Grounding Conductor

[ed downey]

This Project Is Under The 1999 NEC.

I Have A 2500A Double Ended Switchboard With A Full Sized 2500A Tie Breaker.

I Have Run A #6 Copper Wire To A 3/4" x 10' Driven Ground Rod. A #3/0 Copper Wire To The Incoming Water Service. And A #3/0 Wire To The Building Steel.

There Has Been A Claim That The #3/0 To The Water Line Is No Good Because It Does Not Have Contact With At Least 10' Of Earth {1999 NEC 250-50(a)}. Also The Building Steel Is Not Acceptable Because It Is Isolated And Supposedly Not Effectively Grounded {1999 NEC 250-50(b)}.

If All Of The Above Is True Does The Wire To The Driven Ground Rod Need To Be Larger Than #6 Wire {1999 NEC 250-50(a)(2)} (ie Does It Have To Be #3/0 Per Table 250-66)

Also The System Was Tested With A Three Point Fall Of Potential Method And Is Below The 5ohms Required In The Specifications.


Any Help With This Matter Is Appreciated.

-Ed

 

[don_resqcapt19]

Re: Grounding Conductor

The GEC to a driven rod is never required by the code to be larger than #6 copper.
Don

 

[physis]

Re: Grounding Conductor

Hi Don,

I know you and I have been over this section a couple times, but, 250.66(B) requires 4 AWG copper. That's a made electrode. Do I have this one right?

Forgive me Ed, I'm practicing.

 

[don_resqcapt19]

Re: Grounding Conductor

physis,
250.66(A) is for the driven rod, 250.66(B) is for a concrete encased electrode.
Don

 

[brian john]

Re: Grounding Conductor

Ed:

I would review how the three-point test was made. If the building steel was measured as part of the electrode system either separate or connected to the driven electrode the sphere of influence is quite large. Typically the current or C2 probe is taken out ten times the diagonal of the electrode for a ground grid or building utilized as an electrode or 10 times the depth of a driven electrode. I.E. for a ten foot rod 100 feet for a building with a diagonal of 100 feet, C2 is taken out 1000 feet.

When the test is performed multiple measurements are taken usually moving P2 out 10% steps of the total C2 measurement. As you approach the 66% of C2 distance one should arrive at what is considered the effective earth resistance value. As the C2 probe is moved further out the resistance value should increase substantially from the readings taken up to the 66% mark. If the readings do increase you have measured the effective earth resistance value. If the readings remain flat the potential test lead should be moved further out and the test performed again. Normally we perform the test in three different directions (if practical).

It has been me expierence that short cuts are often taken in performing the test that result is less that accurate test results.

 

[roger]

Re: Grounding Conductor

Ed, regardless of the argument of the 3/0 not being any count as far as the GEC is concerned, it would still have to be in place as far as 250-104(a)(1) if applicable

As a more indepth explanation of Brian John's reference to the test proceedures, any one interested can go here http://www.tradeport.on.ca/ApplicationNotes/aemc.pdf for some good information.

This is pretty close to J.G. Biddle information.

Roger

 

[dereckbc]

Re: Grounding Conductor

Ed, I don't know if I can add much to what has already been stated, but here is my oppinion from a code standpoint.

It appears you met the intention of the code. You have proper sized conductor bond to the water pipe, and building steel. Although they might not be considered electrodes, they are still required to be bonded per 250.104 as Roger stated.

You have one electrode defined by 250.50(A)(6) which is a rod. As don stated a rod is not required to be larger than a #6 AWG copper per 250.66(a).

The only thing an inspector might insist on is another supplementary electrode if he doubts the validity of your test.

I reflect the doubts that Roger and Brian have expressed about the impedance you have claimed. I asume this is a large building. In order for you to get an accurate reading on the three point test, would require test lead lengths of at least 10 times the diameter of the building if the building steel were connected at the time of the test. I suspect the inspector would have the same doubts and require you to prove it. It would be easier to just add another rod and walk away.

Your other problem is related to the spec of 5 ohm's or less, but that is not a code issue.

 

[physis]

Re: Grounding Conductor

Thank you Don, I was considering made and driven to be the same.

 

[ed downey]

Re: Grounding Conductor

Thank You All For Your Responses To This Matter. I Believe I Can Confidently Write A Letter Confirming The Installation Is Correct.

Brian Regarding The Building Steel Issue These Tests Where Performed By Electro Test (Independent Testing Agency) And They Did A Test With The Building Steel Removed From The System And One With The Building Steel Attached To The System. The Test Results Where Fairly Close To Each Other For Each Of These Tests. They Went Out 100' From The Building For Their Fall Of Potential Method.

Thanks Again,
Ed