Field Guide • Power & Distribution
How to Size a Grounding Electrode Conductor
Updated July 16, 2026 • Written by the field team at Arizona Electrical Solutions. All field guides →
The grounding electrode conductor (GEC) ties your service's grounded conductor and ground bus to the earth — through ground rods, the Ufer, building steel, or the water pipe. It's not sized for load. It's sized from the largest ungrounded service-entrance conductor using NEC Table 250.66, because its job is to stabilize voltage to earth and carry lightning and surge events, not fault current.
On commercial work this comes up on every new service, every service upgrade, and every separately derived system, and inspectors check it almost every time. It's also one of the easiest red tags to avoid: one table lookup in NEC 250.66 (2023 numbering here), plus two or three caps you have to know cold. Here's the whole process, including the electrode system itself and the jumpers that tie it together.
Safety first. Grounding and bonding work on a service is for qualified, licensed electricians only. Service equipment stays energized on the line side even with the main open — de-energize, apply lockout/tagout, and verify absence of voltage with a tested meter before touching any conductor or bus. Wear appropriate PPE per NFPA 70E. Pull the required permits, and remember the locally adopted NEC edition and any local amendments govern — verify with the AHJ before you order material.
What you'll need
- Locally adopted NEC, Table 250.66 tabbed
- Bare or insulated copper GEC — 8 AWG through 3/0 depending on the service
- Ground rods (5/8 in × 8 ft copper-bonded) or access to the Ufer stub-up
- Listed acorn clamps, rebar clamps, or exothermic weld kit and molds
- Listed water-pipe ground clamps sized to the pipe
- Irreversible compression connectors and crimper if the GEC will be tapped
- Schedule 80 PVC or bonded ferrous raceway for physical protection where required
- Clamp-on ground resistance tester if the AHJ wants a 25-ohm reading on a single rod
Code references
| NEC 250.66 | GEC sizing rules, with the 6 AWG rod cap in (A), 4 AWG Ufer cap in (B), ground ring cap in (C). |
| NEC Table 250.66 | GEC size keyed to the largest ungrounded service-entrance conductor or equivalent parallel area. |
| NEC 250.50 | All electrodes present must be bonded into one grounding electrode system. |
| NEC 250.52 | What qualifies as an electrode — water pipe, building steel, Ufer, ground ring, rods, plates. |
| NEC 250.53 | Electrode installation: rod supplementation and spacing, water-pipe supplement, bonding jumpers. |
| NEC 250.64 | GEC installation — protection, continuity, splices, bonding of ferrous enclosing raceways. |
| NEC 250.70 | Connection methods to electrodes: exothermic weld, listed lugs, pressure connectors, clamps. |
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
Step by Step
How to Size a Grounding Electrode Conductor
1. Find the largest ungrounded service-entrance conductor
Table 250.66 keys off the size of the largest ungrounded service-entrance conductor, not the breaker or the calculated load. A 400 A service fed with 500 kcmil copper is a 500 kcmil entry into the table, period.
If the service conductors are paralleled, use the equivalent area: add the circular mils of one phase across all sets. Two parallel 250 kcmil copper per phase enters the table as 500 kcmil — that's spelled out in the table heading itself.
2. Read Table 250.66
The common commercial lookups: 2/0 or 3/0 copper service conductors take a 4 AWG copper GEC; over 3/0 through 350 kcmil takes 2 AWG; over 350 through 600 kcmil takes 1/0; over 600 through 1100 kcmil takes 2/0; over 1100 kcmil takes 3/0 copper. That 3/0 is the ceiling — the GEC never has to be larger no matter how big the service gets.
Aluminum service conductors and aluminum GECs each have their own column — don't cross them up. A service with 750 kcmil aluminum conductors (over 500 through 900 kcmil Al) takes a 1/0 copper or 3/0 aluminum GEC.
3. Apply the electrode caps in 250.66(A), (B), and (C)
The table result is the default, but three caps apply when the conductor is the sole connection to certain electrodes. Per 250.66(A), the sole connection to a rod, pipe, or plate electrode never has to be larger than 6 AWG copper (4 AWG aluminum). Per 250.66(B), the sole connection to a concrete-encased electrode — the Ufer — caps at 4 AWG copper. Per 250.66(C), the sole connection to a ground ring never has to exceed the ring conductor itself.
Read "sole connection" carefully. If one GEC runs from the service to the Ufer and continues on to a rod, the full table size applies up to the Ufer; only the tail beyond it, serving nothing but the rod, can drop to 6 AWG.
4. Build the electrode system — use everything that's there
NEC 250.50 says all electrodes described in 250.52(A) that are present must be bonded into one grounding electrode system — you don't get to pick your favorite. On a typical commercial building that means metal underground water pipe with 10 ft or more of earth contact, qualifying structural steel per 250.52(A)(2), and the Ufer: 20 ft of 1/2 in rebar or bare 4 AWG copper, in at least 2 in of concrete, in a footing or foundation in contact with earth. Ground rings, rods, pipes, and plates round out the list.
