Grounding Electrode Conductor Sizing Guide

What a Grounding Electrode Conductor Is — and What It Is Not

Every grounded electrical system needs a connection between the grounded conductor (usually the service neutral) and earth. The grounding electrode conductor is the wire (or bus) that makes that connection from the grounding electrode system — rods, plates, concrete-encased conductors, metal water pipe, building steel, or a ground ring — back to the grounded conductor at the service equipment or separately derived system source.

That is fundamentally different from the equipment grounding conductor (EGC). The EGC runs with circuit conductors in raceways and cables. It bonds metal enclosures, devices, and frames together and provides a path for fault current to return to the source so breakers and fuses can operate. Under normal operation, neither the GEC nor the EGC carries load current. Under a ground fault, the EGC carries fault current; the GEC is not sized for that role.

Inspectors frequently cite installations where the branch circuit ground was upsized but the service GEC was left at an old 6 AWG while the service entrance conductors were upgraded to 2/0 or larger. Table 250.66 ties GEC size directly to the largest ungrounded service conductor — when the service grows, the GEC must be revisited.

How to Read NEC Table 250.66

Table 250.66 has separate input columns for copper and aluminum (or copper-clad aluminum) ungrounded conductors, plus output columns for copper and aluminum GEC. First identify both the size and material of the largest ungrounded conductor or equivalent parallel set, then read the desired GEC material column.

The table is not based on service amperage directly. A 200A service with 4/0 AWG copper entrance conductors requires a larger GEC than a 200A service with 3/0 AWG conductors, because the table keys off conductor size, not breaker rating. When service conductors are installed in parallel, NEC 250.66 uses the equivalent total circular mil area of one conductor in the set as if it were a single conductor.

Worked example: 200A residential service

Largest ungrounded conductors: 4/0 AWG copper. Table row: over 3/0 through 350 kcmil copper → 2 AWG copper GEC (or 1/0 aluminum GEC). If the conductor is connected solely to rod, pipe, or plate electrodes, 250.66(A) means it is not required to be larger than 6 AWG copper or 4 AWG aluminum.

Electrode-Type Size Caps — 250.66(A), (B), and (C)

250.66(A) — Rod, pipe, and plate electrodes

For rod, pipe, and plate electrodes, the GEC shall not be required to be larger than 6 AWG copper or 4 AWG aluminum. Physical reality drives this rule: a ground rod or driven pipe electrode has limited contact area with soil. Running 2/0 copper to a single rod does not meaningfully lower grounding resistance compared to 6 AWG. When Table 250.66 yields 4 AWG or 2 AWG, the cap still allows those sizes if smaller than the cap — the cap is a maximum, not a minimum. When the table demands 1/0 or larger, the cap pulls the size back down to 6 AWG copper or 4 AWG aluminum for the rod connection.

250.66(B) — Concrete-encased electrodes (Ufer grounds)

Concrete-encased electrodes use a minimum 20 feet of bare copper in the foundation footing or foundation. The GEC to a concrete-encased electrode shall not be required to be larger than 4 AWG copper. Ufer grounds are common in new construction in many jurisdictions because they are cast into the foundation — the cap reflects the effective contact area of the encased conductor. The section text addresses copper specifically; aluminum GEC terminations to Ufer electrodes require careful attention to listing and local practice.

250.66(C) — Ground ring electrodes

A conductor connected solely to a ground ring is not required to be larger than the conductor used for the ring. A ground ring electrode itself uses at least 20 feet of bare copper not smaller than 2 AWG and is installed in accordance with 250.53(F). Do not turn this rule into a generic 1/0 aluminum cap; verify the ring conductor and connection materials.

Common Code Violations

Undersized GEC after service upgrade. The most frequent violation: new 200A or 320A service conductors with an original 6 AWG GEC left in place. Always recalculate Table 250.66 when ungrounded conductors change.

Wrong material for the run. Aluminum GEC is permitted when sized per the aluminum column, but aluminum may not be installed within 18 inches of earth unless the termination is listed for direct burial in contact with the earth. Many rod clamps are copper-only. Using aluminum GEC in a buried run to a rod without listed fittings is a common fail.

Confusing GEC with EGC. Running a 6 AWG green wire from a subpanel as the only connection to a ground rod is not a substitute for a properly sized service GEC. Subpanel grounds return to the main bonding point; electrode connections originate at the service.

Unlisted connections at the electrode. NEC 250.70 requires listed connectors for GEC terminations to rods, pipes, and plates. Field-fabricated wraps, solder-only joints, or non-listed clamps fail inspection even when wire size is correct.

Connection Requirements at the Electrode — NEC 250.70

Grounding electrode conductor connections must be made with listed pressure connectors, listed clamps, exothermic welding, or other listed means. Connections to rods, pipes, and plates must be accessible unless buried or encased in concrete. Where anti-oxidant compound is required for aluminum, omitting it produces a high-resistance joint that may pass a visual inspection but fail under fault conditions.

Torque matters. Manufacturer specs for panel lugs and ground rod clamps specify ft-lb values. Under-torqued lugs heat and loosen; over-torqued lugs crack conductors. After sizing the GEC, verify the termination is listed for that conductor size and material, torque to spec, and leave the connection accessible for future inspection where the code requires it.

Pair GEC work with wire ampacity and derating for ungrounded conductors, panel schedule documentation for service equipment layout, and residential load calculations to confirm service conductor sizing before you look up Table 250.66.

FAQ

Does a 200A service always need 6 AWG GEC?

No. Breaker rating does not determine GEC size — ungrounded conductor size and material do. A service using 4/0 AWG copper ungrounded conductors requires a 2 AWG copper GEC per the table before electrode-specific limits. A service using 2/0 AWG copper ungrounded conductors requires a 4 AWG copper GEC. Always use the actual conductor material and size.

Can I use the same wire for GEC and equipment grounding?

At the service, the main bonding jumper connects neutral to the enclosure and grounding electrode system — it is not the same as branch EGCs. Branch equipment grounding conductors are sized per 250.122 based on overcurrent device rating. The GEC is sized per 250.66 based on service conductor size. Different rules, different purposes.

Do I need two ground rods?

NEC 250.53(A)(2) may require a supplemental electrode if a single rod measures more than 25 ohms to earth — but many jurisdictions treat the second rod as standard practice regardless. GEC size to rod electrodes is still capped at 6 AWG copper under 250.66(A) even when two rods are installed.

What about bonding gas pipe or water pipe?

Bonding jumpers to interior metal piping systems are governed by NEC 250.104 — separate from GEC sizing in 250.66. The GEC connects the electrode system to the grounded conductor; bonding jumpers connect other metal systems to the grounding electrode system. Both must be correct, but they are not sized from the same table row.