Grounding Electrode Conductor Calculator
Pick the service conductor size and the electrode type to get the minimum grounding electrode conductor.
Step-by-step
- Enter valid values to begin.
Table 250.66 summarised for quick sizing. Aluminum GEC rules (250.64(A)) and local amendments still apply. Confirm with the NEC edition your inspector enforces.
Grounding Electrode Conductor: Tying the Service to the Earth
Core Engineering Principles
A lot of people think grounding is what makes a breaker trip. It isn’t. That’s the job of the equipment grounding conductor, covered on the next page. The grounding electrode conductor has a different job. It connects the service neutral and the enclosure to the ground rods, the building steel, the concrete-encased electrode or the water pipe. It holds the system near earth potential, drains lightning and surge energy, and keeps the voltage from floating. It doesn’t carry fault current for tripping, so it’s sized from the size of the service, not the breaker.
Why the big caps? A ground rod is a poor conductor to earth and a bigger wire doesn’t make it any better. So the Code stops requiring a larger conductor once you’ve reached what the rod can actually use: 6 AWG copper for rods, pipes and plates, and 4 AWG copper for a concrete-encased electrode, because the concrete does the heavy lifting. If the GEC runs to a water pipe or building steel that’s a better electrode, Table 250.66 applies in full.
Rod / pipe / plate: max 6 AWG Cu • Concrete-encased: max 4 AWG Cu • Ring: not larger than the ring
NEC & Standard References
Table 250.66 reads from the largest service-entrance conductor. For copper, 2 AWG or smaller gets an 8 AWG GEC, 1 or 1/0 gets 6 AWG, 2/0 or 3/0 gets 4 AWG, over 3/0 up to 350 kcmil gets 2 AWG, over 350 up to 600 gets 1/0, and over 600 up to 1,100 gets 2/0. 250.66(A), (B) and (C) are the rod, concrete-encased and ground-ring reductions. 250.64(B) says 8 AWG must be protected in conduit or armor, and anything smaller than 6 AWG unprotected is a violation. 250.64(A) restricts aluminum from direct contact with masonry or earth, and it must stay at least 18 in. above grade outdoors.1. Table 250.66: 4/0 is “over 3/0 through 350 kcmil” → 2 AWG copper.
2. Rod electrode only: 250.66(A) caps it at 6 AWG copper.
3. If a second leg goes to a concrete-encased electrode, 250.66(B) caps that leg at 4 AWG copper, so the rod leg is 6 AWG and the Ufer leg is 4 AWG.
Paralleled sets (e.g. two 250 kcmil per phase) are added up to 500 kcmil and the next row applies.
- No splices. A GEC runs unbroken from the service to the electrode, except for irreversible compression connectors or exothermic welds. Don’t use wire nuts or split bolts to join it.
- Protect it. If a 6 AWG isn’t armored or in conduit, it needs to be secured to the building surface, and it must not be exposed to physical damage.
- Water pipe isn’t enough alone. Where you use a metal water pipe as an electrode it has to be supplemented by another (250.53(D)(2)).
- Check the inspector’s rod rules. A single rod must test 25 ohms or less, otherwise add a second.
- Don’t reuse an old GEC on a bigger service. A service upgrade can change the table row.