I have question for you all reguarding a recent situation. I just completed installing a 600a 3phase 208v service. The customer decided to redue there driveway/parking lot. They found the old galv water main was leaking undergound and decided to replace this with PVC. Now my new service has no grounding other than 2 grounding electrodes. There is no building steel as the building is 200 yrs old, wood and stone. What would be the next best solution? Grounding ring is out as there is no way to loop around this structure.
I thought that was the reason that the code changed requiring two electrodes back in the 70s (?). It was my understanding that the proliferation of plastic plumbing rendered a cold water main unreliable as an electrode. Aren't we pretty much just bonding them now?
Dig a Ufer trench but go beyond #6 bare solid -- use 1/0 stranded instead. ( Presumed 900 kCMIL aluminum feeders ... see 250.122 table. )
Come down out of your Service in PVC with a provision for a bonding lug where the GEC leaves the Service IF IT IS METAL.
If your Service is a NEMA3R pad-mount then break out the roto-hammer and punch down through the pad, tunneling down and away into your UFER trench.
Strictly speaking once you've gone beyond #6 ENCASED in concrete you are no longer installing a UFER, per se. Instead such GEC's are referred to as GROUND RINGS; and no, they don't have to loop entirely around the building.
( You could, but only the government is willing to waste that kind of money.)
I normally place the STRANDED 1/0 within clean, moist sand at the bottom of a UFER-length trench ( at least 30" of cover )and have it inspected. ( The end and the start being still exposed to view, the rest under 3" of sand and in my area the GEC must have AT LEAST 20 feet exposed from start to finish in the trench, the GEC then continues unbroken via deep burial/PVC/concrete cover up to the Service bonding rail.) Then I would normally puddle a rat-slab of concrete over it at least 4" thick. After it dries I hit it with marker paint, typically red or orange. Finally, I bury the work with clean fill starting with the remainder of the sand on hand.
At 600 Amps and without metallic piping a supplemental electrode of this size is necessary.
Further inspection of 250.53 (2002) especially in the Handbook will prove fruitful.
As it stands you have an ungrounded system and no end of troubles can come from that.
Let's back up a moment, and consider just what the water bond is meant to accomplish. I see three things:
1) To serve as an equipment grounding conductor; that is, to ensure a breaker trips if the pipe becomes energized;
2) To let the metal pipe serve as a grounding electrode;
3) To provide nearby buildings, that probably share a PoCo transformer, an alternate path ("neutral") back to the transformer through the shared piping.
Let's look at #3 for a moment. Let us imagine a squirrel has chewed through that tasty ASCR service drop conductor at your neighbor's house. What happens then? Well, maybe ... YOUR plumbing becomes energized.
The 'stranded' neutral current will still try to get back home to the transformer. Since we've grounded everything, and tied the neutral to the ground at the main service disconnect, this means that all the grounds will become "hot." Some of this current might travel through the earth itself to the ground rid for the transformer - but dirt is usually a much worse conductor than metal.
With the neighbor's plumbing now energized, so is yours. His current enters the plumbing, comes over to your house through the water main, and returns to the grid through your water bond. Everytjhing operates just fine.
When the water company removes that connection -the water main- from your house, this can no longer happen. You don't need that water bond any more.
After all, your plumbing is bonded (with smaller wires) at the appliances, and you have a ground rod to serve as the grounding electrode.
In the above situation, the moment the plumber cuts the metal pipe he'll see a spark, might even get a shock. Then, most likely, the house with the bad neutral will start having things burn out, as 120v appliances will be exposed to voltages as high as 240v. The TV will fry to protect the can opener.
IMHO, you really don't need to do anything more, since your installation would be NEC compliant as is. Just be sure to bond the buildings interior cold water piping with a bonding jumper sized according to Table 250.66, which I'm sure you have probably already done anyway.
If for some reason you're uncomfortable with the current setup, you could always install ground plates or even additional ground rods until the cows come home, but I don't know of any NEC requirement for additional supplemental grounding beyond what you already have for this 208V service.
The NEC is quite explicit when it states that the GEC need not ever be larger than #6. When you combine this with the '25 Ohm rule," it's pretty clear that connecting an electrical system to the dirt under it has absolutely nothing to do with either the service size, or clearing faults.
Whatever the ground is for ... 'reference plane,' lightning, whatever ... one rod ought to be plenty.
Yes, I'm aware of engineers who specify absolutely massive wires for the GEC, and ridiculously redundant networks of grounding electrodes. I won't be too harsh on the EE's, though; Article 250 is positively schitzoid in it's language. I believe this reflects the various theories regarding grounding that have held sway over time. Fortunately, the last two cycles have done a lot to untangle this mess.
Unless you have an installation that itself generates a great deal of static electricity - radio towers come to mind - I don't see any benefit to exceeding code on this topic.
Reno: IMHO, your statement above (#6) is compliant, but.... As the rods are the only "available" 'electrode' I would consider sizing the GEC per table 250.66 based on the OCP of the main. Again I stress this is my opinion.
Yes, I have seen & installed quite a few engineered grounding systems, some which you will say are ridiculous, but...that was the spec.
If this was a new job with footings, then the UFER would have been mandatory, enforced by the Building Inspectors here, using the rebar in the footings, or #4. I guess Tesla likes to 'dig'!
Ideally, when the 'customer' decided to upgrade the parking lot, that would have been a nice time to call OB!
John, I'm never really surprised when my 'too simple' grounding statements liven up the discussion a bit. I think we are in agreement that Article 250 is the most confusing part of the code, and the various theories about grounding all seem to have weaknesses. I suppose that's, in part, because you can't really see electricity, so none of us really understand it as well as we would like.
I have my own models ... and, while those models have been influenced by others (especially Mike Holt), I've tossed in a fair amount on my own. making some 'connections' that are a bit out of the mainstream. Still, I recognize them for the models that they are, and am willing to accept that others may differ.
I've also been around the trade long enough to have seen a few different theories wax and wane. Though some of the debates may seem quaint today, they were quite passionate at the time. Unlike many a salon philosopher, I'm not ready to blandly assert base motives to any of the factions; well meaning folks can differ on both form and substance.