GEC Sizing (NEC 250.66)

Look up the minimum copper or aluminum grounding electrode conductor size from the largest service-entrance conductor using NEC 2023 Table 250.66, with the 250.66(A) to (C) electrode-specific limits.

Trades & Construction

Standard: NEC 2023 Table 250.66

Service Configuration

GEC Conductor Material

Largest Service Conductor (Cu size · Al equivalent)

Choose the row by the largest ungrounded service-entrance conductor (or the equivalent area of parallel sets). The material selector sets the material of the grounding electrode conductor itself.

250.66(A)

Rod, Pipe or Plate

6 AWG Cu / 4 AWG Al

Max required for the sole connection

250.66(B)

Concrete-Encased

4 AWG Cu

Max required for the sole connection

250.66(C)

Ground Ring

Ring Size

Not required larger than the ring conductor

A metal underground water pipe electrode has no reduced size in 250.66, so the full Table 250.66 size applies to a GEC terminating on it.

NEC Table 250.66 — Full Lookup (Copper GEC)

Cu 2 AWG or smaller · Al 1/0 or smaller8 AWG

Cu 1 AWG or 1/0 · Al 2/0 or 3/06 AWG

Cu 2/0 or 3/0 · Al 4/0 or 250 kcmil4 AWG

Cu Over 3/0 to 350 kcmil · Al Over 250 to 500 kcmil2 AWG

Cu Over 350 to 600 kcmil · Al Over 500 to 900 kcmil1/0

Cu Over 600 to 1100 kcmil · Al Over 900 to 1750 kcmil2/0

Cu Over 1100 kcmil · Al Over 1750 kcmil3/0

Minimum GEC Size

4 AWG

Copper GEC — NEC Table 250.66

Service Conductors

Cu 2/0 or 3/0

Aluminum equivalent: 4/0 or 250 kcmil

Copper GEC

4 AWG

Per table

Aluminum GEC

2 AWG

Per table

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Worked Example: Service With 3/0 Copper Conductors

Calculator default row, NEC 2023 Table 250.66

Design Scenario

A dwelling service uses 3/0 AWG copper service-entrance conductors. The grounding electrode system includes a metal underground water pipe, a concrete-encased electrode and a ground rod.

Design Parameters

Largest service conductor3/0 AWG Cu

Table 250.66 row2/0 or 3/0 Cu (4/0 or 250 kcmil Al)

GEC materialCopper

Mathematical Solution

1Full-size GEC from Table 250.66

Applies to the GEC run to the water pipe electrode.

Row 2/0 or 3/0 Cu -> 4 AWG Cu (2 AWG Al)

2Concrete-encased electrode, 250.66(B)

Same as the table value in this case.

Not required larger than 4 AWG Cu

3Rod electrode bonding jumper, 250.66(A)

Where the conductor is the sole connection to the rod.

Not required larger than 6 AWG Cu

Engineering Conclusion: A 4 AWG copper GEC to the water pipe satisfies Table 250.66 for this service. The jumper whose only purpose is to connect the ground rod may be 6 AWG copper. Confirm installation and protection requirements in 250.64 and any local amendments.

What is Grounding Electrode Conductor Sizing?

The grounding electrode conductor connects the service grounded conductor and equipment grounding system to the grounding electrode system. Its size is not based on load current; NEC Table 250.66 ties it to the size of the largest ungrounded service-entrance conductor.

Mathematical Foundation

Table lookup (not a formula)

GEC_{min} = f_{250.66}(A_{SE},\ \text{GEC material})

A_{SE}

= Size of the largest ungrounded service-entrance conductor, or the equivalent area of parallel conductors (AWG or kcmil)

\text{GEC material}

= Copper, or aluminum / copper-clad aluminum grounding electrode conductor

GEC_{min}

= Minimum GEC size from the matching row of Table 250.66

Laws & Principles

NEC 2023 Table 250.66 sets the minimum GEC size from the largest ungrounded service-entrance conductor or equivalent area of parallel conductors.
250.66(A): sole connection to rod, pipe or plate electrodes need not exceed 6 AWG copper or 4 AWG aluminum. 250.66(B): sole connection to a concrete-encased electrode need not exceed 4 AWG copper. 250.66(C): sole connection to a ground ring need not exceed the ring conductor size.
250.64(C) requires the GEC to be continuous, with splices permitted only by irreversible compression connectors, exothermic welding or connection to busbars.
250.64(A) restricts aluminum and copper-clad aluminum GECs from direct contact with masonry or earth and requires outdoor terminations at least 450 mm (18 in) above earth.

Step-by-Step Example Walkthrough

" A commercial service uses two parallel sets of 350 kcmil copper per phase. "

1. Equivalent area of the largest ungrounded conductor: 2 x 350 = 700 kcmil copper.
2. Table 250.66 row: over 600 through 1100 kcmil copper.
3. Minimum GEC from that row: 2/0 copper or 4/0 aluminum.

Final Result: A 2/0 copper GEC meets Table 250.66 for this service. Select the 'Over 600 to 1100 kcmil' row in the calculator to reproduce it.

Quick Answer

Use the largest ungrounded service-entrance conductor to find the row in NEC Table 250.66. A 2/0 or 3/0 copper service needs at least a 4 AWG copper or 2 AWG aluminum GEC. A conductor that is the sole connection to a ground rod need not exceed 6 AWG copper.

