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Thread: Ground Rod Spacing Requirement

  1. #1
    Join Date
    Mar 2007
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    Ground Rod Spacing Requirement

    Hi folks,

    My company has required customers to put a supplementary ground rod in for the telecom equipment we manufacture. They require that this supplementary ground rod be bonded to the service entry ground rod using 6 AWG wire (so far so good). They also say that for long wire runs, additional ground rods should be driven every 20 feet between their equipment and the service ground rod and to bond these ground rods together with 6 AWG wire. I have been told that this 20 foot rule is an NEC requirement. I can't find it in NEC 2005. Can any of you point me to the source of this requirement?

    Thanks.

  2. #2
    Join Date
    Nov 2006
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    Pinellas County, FL
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    I do not believe the NEC is requiring this, it is their spec. All electrodes must be bonded to the main grounding system.

  3. #3
    Join Date
    Jun 2005
    Location
    North Logan, Utah
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    They also say that for long wire runs, additional ground rods should be driven every 20 feet between their equipment and the service ground rod and to bond these ground rods together with 6 AWG wire. I have been told that this 20 foot rule is an NEC requirement.
    800.100(A)(4) requies that the primary protector grounding conductor shall be as short as practiable. In one- and two-family dwellings, the primary protector grounding conductor shall be as short as practicable, not to exceed 20 ft in length.

    There is an exception to the length requirement for one- and two-family dwellings where it is not practibale to achieve an overall maximum primary protector grounding conductor length of 20 ft, a separate communications ground rod meeting the minimum dimensional criteria of 800.100(B)(2)(2) shall be driven, the primary protector shall be grounded to the communications ground rod in accordance with 800.100(C), and the communications ground rod shall be bonded to the power grounding electrode system in accordance with 800.100(D).

    This rule doesn't require additional ground rods every 20 feet, just the one ground rod if the length of the primary protector grounding conductor exceeds 20 feet.

    Hope this helps,

    Chris

  4. #4
    Join Date
    May 2003
    Location
    Maryland
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    This is considered a supplementary grounding electrode; it is not required by the NEC so the NEC does not care how it is installed. See 250.54.

  5. #5
    Join Date
    Mar 2007
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    10

    20 foot distance

    I wonder where this requirement comes from? I am seeing a 20 foot ground rod requirement in numerous other specifications. For example, I see it in a Sprint grounding specification as well.

  6. #6
    Join Date
    May 2003
    Location
    Leesburg, VA
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    Using 10 foot rods the perfered spacing would be twice the distance of the lenght of one rod. 10' x 2 = 20'
    Brian John
    Leesburg, VA

  7. #7
    Join Date
    Mar 2007
    Posts
    10

    Ahhhh, now we are getting closer

    Ok, I have seen this rule as well. Where does this rule come from? I am looking for a source that I show my customers.

  8. #8
    Join Date
    Jun 2005
    Location
    North Logan, Utah
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    Quote Originally Posted by ziggle
    I wonder where this requirement comes from? I am seeing a 20 foot ground rod requirement in numerous other specifications. For example, I see it in a Sprint grounding specification as well.
    Specifications don't always come from a code requirment. Specification can come from an engineer, manufacture ect...

    Chris

  9. #9
    Join Date
    Mar 2007
    Posts
    10

    I think I have it now

    You gave the me the clue that I needed. You are right -- the requirement is based on good engineering practice. Basically it looks like you need to space the electrodes far enough apart to ensure that they do not interact. Interaction in this case means that you do not realize lower ground resistance. Here is a quote from MIL-HDBK-419 Section 2.6.2 and 2.6.2.1.

    Most of the resistance of a single electrode (ground rod) is obtained within a reasonable distance from the electrode. For a vertical rod, better than 90 percent is realized within two rod lengths. If two or more electrodes are closely spaced, however, the total effective resistance of neither is realized. This interaction prevents the resistance of N electrodes connected in parallel from being 1/N times the resistance of one of the electrodes. For this reason, the crowding of multiple vertical rods is not as beneficial in terms of dollar cost per ohm as is achievable with fewer rods properly spaced. If the electrodes in a multiple electrode installation are separated by adequate distances, the interactive influence is minimized. The separation between driven vertical ground rods in a group of rods should not be less than the length or greater than twice the length of an individual rod. Consider two rods driven into the earth with their tops flush with the surface. The two rods are electrically parallel, but the presence of one rod affects the resistance of the other.

    If this statement is correct, the spacing is desireable to ensure that you get the best ground contact for a given number of ground rods. Does this seem reasonable?

  10. #10
    Join Date
    Feb 2003
    Location
    Plano, TX
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    Yes it is the universally accepted practice developed from many trials and experiments. So just for general rule rods should be spaced a minimum of 2x the buried depth. However the NEC has no such requirement, it is purely a design issue.

    Now a personal question to you

    I have been a Telecom Power Protection Engineer for about 30 years and have dealt with every Telecom manufacture on the planet. Designed 1000’s of grounding electrode systems, etc. They all require about a 10-ohm GES impedance or less.

    Now for the question. WHY?

    After 30 years I have only heard one good explanation for it (it was what I expected), and 10,000 dumb looks and lies. What is your story? :cool:

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