I could go into series and parallel circuit calculations for current flow and voltage drop but that's a little overpowering. The conservative way to go is to just add them up to get total current and install a fuse that has 20% more rating. Don't put a 40 amp fuse in to protect 10 amps of load, you will probably burn up wires if a fault occurs without blowing the fuse. Just use the next size available after you do the total calculation. attached might help
