fuses in the process. The theory behind it is to try to calculate the expected current on the wire and use the appropriate fuse. Say we are usin the same three amps from above. One is being used as a sub amp (the one with the 80A fuse) and the other two are for mids and highs. You can assume that the sub amp will be bridged and nearly maxed out so assume it actually needs an 80A fuse. Neither of the other two amps will be bridged and they'll both be playing at 4 ohms stereo. Assuming they are both rated for 2ohm bridged you could easily cut the fuses in half and be ok. That would give you 107.5A expected load and you could use a 110A fuse.
So, why does all this matter you may be asking? The fuse/breaker is a safety device intended to offer protection. It provides protection in two ways - from trying to pull too much current through a wire and direct shorts to ground. If you try to pull more current through a wire then it can safely carry the wire WILL heat up and it will eventually start melting and possibly catch on fire. That is why the chart in the link is so important. NEVER use a fuse larger then what is listed in the chart (or larger then the wire manufacture recommends if that number is available). Direct shorts, though, also have to be considered, and this is why using the largest "safe" fuse isn't always the best idea. Fuses are rated to blow based on percentage of load. A 350A fuse may hold 650A for 10 seconds or more before blowing! (http://www.cooperbussmann.com/pdf/08...62dd6ea9a7.pdf
) and will allow ~1200A to pass for 1 second before opening up! This has the potential to damage your battery, your alternator, or any electronic device in your vehicle if you were to experience a direct short.
Sorry for the book but this is probably the most critical part of planning a system out!