Area controversy around sizing of GEC for ground rod

[wbalsam1]

Which is my argument as well, if one rod can not be used as an electrode if it doesn't meet the 25 ohm requirement then it is not an electrode until the second rod is added, at this time a single electrode has been created.

Roger

If it's a conducting object through which a direct connection to earth is established, than it is an electrode according to the definition of grounding electrode. The requirement for a supplemental electrode does not redefine the 1st rod electrode as something else other than an electrode.

 

[roger]

If it's a conducting object through which a direct connection to earth is established, than it is an electrode according to the definition of grounding electrode.

So in your opinion a VHF antena can be a GE.

Roger

 

[wbalsam1]

So in your opinion a VHF antena can be a GE.

Roger

My opinion is to look through the Article on Definitions and if a definition is provided in a particular circumstance, then apply that to the issue at hand. 810.21 (E) (F) & (J) indicate to me that a GE is required for an antenna...so the antenna can't really be the GE and the antenna at the same time, can it?

 

[fredfriarpe]

Another viewpoint.

Another viewpoint.

I would not sign and seal a design of a 600A service with anything less than a #2/0(Cu) grounding electrode conductor, any more than I would a 4000A service with anything less than a #3/0 (Cu) grounding electrode conductor. Even if someone tried to convince me a single ground rod could have a resistance of 25 ohms or less, the odds of an AHJ or Utility Company being on board with a single ground rod (and a #6 conductor, for that matter) are slim, and in the event an authority decides to have someone prove the 25 ohm resistance stated in 250.56, and additional rods are required, the #6 will have to be removed anyway. (If the interpretations discussed previously are correct it would lose its designation of "sole connection" the moment another rod is driven and the two (or more) rods are jumpered together.)

In my professional opinion, I will always defer to the values in Table 250.66 when establishing a neutral for the service; it is too much of a stretch for me to assume that a utility company's grounded service conductor is adequately grounded upstream to do that for me in the event the #6 conductor fails.

There is an Exhibit 250.31 in the 2008 NEC Handbook which I believe to illustrate the situation of a "sole connection."

I would side with the inspector on this one. My $0.02.

 

[jwelectric]

What about my temp service with the ground rod that reads 10Ω?

Well George;

I would say that if you had a service of any kind that achieved 10 ohms with an eight foot rod you should immediately purchase you a lottery ticket before your luck changes. I base this statement on the findings of several test done by various testing labs over the past 25 years and some that can be found right here on this forum. One that comes to mind and you can research was done between 1988 and 1993 by Computer Power Corporation where they published that the average depth to achieve 25 ohms was between 20 and 35 feet nationwide average.

As an inspector I would require you present a documented test by a licensed engineering firm before I would accept the single rod. New to the 2011 cycle in 250.53(A)(2) this is made a little clearer as two rods will be required unless this test is presented at the time of inspection.

Personally I think that all driven electrodes should be tested over a period of five years and the average resistance not to exceed 5 ohms before the driven electrode would be accepted or that all driven electrodes be no less than 50 feet in length. I believe this would bring to an end the use of this useless piece of electrical equipment.

 

[roger]

I would not sign and seal a design of a 600A service with anything less than a #2/0(Cu) grounding electrode conductor, any more than I would a 4000A service with anything less than a #3/0 (Cu) grounding electrode conductor. Even if someone tried to convince me a single ground rod could have a resistance of 25 ohms or less,

Fred, all of that is yor perogative as a designer but, it is well over the code and the performance of a rod or pair of rods. Do you think a set of rods will perform better with a large conductor tied to them if the earth resistance is say 100 or even 25 ohms?

the odds of an AHJ or Utility Company being on board with a single ground rod (and a #6 conductor, for that matter) are slim, and in the event an authority decides to have someone prove the 25 ohm resistance stated in 250.56, and additional rods are required, the #6 will have to be removed anyway. (If the interpretations discussed previously are correct it would lose its designation of "sole connection" the moment another rod is driven and the two (or more) rods are jumpered together.)

If some of the discussions were in fact the case, you could simply run one #6 to each rod. Unless an AHJ has amended the NEC (assuming that is the adopted code) they would have to go pound sand if they didn't like it.

In my professional opinion, I will always defer to the values in Table 250.66 when establishing a neutral for the service; it is too much of a stretch for me to assume that a utility company's grounded service conductor is adequately grounded upstream to do that for me in the event the #6 conductor fails.

