two ground rods negating the requirement to check for impedance

mshields

Senior Member
Location
Boston, MA
An inspector recently told me that if you drive two ground rods as part of your grounding electrode system, it negates the need to test for impedance. He was unable to site the code section. a) is he correct and b) where is it in the NEC?

Thanks,

Mike
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
An inspector recently told me that if you drive two ground rods as part of your grounding electrode system, it negates the need to test for impedance. He was unable to site the code section. a) is he correct and b) where is it in the NEC?

Thanks,

Mike
250.53(2) exception
 

Smart $

Esteemed Member
Location
Ohio
An inspector recently told me that if you drive two ground rods as part of your grounding electrode system, it negates the need to test for impedance. He was unable to site the code section. a) is he correct and b) where is it in the NEC?

Thanks,

Mike
He is correct. See 250.53(A)(2) and the Exception thereto.
 

mwm1752

Senior Member
Location
Aspen, Colo
it actually allows your electrode system to be 2 ground rods only -- usually due to no water pipe ground -- not even needed with stand alones like a UFER
 

topgone

Senior Member
An inspector recently told me that if you drive two ground rods as part of your grounding electrode system, it negates the need to test for impedance. He was unable to site the code section. a) is he correct and b) where is it in the NEC?

Thanks,

Mike
IDK how your inspector got that rule. AFAIK, if you drove one rod and the resistance you measured is higher than 25 ohms (or whatever ground resistance value is required), you need to add another rod/s to lower your ground resistance. If your ground resistance is lower than 25 ohms with just one ground rod driven to the ground, you don't need to drive additional rods. Crystal.
 

John120/240

Senior Member
Location
Olathe, Kansas
IDK how your inspector got that rule. AFAIK, if you drove one rod and the resistance you measured is higher than 25 ohms (or whatever ground resistance value is required), you need to add another rod/s to lower your ground resistance. If your ground resistance is lower than 25 ohms with just one ground rod driven to the ground, you don't need to drive additional rods. Crystal.
The effort & expense to measure the resistance of the first ground rod far out weighs the minimal effort & expense to just drive a second ground rod & be NEC compliant.
 

infinity

Moderator
Staff member
Location
New Jersey
IDK how your inspector got that rule. AFAIK, if you drove one rod and the resistance you measured is higher than 25 ohms (or whatever ground resistance value is required), you need to add another rod/s to lower your ground resistance. If your ground resistance is lower than 25 ohms with just one ground rod driven to the ground, you don't need to drive additional rods. Crystal.
The requirement is to drive one more rod, even if the resistance of the two rods is greater than 25 ohms nothing more is required.
 

kwired

Electron manager
Location
NE Nebraska
IDK how your inspector got that rule. AFAIK, if you drove one rod and the resistance you measured is higher than 25 ohms (or whatever ground resistance value is required), you need to add another rod/s to lower your ground resistance. If your ground resistance is lower than 25 ohms with just one ground rod driven to the ground, you don't need to drive additional rods. Crystal.
If you just drive two rods and be done with it there is no need to measure resistance of the first rod, as you are not required to drive anymore then two rods regardless of the net resistance.
 

paulengr

Senior Member
If you just drive two rods and be done with it there is no need to measure resistance of the first rod, as you are not required to drive anymore then two rods regardless of the net resistance.
NEC is a lousy reference for effective grounding systems. Note that the quoted section refers ONLY to ground rods and not any other type of grounding. In previous versions of the Code, 25 ohms was more or less suggested but not even given as a specific target. The reality is that the grounding electrode resistance has more to do with it's use. With building structures although lightning systems are a separate issue (and required by Codes to be a separate ground plane), that's generally the highest duty that the grounding electrode will ever experience. Maintaining low voltage between Earth and the electrical system only requires that it be less than roughly 1/3 of the system capacitive impedance, or it acts more like it is ungrounded. With substations, IEEE 80 specifies generally 1 ohm or less. With residential/commercial installations usually the rule of thumb is 25 ohms or less.

But the interpretation given above is flat out backwards. What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point. As it says, it's an EXCEPTION, not a RULE. Note though that this is not only an exception but if for instance the installation used a flat plate, then the rule would not apply and only one ground electrode would be necessary. So clearly the purpose here is to require measurement. In previous Code editions where the "25 ohm rule" was more of a suggestion, it was up to the jurisdiction to establish a cutoff.

