Check on Google for "Wire Loss Table". You will find tables that will help you find the wire size required to run a certain current over a certain distance.
If you really plan to use 800W continuous, then you will be drawing somewhere between (800 / 13.8 = ) 57 Amps and (800 / 12 = ) 67 Amps. A quick lookup shows that you're not to use #10 wire for that, it will overheat.
By the way, I'm fairly sure you won't be using 800W continuous, unless you plan to run your kitchen coffee machine from it. Do the math and use a lookup table.