two ground rods

[George Stolz]

Re: two ground rods

One of the fifteen Charlie's said:
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.

Sure. They've gotten a lot more cutthroat since the horseshoe pastime dried up. Now they'll stop at nothing to get their maintain their lavish lifestyles of excess and luxury. I've heard through an anonymous source in a closed door meeting that I couldn't transcribe that they've slowly been cloning an army of "ground rod friendly" CMP members who will see to it that "the 25 ohm requirement remains code for-evah! HA HA HA HA HA!"

Since there are many more Charlies that Scotts, we may have to get some others to help you out (Bills, Georges, Bobs, etc.)

Would you really have any value from an army of slaves whose chief activity is staring at an obtuse legal document at great length for the sole purpose of arguing about it?

 

[charlie b]

Re: two ground rods

We can always find a use for an army of slaves.

 

[petersonra]

Re: two ground rods

I once heard an explanation of where the 25 Ohm rule came from that sounded plausible, but I don't know if its real or not.

Supposedly, it was actually related to MV lines and the 25 Ohms was supposed to be low enough to trip the circuit breakers on those lines. Somehow the same requirement got passed on to LV systems.

 

[charlie]

Re: two ground rods

Supposedly, it was actually related to MV lines and the 25 Ohms was supposed to be low enough to trip the circuit breakers on those lines.

Generally speaking, it works and we have much lower that 25 ohms when you consider that we have a minimum of 4 grounds per mile plus grounding at each transformer, lightning arrester, capacitor bank, service to a customer's premises wiring system, etc. I believe someone said that on average, we have less than one ohm on our system.

 

[laidman]

Re: two ground rods

It seems that the code changes depending how old you are. My 1953 code book states Resistance:Made electrodes shall if practicable have arestance to ground of 25 ohms. If the resistance is not as low as 25 ohms than two or more electrodes connected in parallel shall be used. This seems to say keep adding them till you get under 25 ohms. If the customer will pay for them I'll drive 10 to get it down.

 

[friebel]

Re: two ground rods

I was glad to read the response from John Caloggero. I have attended NEC seminars where John has been in attendance, and when John speaks, I listen. In fact, I printed out his last message on the 25 ohm for the main ground.
Locally, the AHJ does not test the main ground to see if it is 25 ohms. What he states is, drive a supplemental ground rod 6 feet from the first one, and that is the end of the story.
Section 250.53, states the following: A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2)through (A)(7). Where multiple rod,pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 6 feet apart.
FPN: The paralleling efficiency of rods longer than 8 feet is improved by spacing greater than 6 feet.
Personally, I would be very satisfied if my AHJ told me to drive another 8-foot rod, and he would be satisfied.

 

[charlie]

Re: two ground rods

My 1953 code book states Resistance: Made electrodes shall if practicable have a resistance to ground of 25 ohms. If the resistance is not as low as 25 ohms then two or more electrodes connected in parallel shall be used.

I added the above emphasis.

This seems to say keep adding them till you get under 25 ohms.

No, the 1953 Code doesn't say that, see the above emphasis. Even in 1953, common sense was supposed to be applied to the Code.

In my opinion, too many electrician will not do something unless the Code spells it out in black and white. On the other hand, too many AHJs will try to enforce what they think the Code should say. Putting these two together gets you the Code the way it is today. It will continue through the change process until it is purrfict.

 

[laidman]

Re: two ground rods

I agree with you Charlie. It just struck me that at that time they thought more tham two might help. going back and reading that old code book makes for some interesting reading

 

[Ed MacLaren]

Re: two ground rods

If the resistance is not as low as 25 ohms then two or more electrodes connected in parallel shall be used.

Here (Prince Edward Island, Canada), if rods are used as the grounding electrode, we have to drive three 10 ft. rods, 10 feet apart.

Ed

 

[renosteinke]

Re: two ground rods

As I write this, I am looking at chart 8.132 in the American Electricians' Handbook.

Simply put, additional ground rouds do not reduce the resistance to ground by any appreciable amount, unless they are significantly separated. Two rods, spaced about 6 ft. apart, have about 60% of the resistance of one rod. Three rods, 10 ft apart, have about 35% the resistance.

 

[romeo]

Re: two ground rods

How does everyone feel about the 2005 requierment for ufer grounds ?

 

[pierre]

Re: two ground rods

Romeo

I was reading through this "fun" stuff and was going to post the same thought!

It would seem with the new requirement for the concrete encased electrode, that the ground rod issue will become a moot situation.
That is for those who are following the 2005. In NY we will not be required to follow the 2005 until 2009

Pierre

Pierre

 

[charlie]

Re: two ground rods

You might also look here.

 

[charlie tuna]

Re: two ground rods

i believe mike holt has a big enough piece of property to build a second house on it. why doesn't the nec provide him enough money to build a second home provided he uses the ufer ground system and he can test it to see if it will clear a ground fault?

 

[tom baker]

Re: two ground rods

Its about time. The ufer ground is the most cost effective available and no ground rods are required. Its too bad Ufers orginal 1960 IEEE paper is not available on line, I purchased a copy. His orginal concept was to provide a low cost, low maintenance grounding system.

 

[charlie tuna]

Re: two ground rods

tom,
this is great--low cost and low maintainance--but is it low impedance? will it clear a fault? what would the total resistance be back to a typical residential's overhead power source? if it is low impedance it should take the place of ground rods in future code changes!

 

[don_resqcapt19]

Re: two ground rods

will it clear a fault?

That is not a job function of the grounding electrode.

if it is low impedance it should take the place of ground rods in future code changes

There is no requirement to install ground rods in addition to a concrete encased electrode in the current code.
Don

 

[heco]

Re: two ground rods

Question...?

What size GEC did you run to the ground rod/rods?

 

[peter d]

Re: two ground rods

Originally posted by heco:
Question...?

What size GEC did you run to the ground rod/rods?

It depends on the type of electrode. If it is a rod, pipe or plate electrode, then #6 copper maximum is all that is required.

250.66 (A)

 

[saadt]

Re: two ground rods

I have developed a software that gives the grounding resistance for various patterns of ground rods.
For the specific case of a ground rod, 5/8" x 8', the top of which is 2' below grade, driven in a soil of uniform resistivity of 100 Ohms.m, one has:

1- Single rod : 38.0 Ohms.
2- Two rods, 6 ft apart : 22.0 Ohms.
3- Two rods, 8 ft apart : 21.5 Ohms.
4- Two rods,10 ft apart : 21.1 Ohms.
5- Three rods, 6 ft apart : 16.2 Ohms.
6- Three rods, 8 ft apart : 15.6 Ohms.
7- Three rods,10 ft apart : 15.1 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.