The recommendation given from theelectricbrewery.com
We use a 10 gauge wire with 3 separate conductors, called "10/3" wire: Two conductor carry each of the 120 VAC "hot" lines to produce the 240 VAC the element uses, while the third will is used as a ground. It's important that everything be grounded for safety reasons: If one of the hot lines was to be become disconnected and touch a kettle, having the kettle grounded ensures that the current will pass to ground immediately and trip your circuit breaker or fuse instead of passing somewhere else (like through the person brewing!).
Look for cables that oil/water resistant, with a rubber coating, and rated for outdoor use. Use whatever length is needed between the kettle and the control panel. 10 feet is fairly common but don't be afraid to use a bit more just in case. The last thing you want is a cable that's too short.