two ground rods
- Thread starter mh183
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: two ground rods
The inspector doesn't really know what he is doing. The 6' is a minimum seperation, not a maximum. The most effective separation of ground rods is a spacing equal to twice the length of the rod.
Don
The inspector doesn't really know what he is doing. The 6' is a minimum seperation, not a maximum. The most effective separation of ground rods is a spacing equal to twice the length of the rod.
Don
stud696981
Senior Member
Re: two ground rods
I don't have my NEC book in front of me, but if memory serves me correct, when using two or more rods they had to be a MINIMUM of 6 feet apart. Ask the inspector for a reference to which code he is quoting. If the inspector is quoting the NEC then he or she is mistaken and needs to be called out on this!
I don't have my NEC book in front of me, but if memory serves me correct, when using two or more rods they had to be a MINIMUM of 6 feet apart. Ask the inspector for a reference to which code he is quoting. If the inspector is quoting the NEC then he or she is mistaken and needs to be called out on this!
electricmanscott
Senior Member
- Location
- Boston, MA
Re: two ground rods
Ah, got to love the inspectors who have no business inspecting. :roll:
Ah, got to love the inspectors who have no business inspecting. :roll:
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: two ground rods
Call him and ask for the code number.If he refuses or can't (he wont find it)then ask to speak with his boss.There are times when we just can't back down.This giving him a nice way out.Be polite about it but stand your ground.
Call him and ask for the code number.If he refuses or can't (he wont find it)then ask to speak with his boss.There are times when we just can't back down.This giving him a nice way out.Be polite about it but stand your ground.
Re: two ground rods
Hi I'm a retired inspector. 2005 NEC 250-56 states 6' is the minimum. FPN states the paralleling effieincy of rods longer then 2.5 M or 10 feet is improved by spacing greater than 1.8 m or (6 ft) I agree with the 20 feet apart. I think you made this a better installation.
I'm curious is this a supplemental electrode ? No water in the Bldg? or area being served. What's the service for? Did you actually need the second ground rod?
Belive me I'm not sticking up for the inspector. If you did more than the code requires let him know.
JP
Hi I'm a retired inspector. 2005 NEC 250-56 states 6' is the minimum. FPN states the paralleling effieincy of rods longer then 2.5 M or 10 feet is improved by spacing greater than 1.8 m or (6 ft) I agree with the 20 feet apart. I think you made this a better installation.
I'm curious is this a supplemental electrode ? No water in the Bldg? or area being served. What's the service for? Did you actually need the second ground rod?
Belive me I'm not sticking up for the inspector. If you did more than the code requires let him know.
JP
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: two ground rods
in order to make sure that no two are further apart from each other than 6 feet.
But it is, indeed, a ?non-problem.? What you are trying to avoid is a loss of efficiency. If two rods are within each other?s ?sphere of influence,? then neither will perform as efficiently as it could. Once they are further apart than their own depth, putting them even further apart has little additional impact. But it would certainly have no adverse impact. But put them a mile apart, if you wish, provided only that you can connect them properly.
Even if you wanted to offer this solution, it would not ?fix? this ?non-problem.? You would need to add three rods in the middle,
in order to make sure that no two are further apart from each other than 6 feet. But it is, indeed, a ?non-problem.? What you are trying to avoid is a loss of efficiency. If two rods are within each other?s ?sphere of influence,? then neither will perform as efficiently as it could. Once they are further apart than their own depth, putting them even further apart has little additional impact. But it would certainly have no adverse impact. But put them a mile apart, if you wish, provided only that you can connect them properly.
charlie tuna
Senior Member
- Location
- Florida
Re: two ground rods
remember---the ground rod rule is basically a joke! if it is important and acceptable to have a (one) ground rod with a resistance of 25 ohms or less, wouldn't it be just as important to test the resistance after using a second ground rod when the 25 ohm resistance could not be obtained?
who came up with this rule about the second ground rod---the ground rod manufacturers?
remember---the ground rod rule is basically a joke! if it is important and acceptable to have a (one) ground rod with a resistance of 25 ohms or less, wouldn't it be just as important to test the resistance after using a second ground rod when the 25 ohm resistance could not be obtained?
who came up with this rule about the second ground rod---the ground rod manufacturers?
charlie
Senior Member
- Location
- Indianapolis
Re: two ground rods
Charlie Tuna, it is nice to have the 25 ohms with one rod and that is all the Code requires. If you can't get it with just one, the Code requires a second one. Now you ask why not drive more until you get 25 ohms? Yes Charlie, I am answering your question but I am re-framing it as well.
