question

At 5000W you really have to question why you are still at a 12V system design.

Most people start with 12V for very small DC loads. A few lights, a small pump, some mobile phone charging. Then a small inverter is added, maybe up to 1200W.

Beyond that, it is worth moving to a 24V system, and using a 24V to 12V DC to DC converter to run the 12V DC loads and wiring, if they exist.

Then once things reach about 2400W, it is time again to move to 48V. With a 48V to 12V DC converter.

There are almost no circumstances that I can even think of where a 12V supply is required in the 800A range. So what are the reasons for staying there, instead of increasing system voltage and using DC to DC conversion for the 12V loads?

There are also issues drawing 800A from a battery bank. Batteries are chemical reactions, and they can only happen so quickly. A C10 rating is considered a large, but sustainable supply from a lead acid battery. Lithium batteries are more like C1. So to sustain a 5000W load on a 12V battery bank would require ~350A, and a 3500 Ah lead acid battery bank at 12V. Batteries very rarely come in that kind of capacity, so you then need to setup parallel banks, and all the other issues of system complexity, and imbalance that come with that.

Victron makes and sells these very large inverters for low system voltages, and some customers do need them, but the market for them is extremely small, and in the VAST majority of cases, picking a more suitable system voltage is the better alternative from an overall system design perspective.

ACTUAL HELP

With all of that said, the fuses required for this unit to operate at it's full potential would be something like an NH-4, which are available from 500-1000A. They are not cheap.

These are high rupture current slow to blow fuses that are suitable for high current surge loads like from an inverter. You might have to shop around a bit to find them, and the holder, as they are not very common.