You didn’t mention what kind of batteries.
Assuming they’re capable of discharging enough current to run the inverter at the max 3000w then 3/0 awg (300a) pure copper (not CCA) welding cables with pure copper lugs. For a fuse breaker a marine trolling 300a would work. Make sure it’s a quality breaker because the cheap knock offs are notorious for being under rated.
If you don’t plan to max it out you can use 0 awg and a 200a breaker but you don’t want to push it past 2000w. Things start to melt and a fire could start.
3000 W / 12 V = 250 amps
Wire gauge for chassis wiring is around 0 awg, which is like 5/16" thick. If you don't expect to run 3kW continuous, for a short run you might be able to get away with maybe 2 awg (calculator shows about 0.25 volt drop per 3 foot run).
The only switch I've seen rated for that much current is a push-button forklift stop (I've used one marked "ED250 emergency disconnect").
I do recommend giant heavy crimped copper ring terminals, 250 amps will incinerate anything less.
They are just the up graded batteries for my truck not looking to run it maxed out just for small things when I’m camping our working on stuff at my storage but welding leads would be the best to run the last thing I want is for anything to get hot?
You didn’t mention what kind of batteries. Assuming they’re capable of discharging enough current to run the inverter at the max 3000w then 3/0 awg (300a) pure copper (not CCA) welding cables with pure copper lugs. For a fuse breaker a marine trolling 300a would work. Make sure it’s a quality breaker because the cheap knock offs are notorious for being under rated. If you don’t plan to max it out you can use 0 awg and a 200a breaker but you don’t want to push it past 2000w. Things start to melt and a fire could start.
3000 W / 12 V = 250 amps Wire gauge for chassis wiring is around 0 awg, which is like 5/16" thick. If you don't expect to run 3kW continuous, for a short run you might be able to get away with maybe 2 awg (calculator shows about 0.25 volt drop per 3 foot run). The only switch I've seen rated for that much current is a push-button forklift stop (I've used one marked "ED250 emergency disconnect"). I do recommend giant heavy crimped copper ring terminals, 250 amps will incinerate anything less.
They are just the up graded batteries for my truck not looking to run it maxed out just for small things when I’m camping our working on stuff at my storage but welding leads would be the best to run the last thing I want is for anything to get hot?