And that's fine as long as that is in your design so that it can be included in the quote or bid.


Roger

 

[roger]

As an inspector I would require you present a documented test by a licensed engineering firm before I would accept the single rod. New to the 2011 cycle in 250.53(A)(2) this is made a little clearer as two rods will be required unless this test is presented at the time of inspection.

So if I had the test set present and showed you the test in progress while you were inspecting you wouldn't accept it because you would want a third party test, where can we find that requirement?

I believe this would bring to an end the use of this useless piece of electrical equipment.

Roger

 

[George Stolz]

I would say that if you had a service of any kind that achieved 10 ohms with an eight foot rod you should immediately purchase you a lottery ticket before your luck changes.

I wouldn't call it luck, I would expect there are places here and there all over the U.S. where a ground rod would show a low reading.

As an inspector I would require you present a documented test by a licensed engineering firm before I would accept the single rod.

Man, finding an engineer with a FOP meter would be more of a feat, I would think.

New to the 2011 cycle in 250.53(A)(2) this is made a little clearer as two rods will be required unless this test is presented at the time of inspection.

Bless their hearts.

Personally I think that all driven electrodes should be tested over a period of five years and the average resistance not to exceed 5 ohms before the driven electrode would be accepted or that all driven electrodes be no less than 50 feet in length. I believe this would bring to an end the use of this useless piece of electrical equipment.

From my little Google-fest from a moment ago, I would think the overall lifespan of the ground rod would be a question for greater than five years from now.

 

[fredfriarpe]

If some of the discussions were in fact the case, you could simply run one #6 to each rod. Unless an AHJ has amended the NEC (assuming that is the adopted code) they would have to go pound sand if they didn't like it.
Roger

I'm still not convinced that could be that case. If the intent of the code is such that for a grounding electrode conductor, all we need is one #6 for each ground rod - no matter if the current-carrying conductors entering the equipment are for a 100A service or a 4000A service or higher, why bother having Table 250.66 at all? Why wouldn't the code clearly state something like "For each grounding electrode provide a #6 conductor"? How could we justify that two ground rods with two #6 would provide an equal grounding electrode system to a #3/0 connected to two rods?

Believe me I'm not a fan of "big timing" projects and adding a bunch of needless expense, and being "over code," but in my short 20 years in this game and hundreds of facilities, if not thousands of grounding electrode systems - I've never once had someone question a grounding electrode conductor size - in peer review, value-engineering exercises, or curious electricians asking "Why is it so oversized?" That doesn't mean I'm not enjoying the discussion and would enjoy seeing a clearer cut justification, or the reasons given by the original inspector in the initial post.

 

[don_resqcapt19]

... How could we justify that two ground rods with two #6 would provide an equal grounding electrode system to a #3/0 connected to two rods? ...

Fred,
You are an engineer. That should be a very simple calculation.
The fact that you ran a large conductor to the grounding electrodes will have very very little influence on the resistance of that grounding electrode system.
The grounding electrode conductor is in series with the grounding electrode system. The resistance of #6 is 1.61 ohms per 1000' and 3/0 is 0.253 ohms per 1000'. If the GEC is 25' long and the ground resistance of the rod is 25 ohms, you would have a resistance of 25.04 ohms for the rod connected with a #6 and 25.006 ohms for the rod connected with the 3/0. How can this make any real difference in the performance of the grounding electrode system?

 

[fredfriarpe]

Fred,
You are an engineer. That should be a very simple calculation.
The fact that you ran a large conductor to the grounding electrodes will have very very little influence on the resistance of that grounding electrode system.
The grounding electrode conductor is in series with the grounding electrode system. The resistance of #6 is 1.61 ohms per 1000' and 3/0 is 0.253 ohms per 1000'. If the GEC is 25' long and the ground resistance of the rod is 25 ohms, you would have a resistance of 25.04 ohms for the rod connected with a #6 and 25.006 ohms for the rod connected with the 3/0. How can this make any real difference in the performance of the grounding electrode system?

It was a rhetorical question; the 3/0 is indeed about twice the cross sectional area of two #6 (assuming for a moment it is even permissable to parallel ground conductors smaller than #1/0 - 310.4 - which could turn into a whole other argument).

