12 volt systems don't typically experience number 2 because there's no such thing as a ground fault when "ground" is exactly the same potential as the negative lead.The equipment ground serves 2 functions, maybe more.
1 Keep the system at the same potential as the planet its attached to.
That usually doesn't take a thick wire.
2 Clear a ground fault(allow the over-current protection to trip).
Wire should be big enough to clear the fault without the insulation melting.
This will probably require a thicker wire.
If you're grounding a higher voltage AC system to the chassis, that's a whole different ball game which is subject to its own particulars in order to ensure things such as GFCI and circuit baskets function properly.