Well, I see that some folks continue to assume that I'm doing things wrong, etc. Well, that's far easier to assert than to support your own bias with the code. Maybe I wasn't clear enough, so I'll try to develop (explain) my reasoning, using smaller words and shorter sentences.
I'm not going to reopen the "25 ohm" debate. This was made irrellevant when the 2011 code added 250.53, which mandates that a single rod be supplimented. Yet, this addition has further relevance, and I'll return to it later.
Let's first look at the the requirements for services. Our GEC has to go back to the first OCPD, where we bond the neutral to the GEC. After then, we keep the two completely separate. This works fine, at least when there is only one building. The question arises: what to do when there is a second building served by the same service?
At one time it was common to treat the feeder to the second building as if it were its' own service. These days, for all practical purposes, we can't do that anymore. Instead, we are required to run a ground wire with the feeder, and to continue to keep the 'grounds' separate forn the neutral.
As a reminder, there are two types of "grounds.' There are the equipment grounds, which help breakers trip, and there is the GEC- the wire from the ground rod.
The next question we need to ask is whether ANY 'ground rod' is required. I'd like to point out that the NEC does not require that any building have a 'ground rod.' One could - and probably would- rebuild the World Trade center without there being a single ground rod anywhere. Think about that for a moment.
We are required to have a grounding 'electrode,' not a 'rod.' A 'rod' is but one of the alternatives if none of the other grounding electrodes are used. If our detached building has a slab, one might easily make that slab into a concrete-encased electrode ("Ufer") and not need any ground rod at all. If the detached building has a metal water line, that might be the only electrode we need (as the pipe is not under the control of the water department).
If someone wished to suppliment the water pipe (or any other electrode) with another electrode, by banging in a rod .... I don't see any requirement that they use two rods. Instead, I see 250.53, which requires a single rod to be supplimented.
Now, let's look at 250.53 in greater detail. How do we suppliment the single rod? Well, 250.53(2)(5) says we can bond it to any grounded service enclosure. That sounds a lot like the panel for the detached building to me.
This might appear as if we are using the green wire of the feeder as both the 'grounding electrode conductor' and the 'equipment grounding conductor.' I submit that the NEC needs to clarify this point, as it's my view that the 'equipment grounding conductor' ends with the branch circuit. As I see it, the green wire with the feeder is simply a 'grounding conductor.'
The alternative to accepting my understanding is to not tie the ground rod into the panel at all, but to instead run a completely separate wire from our new ground rod to a ground rod at the first building.
What about the size of the green wire we run with the feed? Well, we size that by 'neutral' rules, which gives us a larger wire than if we were to size it by 'GEC' rules. I will concede that it's possible that a small panel in the detached building might have a feeder smaller than #6, so it is possible that the green wire will be smaller than the #6 that the NEC might require for the GEC of the main service.
That might present us with a problem. As long as the GEC for the detached building is large enough to serve the entire service, I see the grounding electrodes of the two structures as forming one single grounding network. In electrical terms, the two are not 'detached' at all.
Unorthodox view? I don't think so. After all, this is exactly what we've been doing with swimming pools, and the area around them, for years. Indeed, if you compare the 'equipotential plane' requirements, you will find that it also qualifies as a grounding electrode.
To sum up my view .... as long as that green wire for the feeder is #6 .... the rods at the separate buildings are part of the same grounding system, and fulfill any need there might be for a second electrode. Just think of them as being more -way more than- six feet apart.
