Solar Earthing Electrode Design: Types, Distance and Testing

Last updated: November 2026.

Two ground rods placed too close act like one. This guide shows how far apart to put them, how soil changes the answer, and how to test the result.

A NYSERDA NY-Sun review of 287 PV system inspections found at least one grounding deficiency in 53% of the systems. Labeling was more common, at 78%. The grounding category covers racking and enclosures as well as the electrode, and the second most common finding inside it was an electrode whose top was not flush with or below ground level. This guide treats the solar earthing electrode as a design object. You will see how far apart electrodes should sit, how to read soil resistivity, how to estimate rod resistance, and how to test the result.

Standards come from different regions, and most of their full text is sold. This page cites only what we could read in a public document, and it says so where we could not. The US NEC wording quoted here comes from an IAEI Magazine article that quotes the 2002 edition, so check the edition your authority has adopted. We did not read IS 3043 or IEC 62561 and make no claim about their clauses.

Solar Earthing Distance: How Far Apart Should Electrodes Be?

Electrode spacing has two sides. Rods too close together share the same patch of soil, so the second rod adds little. Rods too far apart cost more cable and trenching. The published guidance agrees on a floor and differs on the upper end.

Sources: MIL-HDBK-419A, section 2.6.2, IAEI Magazine, “Supplementary Grounding Electrodes” (September 2002), the Megger earth electrode and loop impedance booklet and the PLP Australia earth rod brochure. The Megger and PLP documents are manufacturers’ technical material. Earlier versions of this page said IS 3043 requires twice the rod length. We could not confirm that, so we removed it.

What Spacing Does to Resistance

Use the two-rod equation from MIL-HDBK-419A (equation 2-21) and a single-rod equation from the PLP brochure to see the effect. Take a 3 m rod of 16 mm diameter in soil of 100 ohm-m.

• One rod: R = ρ ÷ (2πL) × (ln(8L ÷ d) − 1). That is 100 ÷ (2π × 3) × (ln(8 × 3 ÷ 0.016) − 1) = 5.305 × (7.313 − 1) = 33.5 ohms.
• Two rods, spacing s: the MIL-HDBK-419A equation gives the results below.

The ideal is half of one rod, 16.7 ohms, which two rods would give if they did not interact. Moving from 1.8 m to 6 m cuts 1.9 ohms. Moving from 6 m to 12 m cuts 0.6 ohms. That is why the guides stop at about twice the length.

The same example shows why the 25 ohm rule exists. A rod of 2.44 m (8 ft) and 16 mm in 100 ohm-m soil gives 100 ÷ (2π × 2.44) × (ln(8 × 2.44 ÷ 0.016) − 1) = 39.8 ohms. That is above 25 ohms, so the NEC would ask for a second electrode.

What a Solar Earthing Electrode Actually Does

A solar earthing electrode has one job: create a reliable electrical connection to the earth. It does not normally carry current. During a fault, a lightning strike, or a utility surge, it provides a path for energy to flow into the ground. This limits the voltage that appears on metal frames, inverter enclosures, and conduit.

The electrode is connected to the system through a grounding electrode conductor, or GEC. NYSERDA’s guide says the NEC sizes the GEC under Table 250.66. The GEC is not the same as the equipment grounding conductor, or EGC. The EGC does not carry current under normal conditions. It gives fault current a path back to the source so the breaker trips. The GEC connects the bonded system to the electrode. Confusing the two is a documented cause of inspection findings.

Think of it this way: the EGC is the emergency exit path inside the building. The GEC is the door from the building to the ground. The earthing electrode is the ground itself. All three must be present and continuous, but they perform different jobs.

For the full grounding system, see our solar PV grounding system design guide.

Rooftop vs Ground-Mount Electrode Design

Rooftop systems usually connect to the building’s existing grounding electrode system. NYSERDA’s inspection checklist asks that the GEC be properly bonded to the main premises grounding electrode system. The design focus is on conductor sizing, bonding continuity and proper termination.

Ground-mount systems are different because the array can sit away from the building. Whether you need a separate electrode at the array depends on the NEC edition and local amendments. We could not read the current text of NEC 690.47, so we do not state a distance. Check it in your adopted edition. NYSERDA’s sample wiring diagram for a string inverter lists “Grounding Electrode for Array” as a schedule item, which shows that array electrodes appear on permit drawings. For more on ground-mount layout, read the ground-mount solar design guide.

Utility-scale plants use an earthing grid designed from a soil survey and fault-current data. IEEE 80 is the guide for safety in AC substation grounding, and it covers generating-plant substations. Touch and step voltages are checked across the site.

