For the two 8G runs you dont need fuses if your amplifier has internal fuses. Otherwise if it uses external fuses, use the recommended rating by the manufacturer of the amps. I have a system comprising of 2 amps, pulling a maximum of 90 Amps at the mo that I am installing. Fuse at the battery is 100 amps, and is on 4 gauge wire down to the block where it splits into two eruns of 8g just like you.
