Ground Rod Resistance & Supplemental Rule Calculator

The 25-ohm exception, and why two rods is the default

NEC 250.53(A)(2) starts from a firm position: a single made electrode is not enough on its own. A rod, pipe, or plate electrode must be supplemented by a second electrode of a type named in 250.52(A)(2) through (A)(8). The one way out is the exception, and it is narrow: the single electrode has to be shown to have a resistance to earth of 25 ohms or less.

Two things make that exception impractical in the field. First, "shown" means measured. Without a fall-of-potential or clamp-on reading in hand, you cannot claim the exception, so an unmeasured single rod always needs a supplement. Second, 25 ohms is hard to hit with one rod in ordinary soil, and the reading moves with seasonal moisture. That is why most installers skip the test entirely and drive a second rod at least 6 ft away: the second electrode is the supplement the code asked for, so the 25-ohm requirement is deemed satisfied and never has to be proven.

The 6 ft spacing rule

NEC 250.53(A)(3) sets the minimum separation between rod, pipe, or plate electrodes at 1.8 m (6 ft). This is the code floor. From an electrical standpoint, two rods placed close together overlap the same volume of soil, so the second rod barely lowers the combined resistance. Spacing the second rod at least one rod length from the first, and often twice the rod length, lets it reach fresh soil and do real work. Six feet keeps you code-compliant; more separation is what actually reduces resistance. That best-practice guidance is not an NEC requirement and does not change whether the install passes.

What 25 ohms is, and is not

The 25-ohm figure is a maximum for a single made electrode, tied only to the supplemental-electrode decision. It is not a design target for the grounding system, and the NEC does not require the completed grounding electrode system to reach any particular resistance value. Once a second electrode is installed, the code treats the supplemental requirement as met regardless of the measured resistance, and no ceiling applies to the pair. Facilities with their own performance targets, such as sensitive electronics or utility substations, set those by specification, not by 250.53(A)(2).

Frequently asked questions

Can I stop at one ground rod?

Only if you measure it and it comes in at 25 ohms or less. Otherwise NEC 250.53(A)(2) requires a second electrode. Because the measurement is extra work and the number is hard to hit with one rod, driving a second rod 6 ft away is usually faster than proving a single rod qualifies.

Do I have to test resistance if I install two rods?

No. Two or more rod, pipe, or plate electrodes bonded together satisfy the supplemental-electrode requirement on their own, so no 25-ohm resistance measurement is required. You only need to keep them at least 6 ft apart per 250.53(A)(3).

Does a driven pipe or a plate electrode follow the same rule?

Yes. Section 250.53(A)(2) names rod, pipe, and plate electrodes together, so the supplemental rule, the 25-ohm exception, and the 6 ft spacing apply identically to all three. This tool lets you pick the type, but the determination does not change.

What about a concrete-encased or water-pipe electrode?

Those are different electrode types with their own rules. The 250.53(A)(2) supplemental requirement and the 25-ohm exception are written for made rod, pipe, and plate electrodes. A metal underground water pipe electrode has its own supplemental requirement (250.53(D)(2)), and a concrete-encased (Ufer) electrode is not subject to the 25-ohm rule. This calculator covers the rod, pipe, and plate case.

Related calculators

• Grounding Electrode Conductor Size Calculator: size the conductor from the service conductor per NEC Table 250.66, after you have settled the electrode itself.
• Equipment Grounding Conductor Size Calculator: minimum EGC from the OCPD rating per NEC Table 250.122.
• Bonding Jumper Size Calculator: main, system, and supply-side bonding jumpers per NEC Table 250.102(C)(1).
• Service Size Calculator: service disconnect, conductor, and grounding electrode conductor from a known load.