When you drive a second rod, it is not the same as having a resister in parallel (two 50 ohm resisters in parallel will give you 25 ohms total). In the case of two 50 ohm ground rods in parallel, you may get 35 ohms. Adding a third one may get you to 30 ohms. It may take a forth one to get you to 25 ohms.
Charlie, the concept is to require what is reasonable. There are some cases where getting 25 ohms is not even possible. It is reasonable to require a second rod to be driven but it is not reasonable to require 25 ohms regardless of how many rods are driven.
Look at the big picture of all the grounding. Every service has close to 25 ohms or less, the multi-grounded system neutral is very low (around one ohm) and it is in parallel with all of the service grounding. Lightning and higher voltage to ground connections do have a place to go.
The biggest problem with lightning is that it doesn't like to turn corners and it will not travel far. With that in mind, the lower the ground resistance in any given area, the better. That is why the second rod is required when 25 ohms is not achieved. In this case, I do not believe the ground rod manufacturers had much to do with it. Also, the makeup of the Code Making Panels do permit more than 1/3 of the members to be from the manufacturing sector.
Charlie Tuna, it is nice to have the 25 ohms with one rod and that is all the Code requires. If you can't get it with just one, the Code requires a second one. Now you ask why not drive more until you get 25 ohms? Yes Charlie, I am answering your question but I am re-framing it as well.
When you drive a second rod, it is not the same as having a resister in parallel (two 50 ohm resisters in parallel will give you 25 ohms total). In the case of two 50 ohm ground rods in parallel, you may get 35 ohms. Adding a third one may get you to 30 ohms. It may take a forth one to get you to 25 ohms.
Charlie, the concept is to require what is reasonable. There are some cases where getting 25 ohms is not even possible. It is reasonable to require a second rod to be driven but it is not reasonable to require 25 ohms regardless of how many rods are driven.
Look at the big picture of all the grounding. Every service has close to 25 ohms or less, the multi-grounded system neutral is very low (around one ohm) and it is in parallel with all of the service grounding. Lightning and higher voltage to ground connections do have a place to go.
The biggest problem with lightning is that it doesn't like to turn corners and it will not travel far. With that in mind, the lower the ground resistance in any given area, the better. That is why the second rod is required when 25 ohms is not achieved. In this case, I do not believe the ground rod manufacturers had much to do with it. Also, the makeup of the Code Making Panels do permit more than 1/3 of the members to be from the manufacturing sector.
charlie tuna
Senior Member
- Location
- Florida
Re: two ground rods
charlie #59,
i follow your comments and agree with the theory, but basically you have not answered the question:
if the ground rod is required? which it is! and if one ground rod is sufficent if it measures 25 ohms or less to meet code as stated by the nec.
then why isn't there a required resistance when the second rod is driven?
lets say in your example that the first rod was 1600 ohms and adding a second one brought the total resistance down to 800 ohms. why is this acceptable?
i have only worked on maybe ten or twelve jobs connected with lightning damage in my career. i really never followed the resulting damage back to the ground rod. most of the time the lightning is carried back into the electrical distribution via an outside metal raceway or conductor and then damages the surrounding equipment when you would think that it would continue back thru the metallic bonded system? i have seen large conductors melted and the main ground wire to the ground rod untouched? meter socket blown off the wall and the main ground wire remains in perfect condition? i have seen lightning hit a metal parking lot pole (with a driven ground)and come back into the electric room and blow up the lighting contactor? i have seen it enter a residence via the cable t.v. conductor and jump onto an electrical metallic tube raceway and blow the main breaker to shreds and not remain on that raceway back to the ground rod.
