Grounding electrode conductor calculator
Enter the largest service conductor and pick the electrode. You get the minimum grounding electrode conductor, including the size caps for rods and concrete-encased electrodes.
Built on the 2026 NEC · Free, no sign-up · Updated October 2026
Result
4 AWG Cu
Table 250.66, the row used
- Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil) — Copper
- 2/0 or 3/0
- Size of Grounding Electrode Conductor (AWG/kcmil) — Copper
- 4
- Size of Grounding Electrode Conductor (AWG/kcmil) — Aluminum or Copper-Clad Aluminum
- 2
How it was calculated
- Find the row in Table 250.66 where ungrounded conductor matches "2/0 or 3/0".
- Read the copper GEC column: 4.
- No electrode exception applies; final size is 4.
Assumptions
- Ungrounded conductor 3/0 (copper) → row "2/0 or 3/0".
- Base Table 250.66 GEC: 4 copper.
- No electrode-type cap applied (electrode = "other").
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
How the GEC is sized
Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service-entrance conductor, or the equivalent area of parallel sets. Find the row for that size and material, then read the copper or aluminum GEC column.
Electrode size caps
Some electrodes can't carry much fault current into the earth anyway, so 250.66 caps the GEC that runs only to them:
- Rod, pipe, or plate electrodes: when the conductor is the only link to them, it never has to be larger than 6 AWG copper (250.66(A)). Run copper to a rod: aluminum and copper-clad aluminum outside a building or enclosure can't end within 18 in. of the earth (250.64(A)).
- Concrete-encased electrode (Ufer): never larger than 4 AWG copper (250.66(B)).
- Ground ring: never larger than the conductor used for the ring (250.66(C)).
The cap applies only to the part of the GEC that goes to that electrode and nothing else. A GEC that also continues to the water pipe or another electrode has to be full size from Table 250.66.
2023 to 2026: our list has 12 changes in Article 250. See the changes
Worked example
A service with 3/0 copper service conductors and a copper GEC, general electrode:
- Largest ungrounded conductor: 3/0
- Ungrounded conductor material: Copper
- GEC material: Copper
- Electrode type: Other or general
Result
4 AWG Cu
Table 250.66, the row used
- Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil) — Copper
- 2/0 or 3/0
- Size of Grounding Electrode Conductor (AWG/kcmil) — Copper
- 4
- Size of Grounding Electrode Conductor (AWG/kcmil) — Aluminum or Copper-Clad Aluminum
- 2
How it was calculated
- Find the row in Table 250.66 where ungrounded conductor matches "2/0 or 3/0".
- Read the copper GEC column: 4.
- No electrode exception applies; final size is 4.
Assumptions
- Ungrounded conductor 3/0 (copper) → row "2/0 or 3/0".
- Base Table 250.66 GEC: 4 copper.
- No electrode-type cap applied (electrode = "other").
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
Questions electricians ask
What size grounding electrode conductor do I need for a 400 A or 600 A service?
The amp rating does not set it. Table 250.66 sizes the GEC from the largest ungrounded service conductor, or the equivalent area of parallel sets. Find your conductor size in the table, then read across. For copper, 500 kcmil falls in the row over 350 through 600, which gives a 1/0 AWG copper GEC. Two parallel sets of 3/0 copper add up to about 336 kcmil, which falls in the row over 3/0 through 350 and gives a 2 AWG copper GEC.
What size grounding electrode conductor do I need for a 100 A service?
Look at the service conductors first. Copper conductors of 2 AWG or smaller fall in the first row of Table 250.66, which gives an 8 AWG copper GEC. The 250.66 caps still apply when the conductor runs only to a rod or to a concrete-encased electrode.
Is there a grounding electrode conductor size chart?
Yes. Table 250.66 is the chart. It has seven rows, from 2 AWG or smaller up to over 1100 kcmil copper, with separate columns for copper and for aluminum or copper-clad aluminum. The calculator on this page reads the same table for you.
How do I size the grounding electrode conductor for parallel service conductors?
Add up the circular mil area of the parallel conductors on one phase, then use that equivalent area in Table 250.66. Pick the row that holds that area and read the GEC size from it.
What size ground wire do I need for a 200 A service?
It depends on the service conductors, not the 200 A rating. With 2/0 or 3/0 copper, or 4/0 aluminum, Table 250.66 gives a 4 AWG copper GEC. A conductor that goes only to ground rods never has to be larger than 6 AWG copper.
What size wire goes to a ground rod?
When the conductor is the only link to a ground rod, pipe, or plate, it never has to be larger than 6 AWG copper (250.66(A)). Run copper to a rod: aluminum and copper-clad aluminum outside a building or enclosure can't end within 18 in. of the earth (250.64(A)).
Is the GEC the same as the equipment ground?
No. The GEC connects the system to the earth electrodes and is sized from Table 250.66. The equipment grounding conductor runs with each circuit and is sized from Table 250.122.
Practice it hands-on
Drive rods, clamp the water pipe and footing, size the GEC, and fix a failed inspection.
Teaching this in a class?
Build your own cited lesson, quiz, and practice on any part of the NEC, in your order.
How this calculator was built
The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.
How we test the calculators. See something wrong? Tell us and we will check it against the code.
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