Ground rods
- Thread starter GReed
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
Welcome to the forum. Hope you're trade related.
See 250.53 in the NEC.
oldsparky52
Senior Member
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- Wilmington, NC USA
This is a guess on my part. I wonder if the reason for the 2nd is to lower the overall resistance to earth ground. The two ground rod connection are parallel paths to ground. When you have to parallel paths of equal resistance the overall resistance is 1/2 of one of the paths, so it's an easy way to lower the resistance to earth ground. How much of a difference it makes to the functionality of the service is unknown to me.
And ... you don't have to install the 2nd ground rod if you can prove your one ground rod provides 25 ohms or less of resistance to ground. It's much easier and quicker to install a 2nd ground rod than to prove the resistance of one ground rod.
ptonsparky
Tom
- Occupation
- EC - retired
Pretty good guess.
hhsting
Senior Member
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- Glen bunie, md, us
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- Junior plan reviewer
As it was said in on of the Star Trek episode or movie: resistance is futile but maybe not in your case 
NEC 2014 section 250 forget exactly which one supplemental rod is required to lower resistance 25 ohms or less. However this maybe your case not sure I am guessing. Why not ask the inspector.
NEC 2014 section 250 forget exactly which one supplemental rod is required to lower resistance 25 ohms or less. However this maybe your case not sure I am guessing. Why not ask the inspector.
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It’s to meet the dumbest grounding Code rule.
NEC requires either:
1. Ground it using almost any method they lust and then prove (test) it is under 25 ohms. This requires a 3 point ground tester in single rod cases, plus the time to set up the stakes, verify everything works, etc. Plus unless you own one 3 point testers are not cheap.
2. Same as #1 but if you have supplemental grounds like running a ground to water piping, structural grounds, or additional rods, use a clamp on tester. Quick and easy. Read up on the AEMC web site (they invented it). Also not cheap but much faster.
3. Drive a second rod. No test needed. You already have the tools and it’s faster than doing a three point test.
The inspector is telling you to do option 3. Obviously this is a stupid rule. Some day someone is going to get the rule changed to requiring a test for all grounds just like we already have with utility substations and mining. Then every contractor will buy a ground tester and I’ll have competition on grounding surveys. Until then just drive 2 rods. The minimum distance is supposed to be one rod apart. If you put them too close together they act like one rod. So we have the “six foot rule” but the correct rule should be one rod length apart.
The stupid part is that rods don’t “add”. One extra rod is about 80% effective. The third rod is 50% effective and it keeps dropping from there, I once set 10 rods in a sand mine trying to get 25 ohms where testing is mandatory. Does not matter if they are in a grid, a line, or a circle, if you want half the resistance buy threaded rods and stack them. And it depends on the area. People living in the desert parts of the Southwest can never reach 25 ohms on ground rods alone no matter how many you use. That’s what I ran into at the mine...high soil resistivity with zero clays or organics. People will figure this out too when NEC gets their heads on straight about effective grounding. Or they will realize bonding and the type of grounding is a lot more critical than ground rods.
Thanks for the response. I knew it was a resistance issue. The inspectors answer, that is the code. I have already installed the second rods on all of the boxes since it is cheaper and faster than proving my point to the inspector. I just want this project to pass with least resistance, ha ha, this is the first inspection and he already hates the project, might be the 200 miles he has to drive since we no longer have a inspector for our part of the state.
Thanks Gary
Thanks Gary
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
So my guess in post 3 is inaccurate?
With 2 ground rods, what would be the reduction in resistance to earth potential from 1 ground rod. I don't know how to interpret the "80% effective" statement.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Your words describe a common misconception of the NEC rule. You are not required to add a second ground rod to get the total effective resistance below 25 ohms. You are required to add a second rod if the resistance of the first rod is not proven to be below 25 ohms, but there is no requirement whatsoever as to what the resistance should be after adding the second rod.As it was said in on of the Star Trek episode or movie: resistance is futile but maybe not in your case
NEC 2014 section 250 forget exactly which one supplemental rod is required to lower resistance 25 ohms or less. However this maybe your case not sure I am guessing. Why not ask the inspector.
If the soil is bone dry and the resistance with one rod is 500 ohms, the resistance with a second identical rod 6 feet away from the first will be somewhere a bit above 250 ohms. And that is acceptable to the NEC. Some commercial customers add a contractual obligation to reach a certain ground resistance level though.
I’ve always read and understood different numbers.
the rule for resistances doesn’t apply for ground rods.
electrical-engineering-portal.com
the rule for resistances doesn’t apply for ground rods.
