How to Choose the Right Earthing Electrode Size

The three things that actually decide size

Electrode selection comes down to soil resistivity, the fault current the system has to carry, and how long the protection takes to clear that fault. Everything else — building type, load in kilowatts, number of floors — is a proxy people use when they have not measured resistivity. The proxies are usually close enough, but they are proxies, and on a difficult site they will be wrong.

Why length usually matters more than diameter

Doubling the diameter of an electrode reduces its resistance by only around ten percent. Doubling the length can reduce it by close to forty percent. This surprises people who assume a fatter rod is automatically better. Depth reaches soil layers that stay moist year round, which is where the stability comes from. If your resistance reading is too high, going deeper is almost always more effective than going wider — which is why threaded rods with couplers exist.

A practical starting table

For houses and small shops, 17 to 25 mm at 2 to 3 metres. For commercial buildings, 25 to 32 mm at 3 metres. For factories and industrial plant, 32 to 48 mm, or a 48 to 76 mm chemical electrode where fault levels are high. For solar plants, 32 mm with separate lightning protection earthing. For substations and transmission structures, 48 mm and above, usually as part of a designed earth grid rather than individual pits. Treat these as a starting point to be checked against a resistivity reading, not a specification.

Reading your soil

Normal moist black soil might sit around 50 ohm-metres. Sandy soil can be 200 to 500. Rocky and gravelly ground can run into thousands. Since electrode resistance scales roughly with resistivity, a site at 500 ohm-metres needs a very different design from one at 50 — usually multiple electrodes in parallel with compound, rather than a single larger rod. A four-pin Wenner test takes under an hour and removes the guesswork.

Coating thickness, and why it is not a detail

For copper bonded rods, 25 to 30 micron suits normal soil and cost-sensitive work. 100 micron is the common specification for commercial and industrial installations. 250 micron is used where soil is aggressive or design life is long. The failure mode people encounter is not the rod breaking — it is a thin plated layer peeling off underground, after which the steel core corrodes and resistance climbs. Coating thickness is what you are actually buying.

How many pits do you need

Two electrodes in parallel do not halve the resistance — spacing matters, and pits placed too close interfere with each other. As a working rule, space electrodes at least as far apart as they are deep. Where a single pit cannot reach the target resistance, several properly spaced pits bonded together will, and that is usually cheaper than one very large electrode.