Why 6ft Between Grounds?

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Ground readings

Ground readings

We read ground rods very often due to the fact that we are installing systems under a state written(actually it's all copy and paste work by an overpaid consulting group:mad: )spec.Here is the grounding section.
NOTE: Grounding shall meet the minimum requirements of the NEC.





NOTE: Grounding shall meet the minimum requirements of the NEC.



Provide permanent and continuous grounding circuits with a current-carrying capacity high enough and an impedance low enough to limit the potential above the ground to a safe level.

Perform grounding as follows:

1. Bond the grounding circuits to nonferrous metal driven electrodes. Use electrodes that are at least 0.625 inches (15 mm) in diameter, 8 feet (2.4 m) long, and are driven straight into the ground.
2. Use the shortest possible ground lead that leads directly to a grounding source.
3. Ensure that the maximum resistance between the ground electrode and the cabinet ground buss or other point in the grounding system is no greater than twenty five (25) ohms.
4. Connect the ground electrodes and the ground wire with an exothermic weld.
5. Connect neutral conductors to the cabinet buss-bar and ground them at each terminal point.
6. Ground the cabinet with a No. 6 AWG solid copper wire between the buss-bar to the ground electrode. Bends shall not exceed 4 inch (100 mm) radius bends.
7. Permanently ground the poles by bonding the No. 6 AWG solid copper wire to a separate ground rod.
8. Ground pole-mounted accessories to the pole.
9. Underground metallic conduit or down guys are not acceptable ground electrodes. Do not use Snap-On connections.
10. For extended distances between Ramp Meter and IVDS additional grounding may be required by the manufacturer.


Alot of times have been told that we had to get 25 ohms or less( it used to be 5!!!)We have tried multiple rods only to find the readings going up and with sectionals the same thing at times 10'-38.63/20'-40.78/30'-40.58/37'-41.2 and no further penetration. We only use clamp on type meters for one reason how can you do a 3 point in the middle of the concrete jungle.
 
I'm sure Tom knows that in our area, some jurisdictions require ground rods to be 12 feet apart. Magnetic force was mentioned as a reason why!
 
I've wondered where the 6 ft. number came from as well. A grounding electrode has a "sphere of influence". Approximately 95% of its interface with the earth is accomplished in an area with a radius equal to its length. Hence the rule of thumb to seperate electrodes by twice their length. 6 ft. would be appropriate for 3 ft long electrodes!
 
Ensure that the maximum resistance between the ground electrode and the cabinet ground buss or other point in the grounding system is no greater than twenty five (25) ohms.
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If the GEC has a resistance greater than 25 ohms you have a problem. This spec is only calling for you to measure the resistance of the GEC and not of the electrode to the earth.
Don
 
Tom knows why ground rods are installed 6 ft apart. He wants to know who figured this out.
My guess is it was based on an IEEE paper in the early 1920's
 
tom baker said:
Tom knows why ground rods are installed 6 ft apart. He wants to know who figured this out.
My guess is it was based on an IEEE paper in the early 1920's
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In the link I mentioned above, in the paper "Lightning, Grounding and Protection for Control Systems"
You will find in the references:
[13] H. B. Dwight, ?Calculation of resistances to ground,? AIEE Trans., pp.
1319-1328, Dec. 1936.

This could be a starting point.

You can find this and other grounding papers at this site. I'll bet the author (Marcus O. Durham) could give you some good leads as it sounds like he has been at this for a while.

This one paper in particular is: http://www.superb.org/paper/ieee/tran95-lightning.pdf
 
rokyapp7 said:
We read ground rods very often due to the fact that we are installing systems under a state written(actually it's all copy and paste work by an overpaid consulting group:mad: )spec.Here is the grounding section.
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With all due respect to your overpaid consulting group, this spec is terrible. Most of the requirements are vague, some are downright wrong. Take it from an overpaid consultant.:wink:
Ensure that the maximum resistance between the ground electrode and the cabinet ground buss or other point in the grounding system is no greater than twenty five (25) ohms
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This says that the grounding electrode conductor needs to have a resistance no more than 25 ohms instead of the grounding electrode.
Provide permanent and continuous grounding circuits with a current-carrying capacity high enough and an impedance low enough to limit the potential above the ground to a safe level.
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This is something that is a design matter, not something that the contractor can determine.
Bends shall not exceed 4 inch (100 mm) radius bends.
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Would a 6" bend meet this requirement? How about a 1" bend?
 
frizbeedog said:
If you have Soares Book on Grounding and Bonding, Check out appendix B. It will cook your noodle.

I've been reading it since I saw this post and my noodle is cooked.

It talks about doubling the depth of the rod and effectively reducing the resistance by 40%, so I gather that 2 rods equates to a rod twice as deep, 16ft effectively.

And the distance away from the surface of the rod again reduces resistance, something about shells of earth surrounding the rod and how if the rods are too close the shells intersect and reduces the effectiveness of each.

Nothing yet on why only a number six GEC to a rod, but I'm looking.
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Why six
For lowering ground resistance using multiple ground electrodes,each electrode has its own sphere of influence and for additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod.
I would have said at least 8-10 and not 6.
 
adgelectrique said:
Why six
For lowering ground resistance using multiple ground electrodes,each electrode has its own sphere of influence and for additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod.
I would have said at least 8-10 and not 6.
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The requirements of the NEC are absolute minimums. Anyone that follows the NEC *to the letter* is the worst electrician allowed by law.
 
K8MHZ said:
The requirements of the NEC are absolute minimums. Anyone that follows the NEC *to the letter* is the worst electrician allowed by law.
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I agree to the fact that the NEC is MINIMUM standards..the rest of that statement is little harsh on many who wire that way, and I do mean the many..
 
tom baker said:
I just instructed a Grounding vs Bonding class, and while I know why, I wonder who determined and when:
6 ft between ground rods?
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Is anyone going to tell us why 6 feet between ground rods is a minimum? I know what the code states but why????
 
Perhaps it came from the same place the 25 ohm requirement came from - Someone just pulled it out of their ..... well, you know ;) .
 
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