3 good ways to improve earth electrode resistance | EEP
When your earth electrode resistance is not low enough, there are 3 ways you can improve it - to lengthen electrode, use multiple rods and treat the soil
kwired
Electron manager
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- NE Nebraska
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- EC
Reading of clamp on tester is dependent on other electrodes being present. In a utility supplied system you have a pretty large network of other electrodes as the "other electrode". That network will normally have a pretty low resistance making your testing of a single rod somewhat accurate compared to if you were isolated and only had two rods in the test circuit.
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- New Jersey
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- Journeyman Electrician
IMO we just abandon this whole arbitrary 25 ohms or less nonsense and just require an additional electrode. This is one of the dumber code requirements in the NEC.
kwired
Electron manager
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- NE Nebraska
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- EC
I'm not even totally convinced we always need an electrode, but the dirt worshippers do think so.
spyderx92
Member
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- Los Angeles
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- Electrician
Moderator's Note: I deleted the content of this post because it was a duplicate of a separate thread that spyderx92 posted. Please do not post the same question in different places.
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We need it for ground potential rise that occurs due to surges, basically restricting transient overvoltages.
It is also extremely effective at long distances. It is yet another goofy thing about grounds but in a wire resistance increases linearly with length. That makes sense to most people. In a ground this is still true BUT the number of possible paths is increasing too. It turns out to be two dimensional so paths increases with the square of the distance. Now that old adage about electricity taking the path of least resistance is utterly false. We know because Ohms Law works. It follows all paths proportional to the inverse of their resistances. So we basically have the resistance between two ground rods ignoring local effects is equal to the distance divided by the square of the distance. So the curious result is it is equal to the inverse of distance. So over short distances bonding is crucial. Over long distances bonding becomes worthless to us because resistance is dominated by grounding and in this case I mean Earth connections. In practice from experience the Earth ground dominates at around 1 mile or more. So effectively utility substations are all “grounded” together even if utilities generally don’t run grounds or neutrals between substations for example.
kwired
Electron manager
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- NE Nebraska
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- EC
By stating I'm not convinced we always need an electrode I wasn't meaning no grounding at all, just that it may not always be necessary where NEC requires them. Some of what you stated in your reply supports what I was thinking "Earth ground dominates at around 1 mile or more"
How many services don't have an electrode within 1 mile? How many don't have one within 100 feet or so?
If utility pole is right outside my structure, and usually already has a grounding electrode connected to it, does driving a rod (or 2) at my structure really accomplish much?
If I set a temp power post/pedestal for construction site pretty much right next to utility pole or transformer that already has an electrode, does the rod(s) I set for my temp power really do much? Most inspectors I have encountered will lecture you how you are going to kill someone if you don't put an electrode there, I don't believe them one bit. I will put one there because NEC requires something but don't agree it is always necessary.
- Location
- New Jersey
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- Journeyman Electrician
IMO that is an indicator of someone who doesn't really understand how a grounded system works. And this is a common problem amongst many of the people I work with. They think that if the system isn't connected to the dirt then someone will get electrocuted. The MBJ is what really matters in fault clearing, a concept that many don't understand.
Utilities are mujtigrounded. Google the term stray voltage. There is some valid concern in between the snake oil and old wives tales. During a fault all the grounds work together but having a local ground rod controls stray voltage aka ground potential rise. Second you don’t control the utility and vice versa. Plus there is a massive political rivalry between the utility crowd (IEEE NESC) and the insurance companies (NFPA). So sure it’s partly political and partly practical.
The first systems Edison built were single wire with peg grounds using Earth as the neutral. The number of issues, injuries to horses, and such were legendary. Shortly after the introduction of AC grounding and bonding appeared as a safety feature and we quickly ended up with the Edison (GE) “safety system” of 240/120 with a grounded conductor. It’s been that way ever since. In the 1959s a few plants fell back into ungrounded systems until they saw the consequence and by the 1979s low then high resistance grounding came on the scene which is technically the best option from about 10-15 kV or less.
kwired
Electron manager
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- NE Nebraska
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- EC
If there is a medium or high voltage ground fault causing the rise on the grounded conductor I can agree it will help some.
If it is just a 1 to10 volt rise caused by voltage drop on the MGN it does almost nothing in most cases. I have seen this several times, just a couple volts or so on the grounded conductor that won't go away if you turn the main off. Person has a shower in their basement and this grounded conductor is bonded to their water lines so raises those by same couple volts, but their shower floor is concrete on grade and at true earth potential - they feel a slight shock when they use the shower. People get electrocuted if it happens around boat docks with power on them, because the EGC of anything on that dock is energized to whatever voltage rise is on the grounded service conductor, and the victim is usually in the water which is at true earth potential when they make contact. GFCI's do nothing for it because it is all between the EGC and true earth and not any protected circuit conductors. You can drive a thousand ground rods and it still won't solve the problem either.
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