Hugh,
The Prep Your Ride recommends 4-Gauge jumper cables, but I say 2 Gauge is the minimum, and the lower the better. The power is Current Squared over Resistance, so your 800 Amps / 14 V at your end might end up under 6V, under 300A at the car you are trying to jump. It is going over 10-20 feet so even what seems a trivial resistance causes lots of power loss even if the cables don’t heat up.
At least a half dozen times, with one car racing the engine and the other failing to turn over, I swapped their thin cables for my longer 2 Gauge (Sportsman’s Guide but also on Amazon though I can’t find the exact ones), and the car with the dead battery started immediately. – T.Z.