i know "lightning is unpredictable" --- but electrically this does not make sense!
charlie #59,
i follow your comments and agree with the theory, but basically you have not answered the question:
if the ground rod is required? which it is! and if one ground rod is sufficent if it measures 25 ohms or less to meet code as stated by the nec.
then why isn't there a required resistance when the second rod is driven?
lets say in your example that the first rod was 1600 ohms and adding a second one brought the total resistance down to 800 ohms. why is this acceptable?
i have only worked on maybe ten or twelve jobs connected with lightning damage in my career. i really never followed the resulting damage back to the ground rod. most of the time the lightning is carried back into the electrical distribution via an outside metal raceway or conductor and then damages the surrounding equipment when you would think that it would continue back thru the metallic bonded system? i have seen large conductors melted and the main ground wire to the ground rod untouched? meter socket blown off the wall and the main ground wire remains in perfect condition? i have seen lightning hit a metal parking lot pole (with a driven ground)and come back into the electric room and blow up the lighting contactor? i have seen it enter a residence via the cable t.v. conductor and jump onto an electrical metallic tube raceway and blow the main breaker to shreds and not remain on that raceway back to the ground rod.
i know "lightning is unpredictable" --- but electrically this does not make sense!
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: two ground rods
To Charlie #215 from Charlie #27: Charlie #59 did answer this question, though his answer was a bit hidden within the post:
As much as we may like to have a legal requirement that also makes electrical sense, this is a case in which that can?t happen. From the perspective of ?making electrical sense,? it is clear that the lower we can make the ground resistance, the better will be the path for lightning. From the perspective of ?reasonable rules,? we can?t require adding more and more rods until a specific, measurable resistance value is obtained. The rule stops at two, because requiring three would be getting unreasonable. Since the rule stops at two, there is no need to measure the resistance. That is because we are stopping at two, no matter what the measured reading might be. If measuring does not change the outcome, then there is no reason to measure.
charlie tuna
Senior Member
- Location
- Florida
Re: two ground rods
charlie #27 sir,
most rules incorporated within the nec is put in place due to actual figures and facts---not resonable assumtions. your theory is like a parent telling his kid "thats the way it is because i said so"! like you said "electrically the lower the resistance the better" --- this being true i can see where they might have come up with the limit of 25 ohms or less on one ground rod. but also knowing that some areas have very poor soil conductivity and that those services may not benifit the usefullness, if any, of the ground rod!
and if the ground rod's main function is lighning protection shouldn't the installer be required to install the conductors without sharp bends to prevent the lighning from jumping off the ground rod electrod conductor?
charlie #27 sir,
most rules incorporated within the nec is put in place due to actual figures and facts---not resonable assumtions. your theory is like a parent telling his kid "thats the way it is because i said so"! like you said "electrically the lower the resistance the better" --- this being true i can see where they might have come up with the limit of 25 ohms or less on one ground rod. but also knowing that some areas have very poor soil conductivity and that those services may not benifit the usefullness, if any, of the ground rod!
and if the ground rod's main function is lighning protection shouldn't the installer be required to install the conductors without sharp bends to prevent the lighning from jumping off the ground rod electrod conductor?
jwelectric
Senior Member
- Location
- North Carolina
Re: two ground rods
Bless your little hearts. I have to get involved in this discussion. What is the purpose of the ground rod? This question could be better asked if it was, What is the purpose of the grounding electrode system? I bet that it is not for a direct hit from a lighting strike. The #6 copper conductor can?t handle that much current flow. The grounding electrode system is for
?250.4 (A) Grounded Systems. (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.?
The key word in this article is LIMIT, not stop or divert. The one thing to keep in mind is that the NEC is a minimum standard and is the least that can be done. The NFPA would have no objection if the electrical contractor buried a rail road rail and bonded it with a 2000 kcmil copper conductor to it. This would not stop a direct strike from lighting from doing damage to equipment.
