A single ground rod needs a supplemental electrode unless its resistance to earth is 25 ohms or less. Under 2020 NEC 250.53(A)(2), that supplemental electrode can be another qualifying type. It does not always have to be a second rod. When you use multiple rods to meet this rule, space them at least 6 feet apart.
Watch JCR compare ground rods and concrete-encased electrodes
See why the electrode type matters. Then use the three checks below to work through a ground-rod question.
Watch JCR compare ground rods and concrete-encased electrodesWatch the existing JCR lesson on YouTubeWhen is one rod enough?
The exception applies when the single rod’s resistance to earth is verified at 25 ohms or less. Exactly 25 ohms qualifies. Without that verification, provide the required supplemental electrode.
Do not treat the rod as the whole grounding electrode system. Under 250.50, qualifying electrodes present at the building must be bonded together. A concrete-encased electrode or qualifying metal underground water pipe can change the question you need to answer.
How far apart must ground rods be?
At least 6 feet under 250.53(A)(3). Two rods 4 feet apart do not meet that minimum. The informational note explains that spacing rods twice the length of the longest rod improves their effectiveness. That recommendation is different from the 6-foot minimum.
Also check 250.53(A)(4): rod and pipe electrodes generally need at least 8 feet of length in contact with soil. The Code provides alternatives when rock prevents normal driving. Do not cut a rod short and assume it still qualifies.
Practice grounding-electrode questions with JCR
Turn lessons like this into test-day skills. Find the rule, work the calculation and practice with JCR’s mock exam questions.
Live classes teach the 2020 NEC. Check your state’s exam requirements before choosing a course. NC prep includes NC business and law. For another state, also study any business-and-law material required for your license. JCR carries the 2020 NEC softbound code book.
Already licensed? Earn your CE with JCR
Choose in-person classes, live Zoom or recorded courses. Review NEC navigation, grounding and bonding, and other electrical topics. Check the course’s NEC edition, state approval and credit hours before enrolling.
See CE courses and class datesMust two rods together measure 25 ohms or less?
No. The 25-ohm exception determines whether a single rod needs supplementation. Once you install the required qualifying supplemental electrode, 250.53(A)(2) does not require you to keep adding rods until the combined resistance measures 25 ohms.
Project specifications or another applicable standard may impose a separate performance target. Read those requirements as well.
Does a concrete-encased electrode need a ground rod?
The single-rod supplementation rule does not require adding a rod just because you have a concrete-encased electrode. You must still connect the other electrodes required by 250.50. Metal underground water-pipe electrodes have their own supplementation rule in 250.53(D)(2).
Size the connection using our grounding electrode conductor guide. Do not use the breaker-size table for the EGC as a substitute.
Practice question
A single rod measures 32 ohms. The installer adds a qualifying second rod 6 feet away and bonds it correctly. Does 250.53(A)(2) require a third rod if the pair still measures above 25 ohms?
No. The second rod satisfies that section’s supplementation requirement. This example assumes the remaining electrode and installation requirements are met.
Common questions
Will a ground rod make a breaker trip during a fault?
Do not depend on earth for that job. The effective ground-fault current path must return current to the source so the protective device can operate. Review equipment grounding conductors and the main bonding jumper.
Are there North Carolina exceptions?
Yes. NC’s 2020 state amendments include an exception for certain temporary construction services. Apply the adopted edition and the complete conditions for the installation. The state’s 2017-code ground-rod interpretation explains the same 25-ohm principle.
Keep practicing
Try our most-missed NEC mock exam questions and 2020 NEC practice questions. Find the rule for each answer, work the calculation, and repeat the questions you miss.
Prepared October 25, 2026, for the 2020 NEC. Use the edition required by your licensing exam. On the job, use the locally adopted code and amendments.
