Ground Rod - Spacing

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anbm

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Someone please clarify for me for required ground rod spacing, here is what I recalled from NFPA780 and NEC:

NFPA780 (lightning protection system): the spacing between rods shall not be smaller than the sum of all ground rod length..

NEC article 250...spacing between ground rod is min. 6 feet.


So, which rule should we go with? If we go with NEC, we are ok at spacing ground rod 6' apart as minnimum. If we go with NFPA 780, we have to space them at least 24 feet apart (assume 8' long ground rod)...

Or ground rod spacing rule per NFPA-780 only applies to lightning protection system? I saw several electrical grounding details where they showed the rod either 20' or 15' apart for tri-rods system.
 
dSilanskas said:
NEC 6' apart
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That's not necessarily true.

It depends on what you want and what you are installing them for, if you are looking for the most effective use of your driven electrodes you would go twice the lenght of the electrode 8' X 2 = 16' (not 24'?).
 
Let's take note that the NEC says "they shall not be less than 1.8 m (6 ft) apart.", it does not say they shall be 6' apart.



Roger
 
NFPA 780 is the Standard for the Installation of Lightning Protection Systems. The NEC nolonger has a article on lightning protection I believe it was 280???

in 780 you probably would have to extend the spacing to 24 or whatever feet.
 
hey brian what would be the reason you would want to space the ground rods apart that far? i usually space them 8 feet apart for a house. never really had to do any fancy grounding like you do
 
electricalperson said:
hey brian what would be the reason you would want to space the ground rods apart that far? i usually space them 8 feet apart for a house. never really had to do any fancy grounding like you do
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Do a search for "sphere of influence".

Roger
 
Well most jobs or projects we do the ground rods are 30 feet apart in a delta configuration total of three rods and each rod depth 30 foot down each rod in length plus cad welded rods & cad welded electrode conductor to each rod the EC is normally 3/0 minimum. no exceptions this is per engineers specifications it must be 5 ohms or less . and tested with a three point tester only no clamp ons . comments take care be safe
 
electricalperson said:
thank you. will the book getting down to earth help explain ground rod testing and general grounding theory?
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Not really, but it's a good read. It's more to do with the testing of grounding systems. AVRO put the book out, if I remember correctly. I do seem to recall that they did cover spheres of influence and some results of different grounding configurations.

This site has some great information about grounding and lightning protection:

www.polyphaser.com

There is an entire technical section on the site, hundreds of pages of very well written information.
 
http://www.aemc.com/techinfo/html/Tech-GroundTesters.asp

I made my own book copied articles, literature obtained over the years and tons of stuff from the www.com folks.

As for the delta configuration I doubt (never read anything to support this) that this is any more effective than any other layout maybe less due to the aforementioned sphere of influence. And maybe not with 30' spacing this may be the most efficient layout from a lower ground resistance and minimizing material (?). I mention this because I had a customer that swore that the delta configuration held mystical qualities (SLIGHT EXAGGERATION ON MY PART). and would take no other layout.
 
Well we dont design our grounding its done by the electrical engineer and its on our plans and in our specs !!

I personally dont like installing 30 foot rods or placing rods 30 foot apart , Heres why we test our rods when we install them each rod by its self with a three point tester there is no difference in 30 foot to 10 foot in resistance i know this buy testing rods thur the years . and there is no difference in depth kinda after 20 foot its the same in florida soil . so we dont see how a engineer can figure its better to space apart and go deeper in depth . I think depth will keep resistance more constant at meaning over time than a 10 foot rod depth , and space between rods is only good for not putting all your eggs in one basket ? meaning if you lost one you got another in a safer place or location just better coverage !! comments take care make it simple not technical
 
electricalperson said:
article 250 of the nec is my favorite one. im going to learn more about grounding
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Well i dont have a favorite article but i do think grounding is the most important part of any electrical project look at it this way if nothing without a ground were kinda taken a risk that is not a good thing to be without !! In our history of electric tesla said all power must return to the earth ground ive always thought about that statement.
Heres why can you get power back out of the earth ? meaning can we use a rod in the earth to return our power to actually make power from this return and how can we ? well iam a dreamer na never happen take care
 
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