The exception to 250.50 is written for existing buildings — where the rebar isn't accessible without disturbing the concrete, the Ufer doesn't have to be used. Don't count on that pass for new construction: coordinate with the GC for a stub-up before the pour. A water-pipe electrode must always be supplemented per 250.53(D)(2). A single rod must be supplemented unless it measures 25 ohms or less per 250.53(A)(2) — in practice, drive two rods at least 6 ft apart and skip the test.
5. Size the bonding jumpers between electrodes
Bonding jumpers between electrodes are sized the same way as the GEC — per 250.66, as directed by 250.53(C) — with the same caps by electrode. The jumper to ground rods never needs to be larger than 6 AWG copper; the jumper to the Ufer caps at 4 AWG. Jumpers to building steel or the water pipe get the full Table 250.66 size, because those electrodes have no cap.
The water-pipe connection has a location rule: per 250.68(C)(1), connect within the first 5 ft of where the pipe enters the building, with a limited exception for qualifying industrial and commercial buildings where the piping stays exposed and maintained by qualified persons.
6. Route and protect the GEC per 250.64
A 6 AWG or larger copper GEC can run exposed where not subject to physical damage; 8 AWG needs raceway or cable armor. Anywhere it can get hit — the drop down a CMU wall in a warehouse aisle — protect it in raceway.
The GEC must be continuous without splice or joint, with narrow exceptions in 250.64(C): irreversible compression connectors listed as grounding and bonding equipment, exothermic welds, and busbar connections. Wirenuts and split bolts are not on that list. If you sleeve the GEC in ferrous raceway, 250.64(E) requires the raceway bonded to the GEC at both ends — an unbonded steel sleeve acts as a choke and raises impedance exactly when the conductor needs to work. PVC avoids the issue.
7. Make the connections per 250.70
Connections to electrodes must be made by exothermic welding, listed lugs, listed pressure connectors, listed clamps, or other listed means — solder alone is prohibited. Any clamp buried in earth or encased in concrete must be listed for that use; look for the "DB" marking on the clamp itself, not your memory of the brand.
One conductor per clamp unless the clamp is listed for more. Torque to the marked value or per the manufacturer — 110.14(D) makes marked torque mandatory. Keep connections accessible per 250.68(A); buried or encased connections are exempt, as are exothermic or irreversible compression connections to fire-proofed structural metal.
Watch Out
Common mistakes
- Sizing the GEC from the breaker rating — Table 250.66 keys off conductor area, and the two don't always track.
- Forgetting to sum circular mils on paralleled services, which understates the table entry and the GEC by a size or two.
- Running 6 AWG to the Ufer because "grounds are always #6" — the concrete-encased electrode cap is 4 AWG copper.
- Splicing the GEC with a split bolt or wirenut — 250.64(C) only allows irreversible compression connectors, exothermic welds, and busbars.
- Sleeving the GEC in steel raceway without bonding both ends per 250.64(E), which chokes the conductor exactly when it needs to work.
- Using a standard indoor clamp on a buried rod — buried or encased connections need a fitting listed for direct burial or concrete encasement.
- Connecting to interior water pipe 20 ft in from the wall — 250.68(C)(1) limits it to the first 5 ft inside the building in most occupancies.
FAQ
Frequently asked questions
What size GEC do I need for a 400 A commercial service?
It depends on the conductors, not the amperage. A 400 A service fed with 500 kcmil copper needs a 1/0 copper GEC per Table 250.66 (over 350 through 600 kcmil); fed with parallel 4/0 copper, the equivalent area is about 423 kcmil — also 1/0 copper.
Why does the wire to the ground rod only need to be 6 AWG on a big service?
NEC 250.66(A) caps the conductor at 6 AWG copper when it's the sole connection to a rod, pipe, or plate electrode. A rod's resistance to earth is high enough that a larger wire wouldn't move any more current, so the code doesn't make you pay for it.
Is one ground rod ever enough?
Only if it measures 25 ohms or less to earth per 250.53(A)(2). Most crews drive a second rod at least 6 feet from the first instead of testing, since two rods satisfy the rule with no measurement.
Can I splice a grounding electrode conductor?
Generally no — it must be continuous. NEC 250.64(C) allows irreversible compression connectors listed as grounding and bonding equipment, exothermic welds, and busbar connections; reversible connectors like split bolts are not permitted.
Does the GEC size change for a transformer inside the building?
A separately derived system uses the same Table 250.66 lookup, but you enter it with the derived system's largest ungrounded conductor per 250.30(A)(5) — so a transformer's GEC is sized from its secondary conductors, not the building service.
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