Ground Rod Theory

[infinity]

So we all know that if you pound in two ground rods they must be at least 6' apart. When you do some research on the spacing between the rods the prevalent opinion is that further is better, something like twice the length of the rods. So for a standard 8' that is 16' apart at the minimum. So why does the NEC only require 6'? And is there a distance that is too far apart?

 

[LarryFine]

I'm thinking the 6' is based on dissipation of step potential.

 

[Tulsa Electrician]

I would share what I was told Thursday. I just don't want to get banned. I just said ok and went about my business.

On a serious note I would think if they get closer there would be to much overlap. Making them even less effective.

When I read about it i remember a drawing with that in mind.

As a rule I like to drive 8 foot rods at least seven apart for a acorn clamp just below grade and eight foot part for buried. So for me it's 7' to 8'.

 

[jim dungar]

The NEC doesn't really care about the performance of ground rods, otherwise it would contain some actual criteria. Like most NEC requirements, the distance is based on history rather than science.

 

[electrofelon]

The Seattle electrical code modifies the NEC to require the rod spacing be at least 8 feet. They provide an informational note:

Informational Note: The paralleling efficiency of rods
is increased by spacing them twice the length of the
longest rod.

 

[GoldDigger]

The Seattle electrical code modifies the NEC to require the rod spacing be at least 8 feet. They provide an informational note:

Informational Note: The paralleling efficiency of rods
is increased by spacing them twice the length of the
longest rod.

Just running a simple thought experiment rather than a detailed calculation:
Clearly two rods installed parallel and touching each other will not improve on the performance of a single rod, except for a possible interface resistance at the surface of the rod.
And two ground rods an (approximately) infinite distance apart will not interact at all and the resistance will be that of two resistors on parallel.
So the question is where in between to draw the line. The only distance we have on which to scale the answer is the length of the rods, so one rod length apart is reasonable starting point.
Given the tension between effectiveness and practical spacing, that is not a bad arbitrary value to set in the rules.

 

[tom baker]

The NEC doesn't really care about the performance of ground rods, otherwise it would contain some actual criteria. Like most NEC requirements, the distance is based on history rather than science.

I had a publication frm AEMC called Understanding ground resistance testing. Its likely out of print, but I suspect the information was based on research done 100 years ago. There may be some background info in the IEEE Green Book

 

[tom baker]

So we all know that if you pound in two ground rods they must be at least 6' apart. When you do some research on the spacing between the rods the prevalent opinion is that further is better, something like twice the length of the rods. So for a standard 8' that is 16' apart at the minimum. So why does the NEC only require 6'? And is there a distance that is too far apart?

Yes 16 ft for 8 ft rods, see above

 

[mtnelect]

The ground rod's is for lighting strike dissipation only.

 

[tom baker]

The ground rod's is for lighting strike dissipation only.

What does the NEC say that system grounding is for?

 

[letgomywago]

So although it's allowed at the length it is and insulated ground is allowed I personally think the use of bare copper in the ground between them will be the biggest help.

I do feel that one day in the distant futuee theyll do away of this grounding system or add to it to such a point that is impractical to use as we do today.

 

[infinity]

The thing I don't get is that the NEC has gotten to the point where it has become littered with trivial code language yet they let this one stand from 100 years ago. Even when their own informational note in 250.53(A)(3) says that the distance would be better if double the rod length so why not require it?

Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.

 

[ptonsparky]

I was thinking we discussed this before and it had to do with the length of the typical rod carried by TELCO was back when.
Not disputing Post 7, it was just more practical to carry short rods in your push cart.

 

[electrofelon]

The thing I don't get is that the NEC has gotten to the point where it has become littered with trivial code language yet they let this one stand from 100 years ago. Even when their own informational note in 250.53(A)(3) says that the distance would be better if double the rod length so why not require it?

Is a lower ground resistance better? There seems to be a general belief that lower is better, but I am skeptical it matters much for a low voltage premise wiring system.

 

[infinity]

Is a lower ground resistance better? There seems to be a general belief that lower is better, but I am skeptical it matters much for a low voltage premise wiring system.

The fact that two rods can have a resistance much greater than 25 ohms and still be code compliant as an electrode may lead one to believe that no one really knows. It would indicate to me that maybe the CMP has no idea.

 

[jim dungar]

I had a publication frm AEMC called Understanding ground resistance testing. Its likely out of print, but I suspect the information was based on research done 100 years ago. There may be some background info in the IEEE Green Book

Yep. All of the electrical theory says to use 2X the length as the ideal distance between rods. The NEC uses a distance that is less than this theoretical space.

So what part of the NEC ground rod requirements is based on theory versus historical usage?

 

[Speedskater]

The farther apart, the greater the high frequency impedance of the distant rod.
With lightning, you need to think about impedance up into the megahertz range.

 

[infinity]

The farther apart, the greater the high frequency impedance of the distant rod.
With lightning, you need to think about impedance up into the megahertz range.

So how does that translate to the 6' minimum rod spacing required by the NEC?

 

[drcampbell]

Of course, all of these theoretical analyses assume uniform soil conductivity, and whatever actual need there is for grounding is likely to be different in different kinds of soil.

 

[jaggedben]

The fact that two rods can have a resistance much greater than 25 ohms and still be code compliant as an electrode may lead one to believe that no one really knows. It would indicate to me that maybe the CMP has no idea.

Why 25 ohms in the first place?
The rules are arbitrary lines in the sand. And they always will be since the CMP can't know the soil resistivity on every premise, which in turn varies with the weather and seasons. No one can explain what exact threshold of safety the grounding electrode system is supposed to meet for its various functions anyway. And the CMP - rightly - has to balance effectiveness with feasibility and enforceability.

Can anyone point to an electrocution that would have been prevented if two rods had been farther apart?