How Many Ground Rods Do You Need? NEC 250.53 Two-Rod Rule, the 25 Ohm Test, Soil Resistivity and Rod Spacing, Length and Diameter
Sep 10, 2026
Summary: NEC 250.53(A)(2) requires a single rod, pipe or plate electrode to be supplemented by a second electrode unless the single rod measures 25 ohms or less to earth. Because the test takes a fall-of-potential meter and an inspector's time, nearly every jurisdiction now requires two rods at least 6 ft apart by default and skips the test. Rods must be at least 8 ft long and in contact with soil for the full 8 ft, driven vertically or, where rock prevents it, at up to 45 degrees or buried in a 30 in deep trench (250.53(G)). Listed copper-bonded rods may be 5/8 in diameter; unlisted rods must be 3/4 in. Resistance to earth is set almost entirely by soil resistivity: a single 8 ft rod is about 25 ohms in 100 ohm-m soil and about 250 ohms in 1000 ohm-m rock. Two rods 6 ft apart cut that by about 40 percent, not by half; doubling the rod length is more effective than adding a second rod.
The code rules
| Requirement | NEC section | Detail |
|---|---|---|
| Rod length | 250.52(A)(5) | At least 8 ft; the full 8 ft in contact with soil |
| Rod diameter | 250.52(A)(5)(b) | Listed rods 5/8 in minimum; unlisted stainless, copper-coated or galvanized steel 3/4 in minimum |
| Second electrode | 250.53(A)(2) | A single rod, pipe or plate must be supplemented by a second electrode unless it tests at 25 ohms or less; the supplement may be a second rod, a water pipe, a Ufer, a ring or a plate |
| Spacing | 250.53(A)(3) | Rods, pipes and plates at least 6 ft apart; the informational note recommends more than the rod length, so 8 ft or more for 8 ft rods |
| Installation | 250.53(G) | Vertical; if rock, up to 45 degrees; if that fails, in a trench at least 30 in deep. The top end flush with or below grade unless the clamp is protected |
| Clamp | 250.70 | Listed for the rod and, if buried, for direct burial; one conductor per clamp unless listed for more |
| GEC size to rods | 250.66(A) | Never required to be larger than 6 AWG copper |
| Bonding of electrodes | 250.50, 250.53(C) | All electrodes present bonded together; bonding jumper between rods sized per 250.66, 6 AWG copper is the norm |
Why two rods is the default
The 25 ohm test requires a three-point fall-of-potential measurement with the electrode disconnected from the system, two test probes driven out to 60 and 100 ft, and a documented result. The test costs more than the second rod. So in practice: two 8 ft copper-bonded rods, 6 ft or more apart, tied with a continuous 6 AWG copper GEC through two listed direct-burial clamps, satisfies 250.53(A)(2) without a test in every jurisdiction. Drive the second rod at more than 8 ft from the first if space allows; the resistance benefit falls off inside one rod length.
What resistance to earth actually depends on
The resistance of a vertical rod is approximately R = (ρ / 2πL) × [ln(4L/a) − 1], where ρ is soil resistivity in ohm-meters, L is rod length in meters and a is rod radius in meters. For an 8 ft (2.44 m) rod of 5/8 in (radius 0.0079 m) that reduces to about R ≈ 0.325 × ρ. Soil resistivity dominates; rod diameter barely matters, which is why 5/8 in and 3/4 in perform the same.
| Soil | Resistivity, ohm-m | One 8 ft rod, ohms | Two 8 ft rods 6 ft apart, ohms | One 16 ft rod (two coupled), ohms |
|---|---|---|---|---|
| Wet clay, marsh | 10 to 30 | 3 to 10 | 2 to 6 | 2 to 5 |
| Clay, loam, farm soil | 50 to 100 | 16 to 32 | 10 to 20 | 9 to 18 |
| Sandy loam | 100 to 300 | 32 to 100 | 20 to 60 | 18 to 55 |
| Sand, gravel | 300 to 1000 | 100 to 325 | 60 to 200 | 55 to 180 |
| Rock, dry sand | 1000 to 10,000 | 325 to 3,250 | 200 to 2,000 | 180 to 1,800 |
Values are approximate for uniform soil; the top layer dries in summer, so measured values rise seasonally. Two rods 6 ft apart do not halve the resistance because their fields overlap; the improvement is roughly 40 percent at 6 ft and 45 percent at 16 ft. Coupling two rods end to end to reach 16 ft, or using a 10 ft rod, reaches deeper, wetter soil and usually beats a second parallel rod.
Getting lower when the job requires it
- Telecom, cell sites, data centers often specify 5 ohms; utilities 1 to 5 ohms at substations. NEC compliance is not the target; the spec is.
- Longer rods: 10 ft rods, or 8 ft sectional rods with threaded or compression couplers driven to 16 or 24 ft. Depth is the single most effective lever.
- Ground ring: 20 ft or more of bare 2 AWG copper at 30 in depth around the structure (250.52(A)(4)), tied to the rods; large area, low resistance.
- Multiple rods in a line or triangle at 2 to 3 rod lengths apart, bonded with 2 AWG or 4/0 bare copper and irreversible compression connectors.
- Chemical rods and backfill: bentonite or a conductive backfill in an augered hole for rock and sand.
- Ufer: in new construction the concrete-encased electrode is usually 5 to 10 ohms on its own and is required anyway; see Ufer ground.
Driving and connecting
- Locate utilities. Rods go 8 ft deep; that is below most services but call 811.
- Drive vertically with a rod driver or demolition hammer with a rod cup; keep the tip on the rod end to avoid mushrooming the head, which stops the clamp seating.
- If refusal above 8 ft, angle to 45 degrees or trench at 30 in.
- Second rod at 6 ft minimum, 8 ft or more preferred.
- Clamp: bronze direct-burial clamp per rod, GEC continuous from the service through the first clamp to the second rod (250.64(C)); do not cut the GEC at the first rod.
- Top of rod flush or below grade, clamp buried, or above grade in a protected location. Photograph before backfill; most AHJs accept the photo.
Quick answers
Does the NEC require two ground rods? It requires a single rod to be supplemented unless it tests at 25 ohms or less. Without the test, two rods 6 ft apart are required, which is the standard practice.
How far apart should two ground rods be? At least 6 ft by code; 8 ft or more is better and recommended in the NEC note.
What size wire to two ground rods? 6 AWG copper maximum required, continuous from the service through the first rod to the second.
Does a longer rod help more than a second rod? Yes. Doubling length to 16 ft lowers resistance more than a second 8 ft rod at 6 ft spacing, because the deeper soil is wetter and the fields do not overlap.
Conversions Tech stocks copper-bonded ground rods in 1/2, 5/8, 3/4 and 1 in, 5 to 16 ft, bronze direct-burial rod clamps, rod couplers and Figure 8 compression connectors for sectional rods and ground rings. See the install guide.
