Lately I have had a number of questions about battery bank fuse sizing and I wanted to give a brief explanation of how I size these fuses and why they are a good safety feature..
*Simply put you size the fuse to protect the wire, provided your wire is sufficiently sized.
#1 The ABYC Standard:
The ABYC requirement is for a battery bank fuse to be within 7 wire inches of the battery bank. In the real world of boats and battery compartments this is often hard to do. If you can't get within 7" then the next best thing is to be as close as possible.
If you're having trouble meeting the 7" rule the battery terminal fuses shown in this photo are an excellent option. These fuses are excellent but they do add nearly 2", in height, to a battery post so measure your battery compartment height carefully. The limiting factor with the battery terminal fuses is that they only go to 300A so if trying to fuse a bigger engine you may need a Class T or ANL fuse.