Depends on how far they are away from each other, and the routing of the wire. You have to make a risk assessment. For mine there is no battery in the front of the car and the wire had to be run underneath nearly the whole length of the vehicle. In that situation obviously there is a lot of area that a short could happen so a fuse is highly advisable. If the battery is a foot away from the alternator like a lot of vehicles, and is routed correctly you shouldn't need one. Although.. I was putting an unmarked battery in my other car a few years ago.. in the dark.. and thought I saw red on the post so I hooked it up as power and it turned out to be ground. Burnt up the alternator because there was no fuse. Didn't harm anything else. Stupid mistake, should've paid closer attention and got a light.