I appreciate your resistance calculation, but again I'm hanging my hat on the conductor sizes presented in the table, as a fraction of the cross sectional area the service entrance conductor. It is my prerogative to err on the side of the ampacity of the conductors connecting the service to the grounding electrode system - moreso than the resistance. I become more concerned with resistance at the grounding electrode system itself. I've spent many Saturday afternoons at commisionings watching electricians drive 20-30-40' of ground rods to get a meter to read 25 ohms; that meter was never connected to the grounding electrode conductor.

We engineers "hang our hat" a lot on tables like this, especially when they have been codified by federal, state, and local governments, and in court you could imagine how convulted it could be to convince a jury of this sort of technicality. I understand these system come across as "unecessary" or some sort of "black art" to many - although rightly or wrongly the local utility company in my region has been held liable for damage to equipment in residences due to improperly installed grounding systems, and for that reason, they require at least two ground rods and and inspection prior to turning on service. The moment I sign a drawings, I inherit the same potential liability, so I will err on the side of using the tables.

 

[mivey]

Which is my argument as well, if one rod can not be used as an electrode if it doesn't meet the 25 ohm requirement then it is not an electrode until the second rod is added, at this time a single electrode has been created.

Roger

I see it like that also. The joining of multiple electrodes makes them part of a system. To me, the augmenting electrode is really part of a subsystem that makes up one electrode. You can call the primary and augmenting electrode a system if you like, but they are not a system in the same sense as we have with joining single, non-augmented electrodes.

In my professional opinion, I will always defer to the values in Table 250.66 when establishing a neutral for the service; it is too much of a stretch for me to assume that a utility company's grounded service conductor is adequately grounded upstream to do that for me in the event the #6 conductor fails.

And what if the humongous conductor failed? Would you feel any better?

 

[mivey]

...How can this make any real difference in the performance of the grounding electrode system?

Exactly.

...I'm hanging my hat on the conductor sizes presented in the table, as a fraction of the cross sectional area the service entrance conductor.

You are willing to hang your hat on one part but not the other.

We engineers "hang our hat" a lot on tables like this

You are speaking for yourself, not all engineers.

I will err on the side of using the tables.

You err by taking the stance of "bigger is better". Don't think that position will give you any immunity in court as you can find "experts" willing to testify to almost anything.

 

[roger]

(assuming for a moment it is even permissable to parallel ground conductors smaller than #1/0 - 310.4 - which could turn into a whole other argument).

Sure you can parallel a GEC, regardless of it's size, it's probably better if you do, the same applies for an EGC. To clarify the requirements of 310.4, the GEC and EGC are not included in the article section so it does not apply to either.

310.4 Conductors in Parallel.
(A) General. Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, polarity, neutral, or grounded circuit conductor shall be permitted to be connected in parallel (electrically joined at both ends).

Roger

 

[Dennis Alwon]

Roger check out 310.4(B). The EGC is stated there

(B) Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following:
(1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner

 

[roger]

Roger check out 310.4(B). The EGC is stated there

Yeah, it's kind of funny that it is not included in the General requirement but shows up later in (B) and the requirements of (B) would be impossible to meet when metailc conuit is used with a wire conductor as an EGC, you have two different materials in parallel already.

Roger

 

[Dennis Alwon]

the requirements of (B) would be impossible to meet when metailc conuit is used with a wire conductor as an EGC, you have two different materials in parallel already.

Roger

I never thought about that. LOL

 

[fredfriarpe]

And what if the humongous conductor failed? Would you feel any better?

Certainly. I would consider it a much more defensible position from the standpoint of my design. I would much rather see a #3/0 which was designed based on my intepretation of the code burnt in two and speculate what amazing act of nature caused that, vs. a pair of #6 burnt in two with a bunch of us standing around having a discussion equivalent to this thread trying to remind ourselves why we didn't go "humongous" (per the Table, of course). ;-)

 

[Dennis Alwon]

I've spent many Saturday afternoons at commisionings watching electricians drive 20-30-40' of ground rods to get a meter to read 25 ohms; that meter was never connected to the grounding electrode conductor.

If it is 3 point fall of potential test you would not connect to the grounding electrode conductor.

 

[fredfriarpe]

If it is 3 point fall of potential test you would not connect to the grounding electrode conductor.

Exactly. That was my facetious way of saying we were measuring the resistance of the grounding electrode system, and not the resistance of the grounding electrode conductor.