Bless your little hearts. I have to get involved in this discussion. What is the purpose of the ground rod? This question could be better asked if it was, What is the purpose of the grounding electrode system? I bet that it is not for a direct hit from a lighting strike. The #6 copper conductor can?t handle that much current flow. The grounding electrode system is for
?250.4 (A) Grounded Systems. (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.?
The key word in this article is LIMIT, not stop or divert. The one thing to keep in mind is that the NEC is a minimum standard and is the least that can be done. The NFPA would have no objection if the electrical contractor buried a rail road rail and bonded it with a 2000 kcmil copper conductor to it. This would not stop a direct strike from lighting from doing damage to equipment.
- Location
- Bremerton, Washington
Re: two ground rods
JW keep in mind lighting operates at higher frequencies than AC power and the same rules can't be used. We need to evaluate the impedance of a conductor at high frequencies, and a change from 6 to 2 awg does not make much difference.
One question we don't know the answer to (we are perhaps close)is why 25 ohms?
JW keep in mind lighting operates at higher frequencies than AC power and the same rules can't be used. We need to evaluate the impedance of a conductor at high frequencies, and a change from 6 to 2 awg does not make much difference.
One question we don't know the answer to (we are perhaps close)is why 25 ohms?
electricmanscott
Senior Member
- Location
- Boston, MA
Re: two ground rods
Apparently there is no limit on the number of "Charlies" you can have.
Apparently there is no limit on the number of "Charlies" you can have.
charlie
Senior Member
- Location
- Indianapolis
Re: two ground rods
Tom, take a look at Bennie's Post to get a bit of insight. This is something we have been wondering about for some time and I think Bennie has answered it for us.
Tom, take a look at Bennie's Post to get a bit of insight. This is something we have been wondering about for some time and I think Bennie has answered it for us.
charlie
Senior Member
- Location
- Indianapolis
Re: two ground rods
"Apparently there is no limit on the number of "Charlies" you can have."
Well Scott, there is a conspiracy afoot and one of these days . . . in a land far away . . . all of the Charlies will take over all the world and everyone named Scott will be our servants. Since there are many more Charlies that Scotts, we may have to get some others to help you out (Bills, Georges, Bobs, etc.)
"Apparently there is no limit on the number of "Charlies" you can have."
Well Scott, there is a conspiracy afoot and one of these days . . . in a land far away . . . all of the Charlies will take over all the world and everyone named Scott will be our servants. Since there are many more Charlies that Scotts, we may have to get some others to help you out (Bills, Georges, Bobs, etc.)
john m. caloggero
Senior Member
Re: two ground rods
As a Staff Liason to the NEC Code-Making Panels for 22 years, I have never heard the reason for the 25 ohms value for the maximum resistance of a drive ground rod. I have researched it, and spoke with my friend Norman H. Davis III at UL who served on CMP-5 for Article 250 many years ago. We could not find the substantiation for the 25 ohms maximum. The 25 ohm requirement is in the 1920 NEC and it clearly states that if the ground resistance is more than 25 ohms, drive another rod 6 feet from the first one and the resistance does not have to meet the 25 ohm requirement. Maybe there is someone older than me that may have the substantiation.
As a Staff Liason to the NEC Code-Making Panels for 22 years, I have never heard the reason for the 25 ohms value for the maximum resistance of a drive ground rod. I have researched it, and spoke with my friend Norman H. Davis III at UL who served on CMP-5 for Article 250 many years ago. We could not find the substantiation for the 25 ohms maximum. The 25 ohm requirement is in the 1920 NEC and it clearly states that if the ground resistance is more than 25 ohms, drive another rod 6 feet from the first one and the resistance does not have to meet the 25 ohm requirement. Maybe there is someone older than me that may have the substantiation.
charlie tuna
Senior Member
- Location
- Florida
Re: two ground rods
john c. #648 sir,
please state your present age so we can determine what change may(?) be required to put this issue to sleep! i guess the oldest guy can make the code change ---if necessary!!!
john c. #648 sir,
please state your present age so we can determine what change may(?) be required to put this issue to sleep! i guess the oldest guy can make the code change ---if necessary!!!
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