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Can you explain the ground resistance formula developed by Prof. H. R. Dwight?

Hi Nuttanon,

Thank you for your question regarding Professor Dwight’s formula, it is our pleasure to help.

Professor H. R. Dwight of the Massachusetts Institute of Technology developed a simplified formula for calculating the resistance-to-ground of an electrode in uniform soil conditions.  The problem with the formula is that uniform soil conditions simply do not exist in nature and as such, hand calculations using this or other formulas seldom work.  Here is the formula:

R =   p       {(ln 4L) – 1}

2πL             r

  • R = resistance in ohms of the ground rod
  • to the earth (or soil)
  • L = grounding electrode length
  • r = grounding electrode radius
  • ρ = average resistivity in ohms-cm.

Here is another link that may be of use to you:  https://www.esgroundingsolutions.com/blog/846/what-is-the-mathmatical-formula-for-calculating-the-resistance-to-ground-of-parrallel-electrodes

We hope this helps!  If you should have any further questions, please do not hesitate to contact us again in the future.

Best regards,

The Engineering Team at E&S Grounding Solutions

 

Photo credit: http://www.flickr.com/photos/mikehamm/4425743870/sizes/o/in/photostream/

 

1 Response

  1. Paul S Gomes, PE

    In that case what is the formula for non uniform soil conditions?
    Using Dwight formula and maximum soil resistivity, the calculation will yield conservative value, right? That may add additional (1 or 2) extra ground rods. Design will not be unsafe. Is that a right approach?

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