If you are asking about the in-line fuse at the battery. The in-line fuse needs to be the same size or bigger than the fuse in the amp. If both amps are used then you add the two fuses together. The in-line fuse is there to protect the wire and the car. Make sure you used the right gauge wire and don't put a fuse in bigger than the wire is rated for.
yes im askin bout the in line fuse. well thats wierd...one of my freinds has the mtx 251d amp and he had it professionally installed, they put in a 50 amp fuse and that amp only has a 25 amp fuse in it. the wire used was 8 guage tho if that matters.
Well as long as there is no electrical problem in the car, it'll be fine. The reason for fuse in the first place was for safety and to protect the amp. So by having a 50A fuse, it'll guarantee the amp will fry before the fuse blows.