Here's the situation- The contractor tapped the 400a residential meter can twice with Service Entrance conductors. Once with 4/0 Aluminum to feed a 200a Service panel and again with 2/0 Aluminum to feed a 150a Service panel. Then he installed a #4 awg cu. GEC out of the 200a Service panel and ran it to the water service. Out of the 150a Service panel he ran a GEC #6 awg cu to the water service. The water service is about 75 feet from the Electrical Service panels. What is wrong with this approach?
250.66 table says "Size of Largest Ungrounded Service-Entrance Conductor".
Then you read note 1 and it says
"Where multiple sets of service-entrance conductors are used as permitted in 230.40, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set."
so it sounds more like you can have one GEC, but you size it to the sum of the 2/0 and the 4/0 (344Kcmil) rounded up to 350 which gives you a #2 copper GEC to the first electrode which could be tapped by the #4 and #6 going to each panel.
From a practical sense I am not sure what difference it makes.
Roger- I'm not sure I understand your statment about paralleled conductors if I take the purist difinition of paralled conductors as discribed in Article 300. As for Greg's statment, I agree with his calculation approach and that's the way I teach it. I like the illustration in the handbook (I know, I know) because it's clear and cost effective if your trying to save the customer money. Typically, I would see a number 2 coming out of the 200a panel going to the water service and a number 6 coming out of the 150a panel and being split bolted on to the number 2 external to the 200a panel right there at the electrical service location.
Exhibit 250.28 is the paradox. If he put a 6" piece of #2 on the water pipe and tapped that for each panel it would look like the picture in the handbook but I would see that as a spliced GEC. (obviously I don't like exhibit 250.28)
The most elegant solution is to make the GEC connection in the meter base with a #2 but that is not what he did.
I do see the "loop" Roger is talking about but that exists in exhibit 250.28 too. I think one #2 in either service panel would satisfy 250.24(A)(1)
If you think about it, in places where landing the GEC in the meter is not allowed (because it's inaccessible due to the meter tag), there is no way to avoid paralleling the neutral with GEC's with multiple disconnects/panels.
In fact, with metallic service-cable conduits, the conduit itself always parallels the neutral. I wish we could use the meter's lug for the GEC, since it's already outside.
George, It would pass inspection. Only thing "wrong" would possible be cost and minor waste of copper. It is an alternative way of doing things. Multiple services can use a single large GEC with permanent taps to the other services OR each can have a GEC sized for that service*. As far as parallel pathes, having more than one Grounding electrode bonded together would mean parallel pathes,but only to ground (earth). [two rods six ft. apart] Not a violation. Alan-- *I have seen six GEC connected to the same ground rod, with seperate clamps, for a six unit apt. blding. Waste of copper in my opinion. ***Make sure he labels the service disconnects 1 of 2 & 2 of 2 NEC 230.70(B)
[This message has been edited by Alan Nadon (edited 11-29-2005).]
Just read 250.64(D) and I don't see a problem with doing it that way and what I noted is that the GEC "tap" conductors must extend inside the panels involved. So coming off the meter base don't solve the problem that Roger stated about parallel conductors. I'm still struggling with the fact that we are having a parallel situation here. I don't think it is a true parallel condition. We do have conductors that are both returning to the Xformer but the GEC is really just a back-up for the Neutral but even if we, for one fleeting moment, considered them in parallel the different sizes would violate 310.4. If the code considered them to be in parallel then I'd expect them to comply with 310.4.
They get pretty serious about the parallel neutral deal here. If the GEC lands in the meter can there can't be a parallel path to the service disconnect enclosure (3 wire SEC in PVC is the norm). On the line side of the GEC they don't seem to care since there is always some sort of parallel path, even if it is just through the dirt. I will usually see about 2-3a on my GEC and I saw similar currents on the #8cu that comes down the poles from the transformers around the neighborhood. Yeah I know, I have too much time on my hands.
I am still not sure why he couldn't ground the service in one of the disconnects with a #2 and be done with it.