code intent - ground rods

I always thought the smallest grounding conductor you could use was a #4 not #6.

It is not the size of the rod it is the capacity of the dirt to dissipate the current. Even with 2 rods it is unlikely that the earth would carry away more current than a 6 copper can carry. Once you get into the other electrodes you have some serious capacity and that is when 250.66 kicks in.

Doesn't that have to do with the fact that a breaker should pick up the fault very quickly, so there will be insufficient time to do much thermal damage to the copper?

H.

The grounding electrode doesn't really have much to do with clearing a fault. The fault path is from the equipment grounding conductor to the ground bus, then through the main bonding jumper to the neutral in the service, ultimately to the centertap of the transformer. Grounding the service is really only to establish a common potential for the equipment grounding system and to bleed off common mode transients.

This is what I understand. The grounding path associated with a branch circuit has to be big enough to carry fault current, which could be sizeable, to the neutral grounding point (ie, service entrance), whereas the gec needs to be large enough to establish common potential for the equipment grounding system and to bleed off common mode transients (even lighthing induced transients). This last item could, arguably, be accomplished by a fairly small conductor (in relation to the service ampacity), ie, #6.

Yes/No ?

That's the way I see it. 250.66 also sets the size of the main bonding jumper and that does make the size of the service conductors significant. The service conductor size may be the biggest factor when computing available fault current. If you get a real grounding electrode like a Ufer you might be using a pretty good part of the 4ga you would get from 250.66 (on the typical 200a service) I tend to lean towards the Tom W side with my house. I have 8' rods on both ends of the house plus one at the garage, with a ground ring between them. When I poured my patio I bonded the rebar and wire to get some Ufer effect (I have a spa so I had to anyway but I made sure the whole 600 sq/ft was bonded) It gives me a good place to hang my surge protection. Knock on wood, I am 20 years in Florida without a piece of blown equipment and I have lots of 24/7 stuff.

Yes. The GEC carries very little current, even with a fault, as the path through the earth back to the grounded point of the utility transformer is fairly high resistance, perhaps as much as a hundred ohms, and normally has a metal conductor in parallel with it.

The equipment grounding conductor within a facility is a different matter. It carries the total fault current. Its purpose is to allow the protective device to trip, and to maintain touch potentials at safe levels.

Ben Miller