ground rods
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ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: ground rods
5/8"x 8' unless the engineer wants otherwise.
5/8"x 8' unless the engineer wants otherwise.
charlie
Senior Member
- Location
- Indianapolis
Re: ground rods
Or a 1/2" X 8' copperweld rod.
Or a 1/2" X 8' copperweld rod.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: ground rods
Derrick: Yes, I should have said that. If I were doing the install on an 800 amp service I would use the code minimum on a ground rod (if I used one at all) and I would hang my hat on the electrodes that are probably available at a structure large enough to justify the service, such as the building steel, Ufer and water pipe.
Charlie: Is the rod you speak of nonferrous? I don't see how it would satisfy 250.52(A)(5)(b).
Derrick: Yes, I should have said that. If I were doing the install on an 800 amp service I would use the code minimum on a ground rod (if I used one at all) and I would hang my hat on the electrodes that are probably available at a structure large enough to justify the service, such as the building steel, Ufer and water pipe.
Charlie: Is the rod you speak of nonferrous? I don't see how it would satisfy 250.52(A)(5)(b).
charlie
Senior Member
- Location
- Indianapolis
Re: ground rods
They are listed as equivalent to non-ferrous rods, have the rod length, and manufacturer's name and catalog number within 12 in. of the top of the rod. The copper cladding is pretty thick to be called copper-weld but I don't remember how thick it has to be. That is just about all anybody uses around here.
They are listed as equivalent to non-ferrous rods, have the rod length, and manufacturer's name and catalog number within 12 in. of the top of the rod. The copper cladding is pretty thick to be called copper-weld but I don't remember how thick it has to be. That is just about all anybody uses around here.
charlie
Senior Member
- Location
- Indianapolis
Re: ground rods
Either you have local amendments to the NEC or the AHJ is overreaching his bounds.
Either you have local amendments to the NEC or the AHJ is overreaching his bounds.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: ground rods
It?s a good rule of thumb to space ground rods at a distance at least equal to their length. But I do not know the physics and mathematics necessary to discover how much a change in spacing would improve or diminish the effectiveness of the grounding electrode system.
It?s a good rule of thumb to space ground rods at a distance at least equal to their length. But I do not know the physics and mathematics necessary to discover how much a change in spacing would improve or diminish the effectiveness of the grounding electrode system.
charlie
Senior Member
- Location
- Indianapolis
Re: ground rods
The Soares Grounding book highly recommends that rule; however, the Code is a book of minimums. You might also read 250.56 FPN: The paralleling efficiency of rods longer than 2.5 m (8 ft) is improved by spacing greater than 1.8 m (6 ft).
The Soares Grounding book highly recommends that rule; however, the Code is a book of minimums. You might also read 250.56 FPN: The paralleling efficiency of rods longer than 2.5 m (8 ft) is improved by spacing greater than 1.8 m (6 ft).
Re: ground rods
The" Sphere of Influence" and why they shouldn't intersect from multiple electrodes, is covered (in a basic way) on this page from LEM Instruments. Click on Ground Electrodes
Roger
[ November 18, 2004, 07:37 PM: Message edited by: roger ]
The" Sphere of Influence" and why they shouldn't intersect from multiple electrodes, is covered (in a basic way) on this page from LEM Instruments. Click on Ground Electrodes
Roger
[ November 18, 2004, 07:37 PM: Message edited by: roger ]
Re: ground rods
Here in Georgia, we're now required to install two (2) 5/8" X 8' ground rods, spaced 6' apart-on all services
*******************
Either you have local amendments to the NEC or the AHJ is overreaching his bounds.
********************
Well, maybe not. The way I read 250.56, is if you need to have a ground rod, the resistance to ground shall be 25 ohms or less or you drive another ground rod. Once you drive the second ground rod, there is no requirement for resistance to ground.
Where I live, I can guarantee the ground resistance of a properly installed rod will be 100 to 2000 ohms depending on the time of year.
For residential, the AHJ (generally the local utility) recognizes the rod is essentially useless anyway, and generally require just one rod. And they don't require any ground resistnce measurement. The local utility doesn't have any NEC amendments - I don't think they even have the authority of law. They are no going to connect your power unless the service looks like the picture they have.
