Earth Rod and Earth Grid Resistance Calculator with Ground Potential Rise

Estimate the resistance of earth rods and earth grids.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose rods or a grid and enter the soil resistivity.

For rods enter the length, diameter, number and spacing. For a grid enter the area, the conductor length and the depth.

The calculator gives the resistance and the rise in ground potential for the fault current you give.

Formulas used

  • R = \frac{\rho}{2\pi L}\left[\ln\frac{4L}{a} - 1\right]
  • R_g = \rho\left[\frac{1}{L_T} + \frac{1}{\sqrt{20A}}\left(1 + \frac{1}{1 + h\sqrt{20/A}}\right)\right]
  • GPR = I_fR

Worked example

With the default numbers in the calculator above:

  1. Resistance of a single driven rod (Dwight's formula): 40.39 ohm
  2. Rise in ground potential for the fault current (the voltage of the electrode above remote earth): 4038.61 V

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Number of rods in the row 4; Spacing between the rods (use at least the rod length) 3 m.

  1. Resistance of a single driven rod (Dwight's formula): 40.39 ohm
  2. 4 rods in a row with equal spacing (the rods affect one another, so the total is more than R/n): 12.97 ohm
  3. Reduction compared with a single rod: 67.88 % lower
  4. Rise in ground potential for the fault current (the voltage of the electrode above remote earth): 1297.02 V

Common questions

How do I lower the earth resistance?

Use longer rods, more rods spaced at least one rod length apart, a bigger grid, or treat the soil. Longer rods help more than thicker ones.

Why must the earth resistance be measured?

Soil resistivity varies with moisture, layers and the seasons, so a calculation is only a guide.

What is ground potential rise?

The voltage of the earth electrode above remote earth when a fault current flows into it.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Dwight's formula for a driven rod and the IEEE 80 simplified grid formula. Installations must follow AS/NZS 3000 and the network operator's rules.

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