Grounding Electrode Conductor Size Calculator
Enter the largest ungrounded service or feeder conductor and get the minimum grounding electrode conductor from NEC Table 250.66, in copper or aluminum. The tool applies the 250.66(A) rod, pipe, or plate cap and the 250.66(B) concrete-encased cap, and handles parallel conductors by equivalent area.
How to use this calculator
- Select the largest ungrounded service-entrance or feeder conductor and its material (copper or aluminum). This is the conductor area Table 250.66 is indexed on, not the service rating.
- If the conductors run in parallel, enter the number of conductors per phase; the tool sums their circular-mil area before reading the table.
- Choose the grounding electrode conductor material, then pick the electrode type to apply the 250.66(A) or 250.66(B) "need not be larger than" cap.
- Read the minimum grounding electrode conductor. The notes show the Table 250.66 row used, the equivalent area, and any cap that was applied.
NEC reference
Sizing follows NEC 2020 Table 250.66, which sets the minimum copper and aluminum grounding electrode conductor from the size of the largest ungrounded service-entrance conductor (or the equivalent area for parallel sets). The tool also applies 250.66(A) (a conductor to a rod, pipe, or plate electrode need not be larger than 6 AWG copper or 4 AWG aluminum) and 250.66(B) (a conductor to a concrete-encased electrode need not be larger than 4 AWG copper). Table 250.66 and its subsection caps are unchanged across the 2017, 2020, and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
NEC Table 250.66 grounding electrode conductor
| Largest ungrounded service conductor (copper) | Largest ungrounded (aluminum) | GEC copper | GEC aluminum |
|---|---|---|---|
| 2 AWG or smaller | 1/0 AWG or smaller | 8 | 6 |
| 1 or 1/0 AWG | 2/0 or 3/0 AWG | 6 | 4 |
| 2/0 or 3/0 AWG | 4/0 AWG or 250 kcmil | 4 | 2 |
| Over 3/0 through 350 kcmil | Over 250 through 500 kcmil | 2 | 1/0 |
| Over 350 through 600 kcmil | Over 500 through 900 kcmil | 1/0 | 3/0 |
| Over 600 through 1100 kcmil | Over 900 through 1750 kcmil | 2/0 | 4/0 |
| Over 1100 kcmil | Over 1750 kcmil | 3/0 | 250 kcmil |
Values from NEC 2020 Table 250.66 (identical in the 2017 and 2023 editions). Sizes are AWG unless marked kcmil. The aluminum grounding electrode conductor column is aluminum or copper-clad aluminum, subject to the 250.64(A) termination and contact restrictions.
The ground-rod cap most people rely on
The single most-cited part of 250.66 is subsection (A): where the grounding electrode conductor's sole connection is to a made electrode, a rod, pipe, or plate, it never has to be larger than 6 AWG copper (or 4 AWG aluminum), no matter how large the service is. A 400 A service with 500 kcmil copper conductors would call for a 1/0 copper grounding electrode conductor from the table, but if the only electrode is a driven ground rod, 250.66(A) drops that to 6 AWG copper. The tool applies this the moment you select the rod, pipe, or plate electrode type.
Subsection (B) does the same for a concrete-encased electrode (a Ufer ground): that conductor need not be larger than 4 AWG copper. Because 250.64(A) keeps an aluminum grounding electrode conductor out of concrete and earth contact, use copper for a concrete-encased electrode.
Parallel service conductors
When the service-entrance conductors are run in parallel, Table 250.66 is read on the combined circular-mil area of the paralleled conductors in a single phase, not on one conductor. Enter the number of conductors per phase and the tool multiplies the area for you: two 250 kcmil copper conductors per phase equal 500,000 cmil, which lands in the over-350-through-600 kcmil row and calls for a 1/0 copper grounding electrode conductor.
GEC vs. equipment grounding conductor
These two are easy to mix up because both are "grounding" conductors sized from a 250-series table. The grounding electrode conductor (this tool, Table 250.66) ties the grounded service to the earth electrode and is sized on the service-entrance conductor area. The equipment grounding conductor (Table 250.122) bonds equipment and raceways back to the source so a ground fault clears the breaker; it is sized on the overcurrent-device rating. A 200 A service calls for a 4 AWG copper grounding electrode conductor but only a 6 AWG copper equipment grounding conductor. Size the equipment grounding conductor with the Equipment Grounding Conductor Size Calculator.
Frequently asked questions
What size grounding electrode conductor for a 100 amp service?
A 100 A service is commonly 4 AWG copper or 2 AWG aluminum service conductors, which Table 250.66 sizes with an 8 AWG copper (or 6 AWG aluminum) grounding electrode conductor. If the only electrode is a ground rod, 250.66(A) still allows 6 AWG copper, so the 8 AWG figure already satisfies the rod connection.
Does the grounding electrode conductor change if I only have a ground rod?
Yes. For any service, a conductor whose sole connection is to a rod, pipe, or plate electrode need not be larger than 6 AWG copper under 250.66(A). The larger full-table size applies only where the conductor also serves a water pipe, building steel, or concrete-encased electrode that requires it.
Can the grounding electrode conductor be aluminum?
It can, subject to NEC 250.64(A): a bare or insulated aluminum grounding electrode conductor cannot be used where it is in contact with masonry or earth, or within 18 inches of earth, and it cannot be terminated outdoors within 18 inches of the ground. For a concrete-encased or ground-rod electrode, copper is the practical choice.
Why is the grounding electrode conductor sometimes larger than the equipment ground?
Because they are sized on different bases. The grounding electrode conductor is sized on the service conductor area (Table 250.66), while the equipment grounding conductor is sized on the overcurrent-device rating (Table 250.122). On the same 200 A service the grounding electrode conductor is 4 AWG copper and the equipment grounding conductor is 6 AWG copper.
Related calculators
- Equipment Grounding Conductor Size Calculator: the wire-type equipment ground from the breaker rating (Table 250.122).
- Service Size Calculator: size the service disconnect, conductor, and grounding electrode conductor from a known load.
- Dwelling Service Conductor Calculator: the 310.12 83% allowance for dwelling service and feeder conductors.
- Wire Size Calculator: size the ungrounded service or feeder conductors themselves.
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
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