First of all, what is a fuse ? A fuse is an electrical componant which, by design, burns out (usually by melting) when too much current flows through it.
Current + resistance = heat
Enough heat and the metal will melt. Simple ! Thats how a fuse works.
So why do we use fuses ? Fuses in electrical componants (amps, CD players) protect the equipment from power surges and spikes.
Fuses in the electrical system of your car protect you ! The fuses for big car stereo carry a lot of amperage to cope with the demands of the amplifiers, etc. but are sufficiently small that in the event of a dead short (say if there's an accident) that the fuse will blow, preventing the short starting a fire. After all, its one thing to crash your car, its another to crash your car and burn to death because you didnt fit a $20 safety device.
When designing your own electrical system upgrade keep these rules in mind for safety:
1. Keep fuses within 12" (30cm) of the battery.
2. Keep earth cables 18" (45cm) or shorter.
3. Earth amplifiers to separate parts of the chassis, to prevent "hot spots" from excessive power flow.
4. Do not solder ends of cables. Soldered ends lead to un-even contact inside screw terminals, leading to "hotspots".
Hot spots occur when a lot of current tries to flow through not enough metal, and can lead to fires or electronic failures.
When sizing a good rule of thumb is got 10-20% over the total current draw of all the amps in the circuit. The total current draw can be calculated by checking the amplifiers built in fusing. Some very big monoblocks dont have a fusing, but their manual will always have a recommended fuse size.
So if you have 2 amps on one power lead, with 30A of fusing each:
2x 30 = 60A current draw
60A + 20% = 72A. Rounded, thats 80A of fusing on your power cable.
Remember, the power cable fuse is to protect the car, so should be rated slightly higher than the amplifiers total power draw.
Now that you have a correctly sized fuse, you need to know how to choose the right cable for your power. Use this handy conversion table:
Wire Gauge - Fuse Size
00 gauge - 430 amps
0 gauge - 345 amps
1 gauge - 265 amps
2 gauge - 215 amps
4 gauge - 135 amps
6 gauge - 80 amps
8 gauge - 55 amps
10 gauge - 30 amps
12 gauge - 20 amps
14 gauge - 15 amps
16 gauge - 7.5 amps