NEC 2023 Table 250.66 Summary

Largest service conductor, Cu	Largest service conductor, Al or CCA	Copper GEC	Al or CCA GEC
2 AWG or smaller	1/0 or smaller	8 AWG	6 AWG
1 or 1/0	2/0 or 3/0	6 AWG	4 AWG
2/0 or 3/0	4/0 or 250 kcmil	4 AWG	2 AWG
Over 3/0 through 350 kcmil	Over 250 through 500 kcmil	2 AWG	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

Summary for planning. Refer to the adopted edition of the NEC and its table notes for parallel conductors and other conditions.

Grounding Electrode Conductor vs. Equipment Grounding

This calculator determines the size of the Grounding Electrode Conductor (GEC) per NEC Table 250.66, which establishes the connection between the electrical service grounding system and earth electrodes (such as ground rods, concrete-encased Ufer electrodes, ground rings, or metal underground water pipes). GEC sizing is governed exclusively by the circular mil area of the largest ungrounded service-entrance conductors, not by the service overcurrent device rating.

In contrast, Equipment Grounding Conductors (EGC) are sized under NEC Table 250.122 based on the ampere rating of the upstream circuit breaker or fuse protecting the circuit. To evaluate circuit impedance and fault clearing characteristics along downstream branch runs, use our Ground Fault Loop Impedance Calculator.

Frequently Asked Questions

How do I size a grounding electrode conductor?

Find the largest ungrounded service-entrance conductor (or the equivalent area of parallel sets) in the left columns of NEC Table 250.66, then read the minimum GEC size for copper or aluminum in the same row. For a 3/0 copper service, the minimum copper GEC is 4 AWG.

When can the GEC be smaller than Table 250.66?

Under 250.66(A), the portion that is the sole connection to rod, pipe or plate electrodes is not required to be larger than 6 AWG copper or 4 AWG aluminum. Under 250.66(B), the sole connection to a concrete-encased electrode is not required to be larger than 4 AWG copper. Under 250.66(C), the sole connection to a ground ring is not required to be larger than the ring conductor.

What size GEC is needed for a metal water pipe electrode?

No reduced size applies to a metal underground water pipe electrode, so the conductor is sized from the full Table 250.66 row. NEC 250.53(D)(2) also requires the water pipe electrode to be supplemented by an additional electrode.

Can one GEC serve several electrodes?

Yes. A single GEC sized from Table 250.66 can run to any convenient electrode, with bonding jumpers interconnecting the electrodes to form the grounding electrode system. Under 250.53(C) those bonding jumpers are sized per 250.66, so a jumper whose only purpose is to connect a rod may use the 250.66(A) limit.

What restrictions apply to aluminum GECs?

NEC 250.64(A) does not permit bare, covered or insulated aluminum or copper-clad aluminum GECs in direct contact with masonry or earth or where subject to corrosive conditions. Outdoors, aluminum GEC terminations must be at least 450 mm (18 in) above the earth.

Is this the same as the equipment grounding conductor size?

No. Equipment grounding conductors for circuits are sized from the overcurrent device rating using Table 250.122. Table 250.66 applies to grounding electrode conductors and to some bonding jumpers that reference it.

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Calculation Provenance & Validation Record

Method

Direct lookup in NEC Table 250.66: the largest service-entrance conductor row gives the minimum copper or aluminum grounding electrode conductor, with 250.66(A) to (C) electrode-specific limits shown.

Formula

GEC_{min} = f_{250.66}(A_{SE},\ \text{Material})

Assumptions

AC service; Table 250.66 is entered with the largest ungrounded service-entrance conductor or the equivalent area for parallel sets.
The full Table 250.66 size applies unless the GEC is the sole connection to a rod, pipe or plate electrode, a concrete-encased electrode, or a ground ring (250.66(A) to (C)).
Installation rules for protection, splicing and routing (250.64) still apply and are not checked by the calculator.
Local amendments adopted by the authority having jurisdiction may modify these requirements.

References

NFPA 70, National Electrical Code (NEC 2023) Table 250.66 - grounding electrode conductor for alternating-current systems
NEC 2023 250.66(A), (B), (C) - connections to rod/pipe/plate, concrete-encased and ground-ring electrodes
NEC 2023 250.64 - grounding electrode conductor installation; 250.53(C) - bonding jumpers between electrodes

Last substantive review: 2026-10-03

Automated test status: 2 golden test vectors passing (GEC-01, GEC-02)

Method, assumptions & governing standards

Standard Sourced

Calculation Methodology

Governing Standard 2023

Standard:NFPA 70 (NEC) Table 250.66

Statutory building, electrical, and mechanical codes vary by jurisdiction. Confirm local municipality amendments before installation.

Key Assumptions & Constraints

AC service; Table 250.66 is entered with the largest ungrounded service-entrance conductor or the equivalent area for parallel sets.
The full Table 250.66 size applies unless the GEC is the sole connection to a rod, pipe or plate electrode, a concrete-encased electrode, or a ground ring (250.66(A) to (C)).
Installation rules for protection, splicing and routing (250.64) still apply and are not checked by the calculator.
Local amendments adopted by the authority having jurisdiction may modify these requirements.

Regression Tests: 2 golden vectors

Last Verified: 2026-10-03

Primary References: 3 documented

Field Trade Notice: For trade planning and engineering estimates. Final installations must conform to project blueprints, authority having jurisdiction (AHJ) code approvals, and site-specific inspections.