Driven Rod Resistance to Earth

A driven rod's resistance to earth is not in the rod. It is in the soil immediately around it: current leaving the rod spreads outward through shells of earth that get larger, and therefore less resistive, the further out you go, so almost all of the resistance is accumulated within the first metre or so of the electrode. H. B. Dwight put that into closed form in 1936, and the result is the formula on this page — resistivity over the surface area the rod presents, with a logarithm accounting for the geometry. A 3 m rod of 16 mm diameter in 100 Ω·m soil comes out near 33 Ω.

Read what the equation is telling you about improving that number. Resistance is directly proportional to soil resistivity, which spans four orders of magnitude between wet clay and dry sand and swings by a factor of several on the same site between August and February — so soil is the dominant term, and measuring it with a four-pin Wenner survey is worth more than any amount of guessing. Rod radius sits inside a logarithm, so doubling the rod's diameter buys a few percent and nothing more, which is why rods come in one or two sizes chosen for driveability. Rod length helps genuinely but with diminishing returns, and once you are past three metres a second rod set at least one rod-length away from the first almost always beats a longer single one — set closer than that, the two rods fight over the same shell of soil and you get far less than half.

The 25 Ω figure that people quote is a code threshold for when a second electrode is required, not a performance target and not a safety limit — and for a site with electronics, lightning exposure or sensitive equipment, 25 Ω is often nowhere near good enough. Two more cautions: this formula assumes uniform soil, and real ground is layered, so a rod that hits gravel behaves nothing like the equation predicts; and calculated resistance is never a substitute for a fall-of-potential test on the installed electrode. Check the requirements that apply to your installation in your own edition of the code.