Types of Solar Earthing Electrodes

MIL-HDBK-419A lists vertical rods, horizontal rods or wires, plates, stars and circles as simple earth electrodes, with approximate resistance formulas for each. The notes below come from that handbook and from the manufacturer documents named in each row.

Copper-Bonded Steel Rods

A copper-bonded rod has a steel core and a copper layer. PLP describes steel rod clad in copper or stainless steel as combining conductivity, corrosion resistance and strength for driving. PLP’s copper-bonded rods list copper plating of more than 250 microns. That is one maker’s product specification, so check the datasheet of the rod you buy.

Galvanized Iron and Chemical Electrodes

We could not find a public, readable standard or maker datasheet that states a service life or a price for galvanized iron pipe or chemical earthing electrodes. Earlier versions of this page gave lifetimes and a comparison of cost, and we removed them. If a tender specifies either type, ask the supplier for the datasheet and the test report, and compare them with the standard the tender names.

Soil Resistivity: The Hidden Design Driver

Soil resistivity is measured in ohm-meters or ohm-centimeters. It tells you how easily current moves through the earth. The same rod produces a very different earth resistance in clay and in dry sand.

Source: the Megger booklet. The conversion divides ohm-cm by 100. Megger notes that moisture, water table and temperature change with the season, and that resistivity rises when soil is dry or cold. In a hot, dry region, design for the driest expected condition.

Measure resistivity before you design. Megger describes the four-pin Wenner method as the most popular. The formula is ρ = 2πAR, where A is the spacing between pins and R is the tester reading. In metres and ohms it gives ohm-m. For example, if the tester reads 12 ohms with pins 2 m apart, ρ = 2π × 2 × 12 = 151 ohm-m. Megger writes the same formula in centimeters to give ohm-cm. Wider spacing samples deeper soil, so repeat the test at several spacings and several places on the site. Rearranged, R = ρ ÷ (2πA) gives a rough guide to the resistance of an electrode of that size.

Sizing: Length Beats Diameter

PLP and Megger agree that depth matters more than diameter. The single-rod equation shows by how much. Use ρ = 100 ohm-m and a 16 mm, 3 m rod as the base case, which gives 33.5 ohms.

Arithmetic: 6 m rod, 100 ÷ (2π × 6) × (ln(8 × 6 ÷ 0.016) − 1) = 2.653 × 7.006 = 18.6 ohms. 32 mm rod, 100 ÷ (2π × 3) × (ln(8 × 3 ÷ 0.032) − 1) = 5.305 × 5.620 = 29.8 ohms. The equation is for a single vertical rod in uniform soil, and real soil has layers. It is a first estimate, not a design result.

Rod Length and Diameter Requirements

IAEI quotes the 2002 NEC as requiring a ground rod to be in contact with the earth for at least 8 ft (2.44 m). We could not read the rod dimensions in IS 3043 or IEC 62561-2, so we do not list them. Check the standard your project names.

Earth Resistance Targets

Earlier versions of this page gave 1 ohm for large plants and 5 ohms for rooftops as typical Indian figures. We could not confirm them in a public source. NEC does not give one maximum for every case. Many designs set a lower target than 25 ohms, so use the number your contract or your authority states.

Conductor Sizing

For the NEC, NYSERDA’s guide sizes the GEC under Table 250.66 and the EGC under Table 250.122. Check conductor sizes against the local code. We do not reproduce the IS 3043 conductor formula or its material factors, because we could not read the standard.

Installation Best Practices

A correct design can fail if the installation is sloppy. These points come from the NYSERDA inspection findings and the maker and handbook documents above.

Finish the top of the electrode flush with or below grade. NYSERDA lists an electrode top above ground level, in violation of NEC 250.53(G), as the second most common grounding deficiency it found. Use a proper inspection chamber if you need access.

Keep the GEC continuous. NYSERDA lists a GEC that is not continuous or irreversibly spliced, in violation of NEC 250.64(C) and 690.47(C), among the top five. It names compression crimp and exothermic welding as allowed splicing methods.

Drive deep rather than add rods. PLP notes that fewer rods driven deeper usually give a lower resistance than the same number in parallel, because rods in parallel interact. Megger says doubling the depth can cut the value by up to 40%.

Bond to the premises electrode system. NYSERDA’s checklist asks that the GEC be bonded to the main premises grounding electrode system.

Keep lightning earthing in view. See our solar lightning protection guide for air-terminal and surge protection design, and check the rule that applies to your authority on separation or bonding between the two systems.

Testing and Commissioning

Earth resistance testing is the only way to confirm the design works in the actual soil.

