The recommended fuse for this inverter charger is 400 amps. If you look at the Ampacity Tables you will see that 400 amps requires you to use 4/0 wire since the purpose of the fuse is to protect the wire from overcurrent. If you drop the fuse size to 350 amps then you can drop the wire size to 2/0. The reason they recommend a fuse of 400 amps is to prevent nuisance tripping due to surges. The lower you drop the fuse size the more chance there is that it will blow if you accidentally turn two loads on at the same time, or from a surge load of a motor starting.