Originally Posted by btrayfors
There is no need under ABYC specs to fuse the start battery. However, lots of marine electricians believe it's a good thing to do on smaller to medium size boats. Mine is wired with a fuse. For a 30-footer, a 250 to 300 amp fuse would be enough.
For the house loads, you'll want a smaller fuse.
If the battery is, and will never be called upon, to ever start the motor
then this is true. For most boats the house bank could be used for motor
starting, either regularly or in an emergency
. I always fuse both banks for highest potential load and wire... As we get into much larger engines, that can't be reasonably
fused, I wire the starter cables
in loom to protect it.
Folks should keep in mind when sizing fuses that no fuse
will always result in worse outcomes in a catastrophic short than a fuse
even if that fuse is slightly larger than the ABYC max ampacity tables.
I have personally tripped hundreds of fuses OCP testing including a 300A fuse on 15' of 8GA wire directly connected to a 400A lithium battery bank. The wire never even got warm to the touch and the fuse tripped in micro seconds, yet 300A is 375% larger than the ABYC allows for with 8GA wire... In a dead short this is what you want to happen...
If I had the choice of no fuse at all or a 300A fuse on 8GA wire I would take the 300A fuse every time because at least I would stand a "chance". Without the fuse the wire and boat will eventually ignite....
Max Ampacity Chart Non-Bundled Wire: