NEC Grounding Electrode System: Electrodes, Bonding, and the 25-Ohm Rule

The NEC grounding electrode system requirements, set out in Article 250 of the National Electrical Code, come down to one principle: every qualifying electrode present at a building must be bonded together into a single system, each electrode must meet its own size and burial specs, and the connections between them must be permanent. Section 250.52 lists eight electrode types that qualify, and if any of them exist at the site, they must be used.¹

The system has two jobs. It carries off dangerous voltage from lightning strikes or accidental contact with high-voltage lines, and it stabilizes normal operating voltage so equipment sees a consistent reference to earth.¹

The Eight Qualifying Electrodes

You don’t pick one. If any of these are present at the building, all of them must be bonded into the grounding electrode system.¹

Metal underground water pipe with at least 10 feet in direct contact with the earth, electrically continuous to the point where the grounding electrode conductor attaches.
Metal in-ground support structure such as steel pilings or columns, extending at least 10 feet vertically into the ground, whether encased in concrete or driven directly into soil.
Concrete-encased electrode (Ufer ground): at least 20 feet of either bare copper conductor (minimum #4 AWG) or uncoated reinforcing steel bar (minimum ½-inch diameter), encased in at least 2 inches of concrete that sits in direct contact with the earth.
Ground ring: a bare copper conductor of at least #2 AWG, minimum 20 feet long, encircling the building and buried at least 30 inches deep.
Rod and pipe electrodes: rods at least 8 feet long and 5/8-inch in diameter; pipe or conduit at least 3/4-inch trade size, galvanized or otherwise coated if iron or steel.
Plate electrodes exposing at least 2 square feet of surface area to the soil. Iron or steel plates must be at least 1/4 inch thick; nonferrous plates at least 1.5 millimeters.
Other listed electrodes: any commercially manufactured electrode carrying a listing from a recognized testing laboratory.
Other local metal underground systems: underground piping, tanks, or similar metallic structures that meet the code’s conductivity standards.

Concrete-encased electrodes work especially well because the concrete absorbs and retains moisture from surrounding soil, keeping conductivity reasonable even in dry climates.¹

What the Code Prohibits

Section 250.52(B) bars three things from serving as grounding electrodes, and these are the ones people most often get wrong.²

Metal underground gas piping. Running fault current through gas lines creates an ignition and explosion hazard. Gas piping must still be bonded to the grounding system, but it cannot be the electrode.
Aluminum. It corrodes rapidly when buried in soil or embedded in concrete and fails as a long-term earth connection.
Swimming pool reinforcing steel described in NEC 680.26(B)(1) and (B)(2). That rebar is reserved for pool equipotential bonding.

The aluminum restriction extends to grounding electrode conductors. Bare aluminum or copper-clad aluminum conductors cannot contact concrete, masonry, or earth directly, and when used outdoors they must terminate at least 18 inches above ground level. Copper is the safer default for anything touching soil or concrete.

Burial Depth and Spacing

Rod electrodes are driven straight down so the top sits flush with or below grade, and the rod must maintain 8 feet of contact with the soil. When rock prevents driving the full 8 feet vertically, the code gives two alternatives: drive the rod at an angle up to 45 degrees from vertical, or lay it in a trench at least 30 inches deep.³ The trench option is the more reliable workaround in areas with shallow bedrock, because angled rods still tend to deflect off rock faces.

Plate electrodes must be buried at least 30 inches below the surface. Ground rings share that 30-inch minimum.¹ Those depths put the electrode in soil that holds moisture through seasonal changes and sits below the frost line in most of the country.

When multiple electrodes are installed, they must be spaced at least 6 feet apart.³ Each electrode dissipates current into a roughly spherical volume of soil around it, and rods driven too close together have overlapping zones, so the second rod adds little effective capacity. Six feet is the floor; wider spacing performs better in high-resistivity soil.

The 25-Ohm Rule

A single rod, pipe, or plate electrode must be tested for ground resistance. If the reading exceeds 25 ohms, that electrode on its own is not enough.

The fix is simple. Install a second electrode, bond it to the first, and the system is compliant regardless of what the final resistance measures. You don’t have to keep driving rods chasing a number. Two properly bonded electrodes satisfy the code even in dry, sandy, or rocky soil where low resistance is physically hard to achieve.

Testing uses a ground resistance tester. If the first reading comes in at 25 ohms or below, a single electrode is sufficient. Above that, the supplemental electrode goes in.

Supplemental Electrode for Water Pipe Systems

A metal underground water pipe cannot be the sole grounding electrode even if it meets the 10-foot contact requirement. The code requires a supplemental electrode whenever a water pipe is used, because plastic pipe replacements and repairs can silently eliminate the earth contact that made the water pipe effective in the first place. The supplemental electrode can be any other qualifying type: a ground rod, Ufer ground, plate, or ground ring.¹

Bonding and Connection Methods

Good electrodes in the ground accomplish nothing without reliable connections. The grounding electrode conductor (GEC) links every electrode to the main service equipment, and every joint in that chain has to be permanent.

Connections between the GEC and the electrodes must use irreversible methods: exothermic welding (Cadweld), listed pressure connectors, or listed clamps.⁴ Standard twist-on wire nuts are not permitted. A wire nut can loosen from vibration, corrosion, or thermal cycling, and a grounding connection that quietly fails defeats the system.

Mechanical connections such as pressure clamps and listed lugs must remain accessible after installation so they can be inspected. The exceptions are connections specifically listed for direct burial or concrete encasement.¹

Water Meter Bonding Jumper

When a metal water pipe serves as a grounding electrode, the electrical path through the pipe run must remain continuous. Water meters, dielectric unions, and insulated couplings all create breaks in that path. A bonding jumper installed around the meter keeps the grounding connection intact when a plumber removes the meter for service. This is one of the most commonly failed inspection items, because the water system looks physically intact even when it is electrically broken.

Matching Material to Soil

The electrode material affects whether the system still meets the code in ten or twenty years.

Copper-coated steel rods are the most common choice and work in most soil types. The copper layer is thin and can be damaged during installation, especially through rocky ground. When the coating chips, the exposed steel core corrodes sacrificially, which gradually hollows out the rod from inside.

Hot-dip galvanized steel rods handle rocky installation better because the zinc coating is harder. When the coating is breached, the zinc sacrifices itself to protect the steel core, the opposite of what happens with copper-coated rods and generally better for the rod’s structural integrity. Galvanized rods are a poor choice near coastlines, however, because salt attacks the zinc.

Highly acidic soil with a pH at or below 4 is aggressive toward electrode materials. Copper-coated or stainless steel rods are a better choice in coastal environments where galvanized rods would corrode quickly.

Dissimilar metals buried near each other create galvanic corrosion: one metal deteriorates while protecting the other. Keeping materials consistent throughout the system reduces that risk.

Permits and Inspection

Installing or replacing a grounding electrode system is electrical work that typically requires a permit. The grounding system is regulated under Article 250 of whichever NEC edition your local authority has adopted, and electrical inspectors check it during the rough-in and final inspections of new construction or major renovation.

Inspectors look at specifics: whether all qualifying electrodes at the site have been bonded together, whether the GEC is properly sized and routed, whether connections use approved methods, and whether supplemental electrodes were installed where required. Concrete-encased electrodes and ground rings must be inspected before the concrete pour or trench backfill, because they are invisible afterward. Missing that inspection window may force the work to be exposed again.

Permit fees vary by municipality, ranging from under $100 to several hundred dollars depending on scope. Local jurisdictions can adopt amendments to the NEC that add requirements beyond what’s described here, so the local building department is the final authority on what your specific installation requires. Electrical work done without a permit can void homeowner’s insurance coverage and create complications when selling the property.

1
National Fire Protection Association. Understanding Our Electrical World: 8 Items that Form the Grounding Electrode System
2
UpCodes. Not Permitted for Use as Grounding Electrodes
3
UpCodes. Grounding Electrode System Installation
4
UpCodes. Connection of Grounding and Bonding Equipment