I think #12 from the Intellipower chassis ground lug to the airstream body ground. The ground lug is 7/32", I don't think much more than a #10 would fit in the hole.
Safari,
At least 8 AWG rated for 90 degrees Celsius and you are fine as far as safety issues go. Bigger is better and I think you will see better performance in the automatic mode if you use bigger wire.
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