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Thread: Ground Rods

  1. #1
    Join Date
    Feb 2006
    Posts
    3

    Ground Rods

    Why the code does not allow a ground rod to be buried at an angle more than 45 degrees with the vertical axis? This is the case when we find rock and can not be buried straight.

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,197

    Re: Ground Rods

    The deeper you drive the rod, the more of Planet Earth has an influence on the dissipation of current (e.g., from a lightning strike). A rod driven at a 45 degree angle only goes about 70% as deep as one driven straight down. A rod driven at a steeper angle would be even less deep into the ground. The rod becomes less effective the shallower it is driven into the ground.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Feb 2006
    Posts
    3

    Re: Ground Rods

    Thank you Charlie. But then, why the code allows the same rod, to be horizontally buried at 2.5 feet (larger angle...90 degrees.. and a depth of less than 8 feet?

  4. #4
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,197

    Re: Ground Rods

    Because it is better than nothing. If you can't drive a ground rod into the dirt, then this is the best remaining alternative.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  5. #5
    Join Date
    Feb 2003
    Location
    Florida
    Posts
    7,286

    Re: Ground Rods

    I've got a theory one this as I have also never been able to find the exact history of this requirement.

    I don't think it has much to do with lightning. Lightning only drives approximately 24" into the ground before branching out laterally along the surface of the earth. So, I don't see the benefit of the bottom 6' of the rod other than it may help to reduce the overall resistanc of the grounding circuit.

    My theory has to do with land rights. At one point in history, a company or individual only owned the land on the surface. Oil companies would place drilling rigs on property lines and drive the wells at angles into neighboring properties. This created all kinds of litigation and law changing to protect land-owner mineral/oil rights below the surface of thier land.

    Ground rods came into use during the telegraph boom shortly after the civil war. Many of these lines were run along railroads to connect neighboring towns and cities. Most of these lines were owner by Western Union and a few others (gold and stock companies), but the land they were on was leased from the railroads.

    Its my belief that the pole construction, which was heavily regulated, mandated the exact location the poles were permitted to be located. All other equipment associated with the wiring systems were also closely regulated. I believe at this time, it was not permitted to drive the ground rods at angle away from the pole line. It was also mandated exactly were ground plates could be buried in proximity of the poles.

    So in effect, the rule is an old cross-over from a different era and for different reasons. Sort of like the 25-ohm rule. I don't think anyone here can provide evidence that driving a ground rod at angles is any less effective than straight into the ground.

    This is only a theory....
    Bryan P. Holland, MCP

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