As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds. The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone. There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds. I don't see how this is "slower" than driving another ground rod, even if you have an electric vibrator to speed the process up.
 

Smart $

Esteemed Member
Location
Ohio
NEC is a lousy reference for effective grounding systems. Note that the quoted section refers ONLY to ground rods and not any other type of grounding. In previous versions of the Code, 25 ohms was more or less suggested but not even given as a specific target. The reality is that the grounding electrode resistance has more to do with it's use. With building structures although lightning systems are a separate issue (and required by Codes to be a separate ground plane), that's generally the highest duty that the grounding electrode will ever experience. Maintaining low voltage between Earth and the electrical system only requires that it be less than roughly 1/3 of the system capacitive impedance, or it acts more like it is ungrounded. With substations, IEEE 80 specifies generally 1 ohm or less. With residential/commercial installations usually the rule of thumb is 25 ohms or less.

But the interpretation given above is flat out backwards. What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point. As it says, it's an EXCEPTION, not a RULE. Note though that this is not only an exception but if for instance the installation used a flat plate, then the rule would not apply and only one ground electrode would be necessary. So clearly the purpose here is to require measurement. In previous Code editions where the "25 ohm rule" was more of a suggestion, it was up to the jurisdiction to establish a cutoff.
I believe you should re-read 250.53(A). It applies to rod, pipe and plate electrodes, both as "primary" and as supplemental. Also, this Code section does not require increasing the number of electrodes until the resistance is not more than 25 ohms. Installing one supplemental electrode is all that is required. Note the general statement has no resistance specification, only the Exception does. [2011, 2014 NEC]

As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds. The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone. There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds. I don't see how this is "slower" than driving another ground rod, even if you have an electric vibrator to speed the process up.
This assumes the AHJ accepts a clamp-on ground test result. To my knowledge, many do not.
 

kwired

Electron manager
Location
NE Nebraska
NEC is a lousy reference for effective grounding systems. Note that the quoted section refers ONLY to ground rods and not any other type of grounding. In previous versions of the Code, 25 ohms was more or less suggested but not even given as a specific target. The reality is that the grounding electrode resistance has more to do with it's use. With building structures although lightning systems are a separate issue (and required by Codes to be a separate ground plane), that's generally the highest duty that the grounding electrode will ever experience. Maintaining low voltage between Earth and the electrical system only requires that it be less than roughly 1/3 of the system capacitive impedance, or it acts more like it is ungrounded. With substations, IEEE 80 specifies generally 1 ohm or less. With residential/commercial installations usually the rule of thumb is 25 ohms or less.

But the interpretation given above is flat out backwards. What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point. As it says, it's an EXCEPTION, not a RULE. Note though that this is not only an exception but if for instance the installation used a flat plate, then the rule would not apply and only one ground electrode would be necessary. So clearly the purpose here is to require measurement. In previous Code editions where the "25 ohm rule" was more of a suggestion, it was up to the jurisdiction to establish a cutoff.

As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds. The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone. There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds. I don't see how this is "slower" than driving another ground rod, even if you have an electric vibrator to speed the process up.
Lousy reference? depends on needs of the application. NEC is minimum requirements and is really pretty lenient, some places do have pretty strict specifications but those are design issues and not minimum requirements.

Most of the time if you are connected to a utility system that has multiple grounding points everywhere, your local electrode does little or nothing to your advantage except maybe during a lightning incident. NEC still wants a grounding electrode though so we install them.
 