But for industrial, for a isolated structure, where building steel is not available, I have the electricians drive two ground rods. I have never bothered to measure any of them. No use to. Meets code and is certainly no more useless that one ;-) Which turned out to be a good idea, cause our very own inspector got a new clamp-on ground resistance tester ;-)
So, it could be the ground conditions in the area are known to have poor resistivity, the local utility (AHJ) wants to follow the code, and is just having two rods installed.
carl
Here in Georgia, we're now required to install two (2) 5/8" X 8' ground rods, spaced 6' apart-on all services
*******************
Either you have local amendments to the NEC or the AHJ is overreaching his bounds.
********************
Well, maybe not. The way I read 250.56, is if you need to have a ground rod, the resistance to ground shall be 25 ohms or less or you drive another ground rod. Once you drive the second ground rod, there is no requirement for resistance to ground.
Where I live, I can guarantee the ground resistance of a properly installed rod will be 100 to 2000 ohms depending on the time of year.
For residential, the AHJ (generally the local utility) recognizes the rod is essentially useless anyway, and generally require just one rod. And they don't require any ground resistnce measurement. The local utility doesn't have any NEC amendments - I don't think they even have the authority of law. They are no going to connect your power unless the service looks like the picture they have.
But for industrial, for a isolated structure, where building steel is not available, I have the electricians drive two ground rods. I have never bothered to measure any of them. No use to. Meets code and is certainly no more useless that one ;-) Which turned out to be a good idea, cause our very own inspector got a new clamp-on ground resistance tester ;-)
So, it could be the ground conditions in the area are known to have poor resistivity, the local utility (AHJ) wants to follow the code, and is just having two rods installed.
carl
Re: ground rods
I have developed a software that gives the grounding resistance for various patterns of ground rods.
For the specific case of two ground rods, 5/8" x 8', and considering the top of the rods to be 1' below grade level, and uniform soil resistivity of 100 Ohms.m, the resistance of the parallel rods are:
Spacing 6' Resistance 22,50 Ohms
Spacing 8' Resistance 21,95 Ohms.
For a depth of 2' below grade:
Spacing 6' Resistance 22,04 Ohms
Spacing 8' Resistance 21,52 Ohms.
If this topic is of interest to anyone, I can run the software for different rod spacings and/or different depth of the top of the rods.
The software is based on the potential theory, and is valid for uniform and homogeneous soil.
The above resistances can be corrected for different soil resistivities by taking proportional values. Therefore, if soil resistivity doubles, so does the resistance.
[ November 20, 2004, 07:37 AM: Message edited by: saadt ]
I have developed a software that gives the grounding resistance for various patterns of ground rods.
For the specific case of two ground rods, 5/8" x 8', and considering the top of the rods to be 1' below grade level, and uniform soil resistivity of 100 Ohms.m, the resistance of the parallel rods are:
Spacing 6' Resistance 22,50 Ohms
Spacing 8' Resistance 21,95 Ohms.
For a depth of 2' below grade:
Spacing 6' Resistance 22,04 Ohms
Spacing 8' Resistance 21,52 Ohms.
If this topic is of interest to anyone, I can run the software for different rod spacings and/or different depth of the top of the rods.
The software is based on the potential theory, and is valid for uniform and homogeneous soil.
The above resistances can be corrected for different soil resistivities by taking proportional values. Therefore, if soil resistivity doubles, so does the resistance.
[ November 20, 2004, 07:37 AM: Message edited by: saadt ]
charlie
Senior Member
- Location
- Indianapolis
Re: ground rods
Carl, I apologize. The conversation at the time was talking about the listed 1/2" ground rods. My response was about the required 5/8" rods, not the resistance. I was not clear about what I was talking about.
Carl, I apologize. The conversation at the time was talking about the listed 1/2" ground rods. My response was about the required 5/8" rods, not the resistance. I was not clear about what I was talking about.
Re: ground rods
Certainly no appology required. I'd be one of the first to gently, with great humility, and only with the greatest respect, suggest the AHJ get out of the engineering business and get out of the law making business.
(boyth ith hard to talkth withhh ones tongue bit sharply betwithh ones teethhhh ;-)
carl
Certainly no appology required. I'd be one of the first to gently, with great humility, and only with the greatest respect, suggest the AHJ get out of the engineering business and get out of the law making business.
(boyth ith hard to talkth withhh ones tongue bit sharply betwithh ones teethhhh ;-)
carl
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