How to check grounding electrode resistance
The U.S. National Electrical Code does not classify grounding into numbered types the way some countries do. Instead of a type table, the NEC sets rules for how many electrodes you need and what size conductor connects them. Pick your electrode type, enter the measured resistance to ground, and this tool applies both rules at once.
Under NEC 250.53(A)(2), a single rod, pipe, or plate electrode that does not measure 25 ohms or less must be supplemented with one additional electrode, spaced at least 6 ft away. Installing two rods from the start satisfies the exception with no resistance test at all. Concrete-encased electrodes and ground rings are not subject to the supplement rule, but every electrode present at the building must still be bonded together per NEC 250.50.
The conductor size comes from the NEC 250.66 table, based on the largest ungrounded service-entrance conductor. Size caps also apply: the portion running to a rod, pipe, or plate need not exceed 6 AWG copper, to a concrete-encased electrode 4 AWG, and to a ground ring 2 AWG. Soil moisture and temperature move readings a lot, so test in the driest season and have the final design confirmed by a licensed electrician or the authority having jurisdiction before you close out the work.
Frequently Asked Questions
NEC 250.53(A)(2) requires one supplemental electrode at least 6 ft away. Once the second electrode is installed, the code does not require you to reach any specific resistance value, though lower is always better for surge performance.
No. It is only the trigger for adding a supplemental electrode. Sensitive equipment, lightning protection, and utility substations often specify 5 ohms or less, so follow the stricter project specification when one exists.