Ok, let me frame the question a bit better now that I have had a nice glass of wine.
I have attached an image showing my original plan on wiring up the battery bank. It shows fuses for the following:
- 250 Amp for the inverter (as per manufacturers recommendation)
- 100 Amp for the windlass (which is what it has right now)
- 100 Amp for my house panel (which is what it has right now)
- 250 Amp for the Alternator (as per supplier recommendation)
In practice the actual current draw for each item is as follows:
- 150 Amp MAXIMUM for the inverter (but we never gotten there, usually 80 amps for short periods).
- 60-80 Amp for the windlass (observed on my battery monitor)
- 30 Amps for the house panel with typical mix of loads - max observed for short durations.
So at worst case that works out to 170 amps with all loads running which would be pretty impractical.
My battery supplier just looked at my plan and said put a main fuse in between the battery and the switch of 400A and size your wire accordingly - which to me is 4/0. It just seemed wrong.
If I look at a more suitable maximum current draw of say 200A as suggested by djmarchand, then things start to look more reasonable and I should be able to get by with 1/0 or 2/0 if I wish.