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Joined: Jun 2007
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I have a 4000Amp Delta/Y service with parallel 500 kcmil size wire. I have a Delta ground system ,building steel and a cold water pipe. How do I apply Artlicle 250.66. I've been told that my 250kcmil grounding electrode is too small. This Ground runs straight thru a bond bushing and passes thru a bonding lug on a ground plate, then outside to my Delta Ground. The grounding lug and Plate are bonded to building steel via a #6 CU. Separately the cold water pipe at entry is attached to the grounding plate via a #6 CU.

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Is it possible that maybe they were referring to using a 2/0 CU for the bond to the cold water piping instead of the #6 CU?

Joined: Jun 2007
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I believe you could be in the ball park except that i still don,t understand how to accomplish knowing per code which size is the correct size.

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The wire size I mentioned for the bond for the cold-water piping was just given as an example, assuming two conductors per phase. You most likely have more than that.

Basically, you would total the Circular Mil Area of all ungrounded conductors for one phase. Then use table 250.66 to size your grounding electrode conductor. Sounds like you could be okay there.
You would normally do the same for the bonding jumpers for the cold-water piping and the building steel.
Generally, as I read it, that #6 Cu would only be good when used as the sole [only] connection to the ground plate or rod when it is used as a supplemental ground. There may be other issues, but it seems likely that these could at least be part of the problem.

Just FYI, The Soares book on grounding is an excellent source of information. At around $50.00, it’s a great investment that can really help clear the fog surrounding grounding and bonding in the NEC.

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I'm not sure I understand all parts of the question but based on what I do understand I would say that GEC required based on 250.66 would be 3/0 copper or 250 aluminum. This is based on having parallel 500 kcmil Service Entrance conductors. I am confused by the statement "Delta/Y Service". Is it a Delta or a Wye? Next I guess depending on how you routed the GEC, you could run the large GEC to the water pipe and building steel then run a smaller wire (#6) to the plate and rod as a supplement.

The plate and rod are optional electrodes since the water pipe would have a supplement with the building steel. Assuming the water pipe and building steel qualify as grounding electrodes. 250.52(A)(1)(2)

I don't know where his Service conductors are located- Whether he has a step up Xformer or step down or where his GEC originates. Need to have this info to really size the GEC. Customer owned Xformer? Service point?

Last edited by George Little; 09/16/08 07:37 AM.

George Little
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By Delta Y Service do you Mean Primary Delta Secondary y.

By (Delta Ground) I take it your refering to a 3 points grounding system outside.(Looks like a Delta)Used a Lot on Large Industrial projects.
If this is so Grounding wire to this can be smaller then 3/0.
I also take it that this is a 4000 amp 480/277 Service.
With 11 sets of 4- 500 KCMIL.Each in there own conduid.
Transformer sized & owned by Utility Co.
I'd say 3/0 copper for all except plate # 6 there.
only thing that confuses me is the Bonding bushing Statement??(Metal pipe enclosinging the Grounding Electroid conductor prehaps???(250.64 E)
This is what I come out with by what I've read in your post.
One mans opinion



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Thanks Ernie, I guess I'm not the only one confused by lack of info.
I would also add the the bonding of equipment, raceways, etc. could be subject to the 12.5% calculation if the conductors are all in the same raceway. The GEC sizing if limited to T. 250.66 but the bonding is subject to the 12.5% and it's possible to have a bonding conductor larger than the GEC.


George Little
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You have a good since for figuring out the missing details!
I appreciate All your perceptions and in greater details. The service is feed from a transformer from the poco. I can't remember how many parallel sets of 500 Kc Mil. The posts ahead of yours have also shed some light on the supplemental ground. It is possible that The Engineer is wanting redundant Grounding electrodes versus allowing me to have a supplemental in addition to my ground going to the three point system. I'm Still awaiting his explanation. Oh I failed to mention that the lightning protection system is attached to this supplemental ground plate. Also the poco is providing me with 480 from their transformer.

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OK ,So I would have to take 12.5% of the total circular mils
of the 500 KC mil*3 then size my bond wire coming from the main going to the bushing thru to the ground plate, I'm Sorry but, then what?

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The size of your bonding on the line side of the Service is based on the 12.5% of the cross sectional area of the conductors in the raceways. See 250.102(C).

From what I can determine the GEC will probably be 3/0 cu. or 250 kcmil al. and there is a strong possibility that the equipment bonding conductor might be larger than the GEC.

The conductor running to the plate electrode need only be max of #6 cu. if you lay the job out according to 250.66(A)

Last edited by George Little; 09/16/08 10:35 PM.

George Little

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