How do you size a grounding electrode conductor (GEC) under the NEC?

Under NEC 250.66, the grounding electrode conductor (GEC) is sized from the largest ungrounded service-entrance conductor or equivalent area for parallel sets. You find the total circular mil area of the largest ungrounded conductor(s), then use Table 250.66 to select the minimum GEC size. There are special rules for rod, pipe, and plate electrodes (not required to be larger than 6 AWG copper or 4 AWG aluminum) and for concrete-encased and structural steel electrodes.

Is the GEC ever required to be larger than 3/0 copper or 250 kcmil aluminum?

No. NEC 250.66(A)–(C) limit the maximum required GEC size. For concrete-encased and structural steel electrodes, the GEC is not required to be larger than 4 AWG copper. For rod, pipe, and plate electrodes, the GEC is not required to be larger than 6 AWG copper or 4 AWG aluminum. For other electrodes, Table 250.66 stops at 3/0 copper and 250 kcmil aluminum.

How do parallel service conductors affect GEC sizing?

For parallel sets, you add the circular mil area of the largest ungrounded conductor in each parallel set and use that total to size the GEC from Table 250.66. For example, three parallel 500 kcmil copper ungrounded conductors have a total of 1500 kcmil, which falls in the 1101–2000 kcmil range, so the minimum copper GEC is 2/0 AWG.

What is the difference between a GEC and an equipment grounding conductor (EGC)?

The grounding electrode conductor (GEC) connects the service equipment or separately derived system to the grounding electrode system (ground rods, concrete-encased electrode, building steel, etc.). The equipment grounding conductor (EGC) is the conductor that provides a low-impedance fault-return path with branch circuits and feeders. GEC sizing is covered in NEC 250.66, while EGC sizing is covered in NEC 250.122.

Does this calculator replace reading the NEC?

No. This calculator is a study and design aid based on common interpretations of NEC 250.66, but it does not replace the code book or local amendments. Always verify final conductor sizes against the current NEC edition adopted in your jurisdiction and consult the authority having jurisdiction (AHJ) when in doubt.

NEC Grounding Electrode Conductor (GEC) Sizing Calculator

Size the grounding electrode conductor (GEC) from the largest ungrounded service-entrance conductor or parallel sets using NEC 250.66 table logic. Supports copper and aluminum, AWG and kcmil.

This tool is for educational and design-assist purposes only and is not a substitute for the NEC, local amendments, or the authority having jurisdiction (AHJ).

GEC Sizing Calculator

System type

Electrode type (NEC 250.66(A)-(C))

GEC material

Number of parallel sets of ungrounded conductors

Use >1 when service conductors are run in parallel.

Largest ungrounded conductor in each parallel set

Conductor size

Unit

For AWG, enter the AWG number (e.g., 3, 2, 1, 0, 00=2, 000=3, etc.).

Ungrounded conductor material

NEC 250.66 Quick Reference Table (Simplified)

The table below summarizes the common ranges from NEC 250.66 for sizing the grounding electrode conductor from the largest ungrounded service-entrance conductor or equivalent area of parallel sets.

Total largest ungrounded conductor area (kcmil Cu or Al)	Min Cu GEC	Min Al GEC
≤ 2 AWG Cu or ≤ 1/0 AWG Al	8 AWG Cu	6 AWG Al
> 2 AWG to 1/0 AWG Cu, or > 1/0 to 3/0 AWG Al	6 AWG Cu	4 AWG Al
> 1/0 to 3/0 AWG Cu, or > 3/0 to 250 kcmil Al	4 AWG Cu	2 AWG Al
> 3/0 AWG to 350 kcmil Cu, or > 250 to 500 kcmil Al	2 AWG Cu	1/0 AWG Al
> 350 to 600 kcmil Cu, or > 500 to 900 kcmil Al	1/0 AWG Cu	3/0 AWG Al
> 600 to 1100 kcmil Cu, or > 900 to 1750 kcmil Al	2/0 AWG Cu	4/0 AWG Al
> 1100 kcmil Cu or > 1750 kcmil Al	3/0 AWG Cu (max)	250 kcmil Al (max)

This is a simplified representation for study purposes. Always verify against the actual NEC 250.66 table in the edition adopted in your jurisdiction.

