Man, I have been searching all day for a Class T fuse IN STOCK. I found a 350Amp one in stock. Mind if I ask a question? SS tech guy said I should get a 400A T fuse. Will used a 300A T fuse. I figured I needed a 312A so, wondered if the 350A T fuse is too close to that number and the 400A T Fuse would be better? I am stuck on this one item. I want it to go between the Growatt(48V)12KW 250V 120A off-grid inverter and the EGLL 48V 5.12kw batteries (times 6) that I bought from SS. Probably using the wrong conversion and all.
The main factor to consider when sizing a fuse is the battery cables you are using. The cables have the capability to handle specific amounts of amperage, based on their gauge. If there is a short, and a greater amount of amperage is allowed to pass, the wire could melt or catch fire before an oversized fuse blows to interrupt the circuit.
To properly determine fuse size we need to consult NEC table 310.17 for wire in free air, see attached PDF.
Example: according to the table a #3/0 AWG copper wire with 90°c insulation is rated for 350 amps. So for this example a 350 amp T-fuse is sufficient.
Note: this only applies to running battery cables with 90 degree insulation in free air and not running the cable(s) through a raceway or conduit.
For the Inverter 12000w / 0.85 (efficiency of Inv) / 48V * 1.25 (fuse multiplier) = 367Amps.
So for the Inverter a 350 amp T fuse should work.
So with 3/0 90 degree wire with a 12KW inverter the 350 amp T fuse looks like a good choice. If you find the inverter has significant large surges from the loads and the 350 blows then the 400 may be the better choice, but if the 350 blows the loads maybe overloading the inverter.
I personally prefer to have the fuse a little on the lower side so my choice would be a 350 amp T fuse.
I don't know what gauge wire Will is using. Possibly less than #3/0 AWG so a lower amp fuse is used to match the wire between the battery and inverter.