Quote Originally Posted by BanginJimmy
whatever amperage your alt charges it is what you want size fuse you want to use. I know I have a 230 amp alt, and I have a 250 amp fuse inline on the way back to the battery.

As david said, the fuse is to protect the car, not audio equipment. In this case you need to fuse both sides of the wire because you have a power source on both sides of the wire. Instead of a fuse you could use a circuit breaker. It does basicly the same thing, but they tend to be a bit smaller than a fuse and fuseholder would be.
i still dont understand why your saying i'd need one. Im going to having the exact same setup. only difference is my power will be connceted under the hood and brough towards the back....