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Thread: why 6' apart

  1. #1
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    why 6' apart

    why is it that the 2nd ground rod has to be spaced six feet from the first?

  2. #2
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    Quote Originally Posted by jayrush13
    why is it that the 2nd ground rod has to be spaced six feet from the first?
    Lets be clear, it does not have to be spaced 6' from the first, it must be at least 6' from the first, they could be 20' apart.

    The reason they have to be at least 6' apart is because the closer they are together the less effective the second rod is.

    If you placed them side by side the second rod would do almost nothing.

    I believe this is due to the resistance of the soil.

  3. #3
    Join Date
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    I agree with Bob 6' is a minimun . Two times the length of the rod would be better.
    From;


    Bringing Grounding Down to Earth

    Don't let the rules and requirements intimidate you. Grounding isn't as mysterious
    as you might think.

    By Tim Janof, P.E.

    (published in EC&M, February 2005)
    ....The Code requires you to space rods
    at least 6 feet apart [250.53(B)]. However,
    this spacing is a minimum
    and far from
    ideal. When using the typical 8-foot or 10-
    foot ground rod, you get the best results by
    spacing the rods at least 16 or 20 feet apart,
    respectively. This is much greater than the
    Code minimum 6-foot spacing.
    Ground rods spaced less than two
    rod-lengths apart will interfere with each
    other because their effective resistance areas
    will overlap (Figure 2A). For reference, see
    IEEE-142 and Soares Book on Grounding.
    The overlap increases the net resistance of
    each rod, making the grounding electrode
    system less effective than if the rods were
    spaced farther apart (Figure 2b)......

    "America will never be destroyed from the outside.
    If we falter and lose our freedoms,
    it will be because we destroyed ourselves."

    Abraham Lincoln

  4. #4
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    Per A.4.13.2.4 of the NFPA 780, "No benefit is gained from the second ground rod if placed closer than the length of the longer rod. No additional benefit is gained if the second rod is placed over four times the length of the longer rod."
    Bryan P. Holland, MCP

  5. #5
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    When current flows through the rod, it creates "spheres of influence" in the soil. If the spheres of influence touch each other, it effectively makes both rods as one. There are other variables, but that is generally the reason why the rods actually should be apart from each other at least the length of the longest rod as Bryan pointed out.
    Instructor, Industry Advocate

  6. #6
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    Because the Code making panel likes even numbers otherwise it would have been 7 feet.
    Brian John
    Leesburg, VA

  7. #7
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    6 ft is a number that is used a lot in the NEC. distance from a sink, maximum length of flex w/o EGC, maximum length of bonding jumper.
    At some time, long ago, someone figured this all out.
    Moderator-Washington State
    Ancora Imparo

  8. #8
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    Tom

    Are you saying the CMP actually figured out 6' was a minimum effective distance or that 6' was a nice sounding number that works in a variety of instances.
    Brian John
    Leesburg, VA

  9. #9
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    Quote Originally Posted by tom baker
    6 ft is a number that is used a lot in the NEC. distance from a sink, maximum length of flex w/o EGC, maximum length of bonding jumper.
    At some time, long ago, someone figured this all out.

    ...in a land far, far away!!! :grin:
    Instructor, Industry Advocate

  10. #10
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    They where sitting in a meeting and one of them stretched their hands as far apart as possible and said "How 'bout this far"?

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