Greg
We had some very interesting debate here in the great white North contemplating a code change for the Canadian Code. The 2012 edition of the CEC has 1 simple rule for grounding. Not smaller than #6. That is it for low voltage AC systems. No ampacity table, no various electrode types for different wire sizes etc. #6 from a 15 amp to a million amps under 750 volt service.
You can actually have way more ground than enough and when we examine the normal purpose of grounding and the normal fault path for OC protection that we discover the ground wire does very little beyond voltage stabilization and a real poor backup for a failed neutral or other bonded return path.
We looked at European rules, US rules and obviously our own rules and found many surprises in ground wire sizing but most of the larger ground wires (bigger than #6) were sized for fault current and not voltage stability. This is true for some NEC and CEC rules but the CEC was by far the most conservative wire size. In the NEC it seems only a UFER needs a bigger wire than #6.
Unlike the NEC we removed resistance and impedance requirements for grounding low voltage (<750 volts) many codes ago. We also treat water pipes a little differently as we may use then as a ground electrode or just bond it and add an artificial electrode. In the city I work we often only required a #6 to bond the water mains which was conductively connected to dozens of services and by far the lowest ground resistance to the supply. We were requiring 2/0 ground wires connected to a couple of 10 foot rods too as this was the electrode. It does not take much math to realize the bond wire was usually the lowest resistance between the ground and bonding yet even in very high faults the #6 was never damaged even in cases where the electrodes had long ago turned to rust. It was just that conflict that forced this issue. since 1 wire was called a bonding wire it could be smaller than the ground wire yet be 1/10 the resistance of the ground wire.
The upshot is fault current returns on the grounded system conductor (neutral) and rules to require such over sized ground wires just no longer made sense.
We did discuss the secondary open neutral condition but when this happens, the ground seldom takes over the role or does so poorly as to be useless anyway.
OK so it is my opinion that you can easily have too much ground but hardly too much grounded circuit conductor. Lightning strikes notwithstanding. Of course that is not low voltage either ;-)
