Sizing the Grounding Electrode Conductor — Table 250.66 Explained

The grounding electrode conductor (GEC) is the wire that ties your service grounded conductor (or a separately derived system's neutral) to the grounding electrode system — ground rods, concrete-encased electrodes, ground rings, metal water pipes. Size it from Table 250.66 based on the largest service-entrance conductor, then apply the three "max-out" exceptions in 250.66(A), (B), and (C).

What the GEC is — and isn't

From Article 100: the grounding electrode conductor is "a conductor used to connect the system grounded conductor or the equipment to a grounding electrode or to a point on the grounding electrode system." Two roles to keep distinct:

• GEC connects the service or SDS to the earth (the grounding electrode system). Sized per Table 250.66.
• EGC (equipment grounding conductor) connects equipment frames back to the source so a fault has a low-impedance return path. Sized per Table 250.122 based on the OCPD.

Different conductors, different tables, different jobs. Don't confuse them on the exam.

Table 250.66 — the basic table

Table 250.66 sizes the GEC based on the size of the largest ungrounded service-entrance conductor:

For parallel conductor sets, you size by the equivalent conductor area. Two parallel sets of 250 kcmil Cu = 500 kcmil equivalent → fall in the "350 to 600 kcmil" row → 1/0 Cu GEC.

The three max-out exceptions — 250.66(A), (B), and (C)

These three rules CAP the GEC size for specific electrode types. Even if Table 250.66 says you need a 2/0 conductor based on a giant 800 A service, you may not need to run that much copper to a single ground rod.

250.66(A) — connection to ground rods, pipes, plates

If the GEC's sole connection is to a rod, pipe, or plate electrode, it doesn't have to be larger than:

• 6 AWG copper, OR
• 4 AWG aluminum (cannot be installed within 18 inches of earth or where subject to corrosion)

"Sole connection" is the key phrase. If your GEC continues from the rod over to a concrete-encased electrode or a water pipe, the 6 AWG cap doesn't apply to that whole run — only to the section that's the sole tie to the rod. In practice, many installers run 6 AWG Cu from the service neutral all the way to the ground rod and stop there.

250.66(B) — concrete-encased electrode (Ufer ground)

If the GEC's sole connection is to a concrete-encased electrode (per 250.52(A)(3)), it doesn't need to be larger than 4 AWG copper. The concrete-encased electrode itself must be ≥ 20 ft of bare 4 AWG Cu or ≥ 20 ft of ½-in. or larger rebar, encased in ≥ 2 in. of concrete in direct contact with earth.

250.66(C) — ground ring

If the GEC's sole connection is to a ground ring (per 250.52(A)(4) — at least 20 ft of bare copper conductor not smaller than 2 AWG buried at least 30 in. below grade), it doesn't need to be larger than the ring conductor itself. So if the ring is 2 AWG Cu, the GEC can be 2 AWG Cu — no bigger.

The "largest service-entrance conductor" question — what counts

Three subtleties show up on the exam:

• Parallel sets count as one equivalent conductor. Three parallel sets of 250 kcmil Cu = 750 kcmil equivalent.
• SDS (separately derived system): per 250.30(A)(4), the GEC is sized using Table 250.66 based on the largest derived ungrounded conductor (the secondary phase conductor of a transformer, for example).
• The grounded conductor matters too. Even if the ungrounded conductors are huge, if the grounded service conductor is reduced (per 250.24(C)(1)), the table still applies based on the ungrounded conductor size.

Worked example 1 — 200 A residential service, 4/0 Al SE

• Service: 200 A, single-phase 120/240 V.
• Service conductors: 4/0 aluminum SE cable.
• Grounding electrode system: one 8-ft ground rod plus a metal underground water pipe.

Find 4/0 Al in Table 250.66 — falls in the "2/0 or 3/0 Cu / 4/0 or 250 kcmil Al" row → 4 AWG Cu (or 2 AWG Al) GEC required by the basic table.

To the metal water pipe: run the full 4 AWG Cu (no max-out applies — the water pipe is not in 250.66(A)/(B)/(C)).

To the ground rod: per 250.66(A), the portion that's the sole connection to the rod doesn't have to be larger than 6 AWG Cu. Many installers run a 4 AWG Cu directly to the rod anyway — code allows the smaller, but the bigger isn't wrong.

Final: 4 AWG Cu from the service neutral to the water pipe (the primary GEC), with a 6 AWG Cu bonding jumper from the water pipe (or directly from the GEC bus) to the ground rod.

Worked example 2 — 400 A commercial service, 500 kcmil Cu

• Service: 400 A, three-phase 208 V.
• Ungrounded service conductors: 500 kcmil copper.
• Electrodes: concrete-encased electrode (Ufer) + two 8-ft ground rods.

Find 500 kcmil Cu in Table 250.66 — falls in the "350 to 600 kcmil Cu" row → 1/0 Cu GEC.

To the concrete-encased electrode: per 250.66(B), the sole-connection portion doesn't have to be larger than 4 AWG Cu. Run 4 AWG Cu to the Ufer.

To each ground rod: 6 AWG Cu suffices (250.66(A)).

Final: a 1/0 Cu primary GEC tying the service neutral to a grounding-electrode bus, with a 4 AWG Cu tap to the Ufer and 6 AWG Cu taps to each rod. (Or run 1/0 Cu to the Ufer and tap off — the bigger conductor isn't a violation.)

Worked example 3 — parallel sets, 800 A service

• Service: 800 A, three-phase 480 V.
• Ungrounded conductors: 2 parallel sets of 600 kcmil Cu per phase.
• Electrodes: ground ring (2 AWG bare Cu, 30 in. deep) + Ufer.

Equivalent conductor area = 2 × 600 = 1200 kcmil Cu. Table 250.66: "over 1100 kcmil Cu" row → 3/0 Cu GEC.

To the ground ring: per 250.66(C), the sole-connection portion doesn't have to be larger than the ring conductor itself, which is 2 AWG. Run 2 AWG Cu to the ring (and bond to the ring with the same).

To the Ufer: per 250.66(B), 4 AWG Cu suffices.

Final: 3/0 Cu primary GEC → grounding-electrode bus → 2 AWG Cu to the ring, 4 AWG Cu to the Ufer.

Common exam traps

• Confusing 250.66 (GEC) with 250.122 (EGC). If the question is about the wire to a ground rod or water pipe, use 250.66. If it's about a wire that runs with the circuit conductors back to the source, use 250.122.
• Forgetting the sole-connection language. The 6/4/ring rules apply only to the part of the GEC that only ties to that specific electrode. The portion of GEC that continues on to another electrode, or that splits at a bus, has to be the full Table 250.66 size.
• Picking a copper-only answer when aluminum is in play. Aluminum GEC is one trade size larger than copper at every row of Table 250.66. The exam likes to give you choices in both materials and watch you pick the wrong column.
• Service neutral reduction. The grounded service conductor sizing rule in 250.24(C)(1) is independent of the GEC sizing rule. Don't size the GEC by the reduced grounded conductor — use the largest ungrounded.

Related code sections

• 250.52 — Grounding Electrodes (which types are allowed and what each must be)
• 250.53 — Grounding Electrode Installation (rod spacing, depth, supplement requirement)
• 250.62 — Grounding Electrode Conductor material (Cu, Al, Cu-clad)
• 250.64 — GEC installation (continuous, protected from physical damage, where it can be run)
• 250.68 — connection to grounding electrodes (clamps, exothermic, accessibility)
• 250.66 — sizing (this article)