big john

Senior Member
Location
Portland, ME
NEC is a lousy reference for effective grounding systems.
No argument, but since the NEC is primarily a low-voltage code and low-impedance earthing plays very little role in the effective operation of low voltage system, there ain't much point.
Note that the quoted section refers ONLY to ground rods and not any other type of grounding.
Double check, it refers to most types of electrodes except Ufers.
With building structures although lightning systems are a separate issue (and required by Codes to be a separate ground plane), that's generally the highest duty that the grounding electrode will ever experience. Maintaining low voltage between Earth and the electrical system only requires that it be less than roughly 1/3 of the system capacitive impedance, or it acts more like it is ungrounded.
Agreed.
What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point.
I don't follow; are you saying it should say that? If so, why? You just pointed out above that without a LPS, low voltage systems have very little use for a low-impedance earthing connection.
As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds.
Because of the myriad parallel paths around most buildings, clamp on testers are almost useless. They only work when attached to systems where there is nothing in the zone of influence of the electrode.
The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone.
IEEE 81 does not recognize clamp-on tests as an effective method, so we still rely on fall-of-potential when performing grid testing.
There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds.
I can speak from experience that the value achieved with a 3-point test almost never matches what a clamp-on meter will tell you, and unlike a fall-of-potential test, there's nothing the operator can do to verify the clamp-on reading if the value is questionable.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
.... I can speak from experience that the value achieved with a 3-point test almost never matches what a clamp-on meter will tell you, and unlike a fall-of-potential test, there's nothing the operator can do to verify the clamp-on reading if the value is questionable.
big john - Remind us:
What can an operator do to verify a fall-of-potential test if the value is questionable?

ice
 

infinity

Moderator
Staff member
Location
New Jersey
But the interpretation given above is flat out backwards. What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point. As it says, it's an EXCEPTION, not a RULE. Note though that this is not only an exception but if for instance the installation used a flat plate, then the rule would not apply and only one ground electrode would be necessary. So clearly the purpose here is to require measurement. In previous Code editions where the "25 ohm rule" was more of a suggestion, it was up to the jurisdiction to establish a cutoff.
There is no requirement to meet 25 ohms or less if you have two rods.
Also what code version are you using?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
... So clearly the purpose here is to require measurement. ....
Absolutely not. The only requirement for measurement is if you want to use the exception and install as single driven rod as a grounding electrode. The base rule requires that a single rod be supplemented by an additional electrode and does not require or imply that testing or measuring is required.
 

big john

Senior Member
Location
Portland, ME
What can an operator do to verify a fall-of-potential test if the value is questionable?
The first step is because you're dealing with a plotted graph instead of a single value, its easy to see when you've reached remote earth based on the curve of the graph. This is a luxury you don't have with a clamp-on meter, you're simply taking a single measured value on faith.

If the slope of the graph seems suspiciously shallow or too constant you can plot the value with the slope coefficient and see if you get an acceptable remote-earth value out of that.

And if you run a test and you're sure you're getting clearly bad results you can simply move the test. I prefer to re-run a second test perpendicular to my first, because this is measuring the exact same zone of influence, ideally the results should be identical.

Alternately, if you have the room you can extend the test until you finally clear the zone of influence, but this is often a problem testing an electrode system in the city because there simply isn't room.

As a final method you can test from a totally different part of the electrode system and hope that gets you clear of any outside influences.

Getting bad results in ground testing can be a pain in the a**. :D
 

infinity

Moderator
Staff member
Location
New Jersey
Absolutely not. The only requirement for measurement is if you want to use the exception and install as single driven rod as a grounding electrode. The base rule requires that a single rod be supplemented by an additional electrode and does not require or imply that testing or measuring is required.

Exactly and that's why I asked in my last post which code cycle was referenced because the wording changed in the 2008 NEC to require two electrodes and made the 25 ohms or less proof requirement part of the exception.
 

kwired

Electron manager
Location
NE Nebraska
Exactly and that's why I asked in my last post which code cycle was referenced because the wording changed in the 2008 NEC to require two electrodes and made the 25 ohms or less proof requirement part of the exception.
Prior wording essentially made the AHJ have to prove the first rod was more then 25 ohms before he could ask you to drive a second rod. Current wording makes you prove you have 25 or less with one rod if you choose to only use one, if you drive a second rod - no measurements are necessary.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Exactly and that's why I asked in my last post which code cycle was referenced because the wording changed in the 2008 NEC to require two electrodes and made the 25 ohms or less proof requirement part of the exception.
I really don't see any difference in the function of the code rules between 2008 and now. Just a different way of saying the same thing.
250.56 in the 2008 said either test and prove that your single rod meets the 25 ohm rule or install two rods. To me that is the same as what the exception to 250.53(A)(2) says in the 2014 code.
 
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