Ground rod resistance is determined overwhelmingly by the soil the rod sits in, not by how deep you hammer or how hard you try. That is why the wiring rules take a pragmatic approach: either demonstrate the electrode meets a resistance threshold by testing, or install a supplemental electrode and stop worrying about the number.
Soil, not steel, sets the resistance
A driven rod is a small piece of metal in a very large resistive medium. The resistance you measure comes almost entirely from the soil immediately surrounding the rod, where the current density is highest, and that soil resistivity varies enormously. Damp clay gives a low reading. Dry sand, gravel or rocky ground gives a high one, sometimes by an order of magnitude. The same rod installed at two ends of the same property can read very differently. Moisture content also means readings change seasonally, so a measurement taken after spring rain does not describe the same electrode in August.
Why the code offers a two-rod alternative
Requiring every installation to be tested would mean specialised equipment and a competent operator on every service. Instead the rules allow a single rod electrode to stand alone if it is shown by test to meet the specified resistance to earth, and otherwise require it to be supplemented by an additional electrode. In practice almost nobody tests, so almost every installation gets a second rod. That is not because two rods reach a magic value; it is because two rods are cheaper than a test, and the requirement is written to make the practical choice also the compliant one.
Spacing matters more than depth beyond a point
Two rods driven close together barely perform better than one, because their zones of influence in the soil overlap heavily. Separation is what gives each rod access to fresh soil. The code specifies a minimum separation for supplemental rod electrodes, and greater separation generally performs better. Depth helps where it reaches moister strata, which is why a longer rod can outperform two short ones in dry ground. What does not help is driving a rod into fill or rubble, or bending one over when it hits rock. If a rod cannot be driven, the code provides alternatives for that situation.
What a resistance test actually measures
Fall-of-potential testing with a dedicated instrument gives a meaningful figure for a specific electrode at a specific moment. A clamp-on ground tester gives a useful reading in some configurations and a misleading one in others, particularly on a system with multiple bonded electrodes where it measures a loop rather than the rod alone. A multimeter does not measure ground resistance at all. If a number matters to you, use the right instrument and understand what it is reading. For most residential work the number does not matter, because the supplemental electrode route sidesteps it entirely.
What the electrodes are not doing
It is worth restating, because chasing a low rod resistance often comes from a misunderstanding: fault current does not return through the earth. It returns through the equipment grounding conductor and the bonded service neutral. A rod reading five ohms and a rod reading fifty ohms will both fail to trip a breaker, because neither is the fault path. The electrode system limits surge and lightning voltages and stabilises the system with respect to earth. Concrete-encased electrodes in a footing, where present, typically outperform driven rods substantially, and where one exists it must be part of the system.
What to check before you act
- Bond every electrode present at the building into one grounding electrode system.
- Separate supplemental rods by at least the code minimum, and more where practical.
- Use a proper ground resistance instrument if you need an actual figure.
- Confirm clamps are listed for direct burial and suit the electrode material.
Common questions
Is a deeper rod better than two rods?
Often, if the extra depth reaches moister soil. Code requirements still govern what arrangement is acceptable, so depth is not a free substitute for the supplemental electrode rule.
Can I measure ground rod resistance with a multimeter?
No. Measuring resistance to earth requires an instrument designed for it, using a fall-of-potential or equivalent method.
The practical takeaway
The second rod exists because testing is impractical, not because two rods hit a target. Bond every electrode at the building into one system, space supplemental rods properly, and remember that the earth connection is for surges, not for clearing faults.
References and further reading
- NFPA: National Electrical Code overview including grounding electrode requirements
- OSHA: grounding requirements for electrical systems and equipment
- OSHA: Subpart S electrical standards covering system grounding and bonding
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
