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Posted By: Electricmanscott Grond rod questions - 03/29/03 09:51 PM
Installing a service in an old barn today. Could not get the ground rods in. (bedrock?) Instead I laid them in the trench that the service is run in. It is about three feet deep. I used three rods, each one six feet from the other. Reading through the handbook I am not to clear on this exact situation. Appreciate all thoughts.
Posted By: Fred Re: Grond rod questions - 03/29/03 11:39 PM
Scott. According to 250.53(G) you've satisfied the NEC providing the trench you put them in was at least 30" deep. Sounds like you did the best anyone could do and then some.
Posted By: zapped208 Re: Grond rod questions - 03/30/03 12:41 AM
20' of #4 bare copper would be acceptable in the ditch too, if your AHJ has no problems with that.
Posted By: resqcapt19 Re: Grond rod questions - 03/30/03 01:21 AM
Zapped,
Please cite the code section that permits the use of 20' of #4 in a ditch as a grounding electrode.
Don
Posted By: Trumpy Re: Grond rod questions - 03/30/03 02:20 AM
We do this sort of thing all the time in New Zealand, especially where a really good Earth is required, we lay out about 20-30 metres of 35-50mm2, for really poor soils and it works a treat.
This sort of system is really only used for Earthing Distribution Transformers and the like, no one would like a bill for this sort of wire at these lengths. [Linked Image]
Posted By: zapped208 Re: Grond rod questions - 03/30/03 02:51 AM
Don,- While there is no direct code section stating we can install the electrode system as I said, there also is no wording that says I cannot. Basicly we are using 250.52(A3&4) without the concrete. On our larger services we use 2,3,&4/0 copper. Our AHJ's have no problems with these installations. Don't get me wrong, I do use ground rods 90% of the time.
Posted By: DBC1 Re: Grond rod questions - 03/30/03 03:57 AM
Scott, what you did will satisfy code. Another option would have been to use a ground ring or plate electrode.
Posted By: resqcapt19 Re: Grond rod questions - 03/30/03 04:06 AM
zapped,
There is wording that says you can't do that. The code lists the grounding electrodes that are permitted to be used in 250.52. Installing 20' of bare copper in a ditch is not on the list, unless the 20' encircles a building. I see no reason why it shouldn't be just as good as a ground ring, but it is not permitted by the NEC rules.
Posted By: frenchelectrican Re: Grond rod questions - 03/30/03 04:47 AM
don :

i hate to ask you a stupid question but i know i will respect you with this one anyway but you mean that cant bury bare copper wire unless it is around the building that mean all the way around the building area ???
( i never done this one so i am double checking i did see it on nec code say cant bury it in short straght section ..)


thanks for your time


merci marc
Posted By: zapped208 Re: Grond rod questions - 03/30/03 04:55 AM
Don,- Maybe the CMP should take a closer look at 250.52(A4), kinda hard to take 20' of wire and encircle a 25+ 40' building.
Posted By: resqcapt19 Re: Grond rod questions - 03/30/03 05:37 AM
zapped,
The 20' is the minimum length permitted. You could have a 4' x 4' building. The wording also applies to "stuctures" not just buildings, so the 4' x 4' is not out of the question. I know that the 20' in a ditch would be just as good if not better than a 20' ground ring, it just is a violation of the code wording. A good place for a 2008 proposal.
Don
Posted By: frenchelectrican Re: Grond rod questions - 03/30/03 05:44 AM
ok don thanks for explaining that code now and i hope they can do something in 08 code to clear it. so we can understand the wordings more clear with out more confusing facts

merci marc
Posted By: zapped208 Re: Grond rod questions - 03/30/03 12:12 PM
Don,- Thanks for your input about this topic and hope to have more discussions in the future about some more grey areas of our NEC. Zapped208
Posted By: Edward Re: Grond rod questions - 03/30/03 11:46 PM
I had the same bedrock problem on my last 400 Amp panel upgrade. I laid in a 3/4"x10' rod along with 40' of bare copper. AHJ approved the installation.

Edward
Posted By: elecbob Re: Grond rod questions - 03/31/03 02:22 AM
If I lay ground rods in a trench, how do I keep the acorn accessable?
bob
Posted By: sparky66wv Re: Grond rod questions - 03/31/03 03:00 AM
Of course, you must try to drive them, first at a 90º angle, then up to a 45º and hit bedrock...

When I throw them into a trench, I first find a good solid limestone rock embedded in the soil and tap the rod into it at a 90, declare "Oh, I've hit stone" then tap it at a 45 and declare, "Gosh, I've hit stone again" and then I chuck it into the trench...

[Linked Image]
Posted By: stamcon Re: Grond rod questions - 03/31/03 03:12 AM
Bob, the acorn doesn't need to be accessible, as long as it's listed for burial. With a driven rod, unless you're using a 10' rod, the acorn should be below grade, in order to have 8' of rod in contact with the earth.
Posted By: Electricmanscott Re: Grond rod questions - 03/31/03 08:54 PM
The acorn can't be left above grade unless protected from damage. By the way the job was inspected today with no problems.
Posted By: sparky Re: Grond rod questions - 04/01/03 09:35 PM
E-Scott,
was this a barn for animals??
Posted By: Trainwire Re: Grond rod questions - 04/01/03 10:00 PM
I have another question.

Here at the railroad, if we need a long drill bit, we get out the welder, and we make a long drill bit. Would it be legal/kosher to bond a cheap carbide masonary bit to the bottom of yon ground rod, and chuck it up in the Hilti, and go to town? leaving the drill bit at the bottom, hence the reason for using a cheap drill bit.

TW
Posted By: Electricmanscott Re: Grond rod questions - 04/01/03 10:22 PM
No animals anymore. Just a big old historic barn that the local yokels are trying to preserve. The service is the most structurally sound part of the building!
Posted By: sparky Re: Grond rod questions - 04/02/03 10:10 PM
all code quagmires aside, i often wonder what is the better job in ohmic terms.
not having the scratch for a good tester, i'll probably never know.
Posted By: Bjarney Re: Grond rod questions - 04/02/03 11:13 PM
My water-well casing reads a solid two ohms by the three-point method, but the place was built in 1935. Drillers haven't used steel for domestic-well casing for ~30 years in this area.

Regardless of test numbers, nothing can prevent destruction from a direct lightning hit.




[This message has been edited by Bjarney (edited 04-02-2003).]
Posted By: zapped208 Re: Grond rod questions - 04/03/03 12:18 AM
What do they use now?
Posted By: elecbob Re: Grond rod questions - 04/03/03 04:47 PM
Thank you for the info guys. A commercial property is completely covered by a 6 inch thick concrete slab. They saw cut the concrete for plumbing equipment. I will drive the ground rods in the trench area and the acorns will be buried below the concrete. I was worried I'd have to block out the concrete for access to the acorns.
Bob
Posted By: Bjarney Re: Grond rod questions - 04/03/03 10:45 PM
zapped — for 8-inch or less casings [maybe larger] — it's PVC.
Posted By: Bjarney Re: Grond rod questions - 04/03/03 10:50 PM
As far as test instruments, local AEMC and Megger reps may be able to demonstrate their wares on your jobsite. Short-term rental may be another option.
Posted By: Obsaleet Re: Grond rod questions - 04/13/03 02:51 AM
Bury the rods in the concrete! My inspector claims the concrete has the right moisture content and that it stays steady.

Phil
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