Class-T fuse sizing sanity check

[elizabethii]

When I built out my system, I included a Class-T fuse holder inline with the main cable coming off the battery rack bus bar. I'm using 4/0 105C-rated welding cable from the battery bank bus bars to the main distribution bus bars. The total run one-way from the distribution bars to the battery bank bus is less than 5 feet. I installed a 400amp fuse in the holder, and I swear I had some logic behind the sizing at the time, but I don't remember what it was and now I'm really doubting myself.

I'm running 2 currently-installed 3000EHV-48 inverters and will likely install 2 additional future 3000EHV-48 inverters once I expand the scope of the system. At full load, the 4 inverters shouldn't be able to pull much more than 250 amps, which itself makes me wonder if I have heavy enough cable. I recently saw a chart that claimed 4/0 was only good for 200 amps, but also found a different one that it was good for 600 amps for less than 50 feet of total run. I think the discrepancy is related to assumptions based on system voltage and AC/DC, but I never learned the theory behind how to calculate ampacity. I just have amp ratings for the common wire sizes in 120/240 VAC and 12VDC memorized...

Should I adjust my fuse size and/or wire gauge for my system?

 

[rodrick]

According to the Lincoln electric chart the people who make welders 3/0 will handle 400 amps with a 100% duty cycle and that was 75c rated cable so 4/0 105c should be protected just fine with a 400 amp fuse I would think

 

[Danke]

 

[AHTrimble]

A Class T fuse is a fast acting, current limiting, fuse that is rated to interrupt a minimum of 200,000 amps. In other words...really fast-acting high current flow. It is designed to protect everything in that circuit from a high amp flow situation. Normal circuit breakers and fuses are for protecting the wire/cable under normal conditions. A Class T handles that rare occurrence of high amp flow such as 220ka that would melt normal fuses & breakers.
My Victron Quattro 5kw has a recommended fuse size of 200a and I have 2 Quattros.
Together my 2 Quattros should never draw more than about 210aDC max under max continuous draw.
But they theoretically could pull 10kw each IF they each hit a surge max draw at the same time...meaning 420aDC...and that is hitting the upper end of the 4/0 cabling.
I wanted to allow for some surge and to protect the circuit...and be able to handle that rare occurrence that a very high amp flow might occur.
That led me to the 400a Class T fuse between my Victron PowerIn (1000a bus bar) and the inverters.
That being said...I also have a 125a circuit breaker on each battery as well...between the battery and the PowerIn. And the batteries are connected to the PowerIn with 2/0 cable.