I have a 3000w inerter and a 280ah lithium leisure battery which I’d like to use for a night out.
I want to be safe so wondering what size Earth cable to ground it?
Should put a fuse between the inverter and battery?
Read this will help
https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf
All infos you need and more in this pdf but an earth cant be to smal in the earth but in wire short and big is better than long and smal 
Same for the battery cables.
Fuse in the battery cable short as possible to the battery after the fuse a switch is the best idea.
Is use switchable fuses.
You want to earth the battery - pole (GND) or did i understand this wrong?
convention has that the Earth must be able to withstand any fault current. If one pole of the battery is grounded (Theoretically the negative in this case) then the earth wire needs to be at least 50% of the battery cable size. Default minimum for a non earthed battery system of this size would be 4mm^2.
thankyou Maik.
just the Inverter
This is a topic that comes up on multiple forums. The safe answer is that the ground cable (not the negative cable!) should be the same size as the positive/negative cables.
I’ve seen some Victron guidance (that I can’t point to right now) that says the ground cable can be one size smaller than the main cables. So if you use 2/0 for the main cables, 1/0 for the ground.
What I do in practice depends on what cable I have available. Plenty of 2/0? Then that’s what the ground gets.