Fall-of-potential method. Megger describes it as the standard test. Connect the tester to the electrode, drive a current spike away from it, and place a potential spike in between. For a single electrode, Megger says the current spike can usually go about 15 m away with the potential spike about 9 m away. For a small grid of two electrodes it suggests about 100 m and 62 m. For larger systems, up to 200 m and 125 m. Megger also describes a 61.8% rule, which puts the potential spike at 61.8% of the distance to the current spike, and warns that it must be treated with great care. Check for a flat region when you move the potential spike. If the readings keep changing, move the current spike farther out.

Clamp-on and attached-rod methods. Megger describes the attached rod technique, which uses a clamp to measure only the test current in the electrode, so you do not have to disconnect it. A clamp reading can mislead where several earth paths run in parallel, so read the instrument manual.

Two-pole testing. Megger calls the direct two-pole method a last resort, because it depends on a second earth whose value you cannot prove.

Record the electrode number and location, the measured resistance, the soil and weather at the time, the date and the tester serial number. Moisture and temperature move the reading through the year, so a single result in one season tells you little about the driest period. Set re-test intervals from your contract and the rules that apply to your plant.

Common Mistakes and How to Avoid Them

Assuming one rod is enough. The worked example above shows a 2.44 m rod at 39.8 ohms in soil of 100 ohm-m. Measure the soil and calculate.

Installing rods too close together. At 1.8 m two 3 m rods give 19.9 ohms, against 18.0 ohms at 6 m.

Leaving the electrode top above grade. It is a documented inspection finding.

Confusing the EGC and GEC. The EGC bonds equipment and carries fault current. The GEC connects to the electrode. They land at the same bus but do different jobs.

Testing in one season only. Moisture and temperature change the reading, so a single test is a snapshot.

Ignoring local amendments. Authorities add their own requirements. Check the local checklist before you finalize the design.

When to Use Design Automation

Earthing electrode design is a field and soil problem. You cannot automate the soil measurement. You can make the electrical documentation around it clear. A single-line diagram that shows the GEC route, the electrode locations and the bonding points helps an inspector, and NYSERDA’s checklists ask for these items.

Conclusion

A solar earthing electrode is not a commodity item. Its length, spacing and depth have to match the soil and the code. The process is simple: measure the soil, estimate the resistance, pick the layout, install with care and test. Most failures come from skipping a step.

Three actions to take next:

1. Add soil resistivity testing to your site survey before you finalize the electrode design.
2. Write the electrode type, length and spacing on the drawings so the installer cannot substitute a shorter rod.
3. Record earth resistance at commissioning and plan a re-test in the driest season.

To see how SurgePV documents the electrical side of a design, book a demo.

Frequently Asked Questions

How far apart should solar earthing rods be?

The NEC sets a 1.8 m (6 ft) minimum between rod, pipe or plate electrodes, as IAEI quotes the 2002 edition. MIL-HDBK-419A advises between one and two rod lengths, which is 3 to 6 m for a 3 m rod. Megger and PLP advise at least the rod length. Wider spacing lowers the combined resistance, with diminishing returns beyond about twice the length.

What is the solar earthing rod distance for a 3 m rod?

Between 3 m and 6 m under MIL-HDBK-419A, and at least 1.8 m under the NEC. Two 3 m rods in 100 ohm-m soil give about 19.9 ohms at 1.8 m, 18.9 ohms at 3 m and 18.0 ohms at 6 m, against 33.5 ohms for one rod.

What is a solar earthing electrode?

A solar earthing electrode is a metal conductor placed in the earth to provide a low-resistance path for fault currents, lightning surges and leakage currents. It forms the physical earth reference for a PV system and is connected to the system through a grounding electrode conductor.

What is the maximum allowable earth resistance for a solar plant?

It depends on the standard, the plant and the contract. The NEC asks for a second electrode when a single rod, pipe or plate does not measure 25 ohms or less. MIL-HDBK-419A says experience shows 10 ohms gives fairly reliable lightning protection. We could not read the IS 3043 or tender figures for India, so we do not state them.

How does soil resistivity affect earthing electrode design?

Soil resistivity sets how easily current flows into the earth. Megger lists clay at 2 to 100 ohm-m and sand and gravel at 50 to 1,000 ohm-m. Higher resistivity needs longer or deeper rods, more electrodes or a grid. Measure it on site with the Wenner method, ρ = 2πAR.

Does a longer rod or a thicker rod lower resistance more?

Length. For a 16 mm rod of 3 m in 100 ohm-m soil, doubling the length cuts the resistance from 33.5 to 18.6 ohms, which is 44%. Doubling the diameter to 32 mm cuts it to 29.8 ohms, which is 11%.

What is the difference between earthing and grounding in solar?

They describe the same safety function. Earthing is the British and Indian term, and grounding is the North American term. Both mean connecting exposed metal parts and the electrical system to earth through an electrode.