How this GEC sizing calculator works

This calculator follows the logic of NEC 250.66 to size the grounding electrode conductor (GEC) based on the largest ungrounded service-entrance conductor or equivalent area of parallel sets.

1. Determine the largest ungrounded conductor area

For a single set of service-entrance conductors:

Identify the largest ungrounded conductor size (e.g., 3/0 AWG Cu, 500 kcmil Al).
Use its circular mil area to select the GEC size from NEC 250.66.

For parallel sets:

Find the circular mil area of the largest ungrounded conductor in one set.
Multiply by the number of parallel sets to get the total area.
Use that total to size the GEC from Table 250.66.

Parallel example

Service: 3 parallel sets of 500 kcmil Cu ungrounded conductors.

Area per conductor = 500 kcmil
Number of sets = 3
Total area = 500 × 3 = 1500 kcmil
From Table 250.66, 1101–2000 kcmil → minimum GEC = 2/0 AWG Cu

2. Apply NEC 250.66(A)–(C) electrode limitations

The code places upper limits on GEC size for certain electrode types:

Concrete-encased or structural steel electrodes (250.66(A), (B)):
- GEC is not required to be larger than 4 AWG copper.
Rod, pipe, or plate electrodes (250.66(A)):
- GEC is not required to be larger than 6 AWG copper or 4 AWG aluminum.
For other electrodes, the full Table 250.66 applies up to 3/0 Cu or 250 kcmil Al.

The calculator lets you select the electrode type and automatically caps the recommended GEC size according to these rules.

3. Copper vs. aluminum GEC

NEC 250.62 and 250.64 permit the GEC to be copper, aluminum, or copper-clad aluminum, subject to environmental and termination limitations. This tool provides a recommended minimum size for the selected material, but you must ensure:

The material is permitted in the installation environment (e.g., aluminum not in direct contact with earth).
Terminations are listed and identified for the conductor material and size.

Worked examples

Example 1 – 200 A residential service

Assume a 200 A, 120/240 V single-phase service with 3/0 AWG copper ungrounded service-entrance conductors, single set (no parallel).

Largest ungrounded conductor: 3/0 AWG Cu.
From NEC 250.66, 3/0 Cu falls in the “over 1/0 through 3/0 Cu” range.
Minimum GEC: 4 AWG copper.

Example 2 – 800 A service with parallel aluminum conductors

Service with three parallel sets of 500 kcmil aluminum ungrounded conductors.

Area per conductor = 500 kcmil Al.
Number of sets = 3 → total area = 1500 kcmil.
From NEC 250.66, 1500 kcmil falls in the 1101–2000 kcmil range.
Minimum GEC: 4/0 AWG aluminum (or 2/0 Cu if copper is used).

Example 3 – Rod electrode only

If the only electrode used is a ground rod, NEC 250.66(A) limits the GEC size:

Even for very large services, the GEC to the rod is not required to be larger than 6 AWG Cu or 4 AWG Al.
The calculator enforces this cap when you choose “Rod, pipe, or plate” as the electrode type.

Common pitfalls and best practices

Confusing GEC with EGC: GEC sizing is in 250.66; equipment grounding conductors (EGCs) are sized per 250.122 from the OCPD rating.
Ignoring parallel sets: For paralleled service conductors, you must use the total circular mil area, not just one conductor.
Over-sizing beyond code limits: For rods and concrete-encased electrodes, the code does not require GECs larger than the specified maximums.
Local amendments: Some jurisdictions may have stricter requirements; always check with the AHJ.

Disclaimer

This calculator is based on typical interpretations of NEC Article 250, especially 250.66 and 250.52–250.68. It is intended as a learning and preliminary design tool only. It does not replace:

The actual text of the NEC (including the specific edition adopted in your area), or
Local codes, amendments, or engineering judgment.

Always verify final conductor sizes and installation details with the current code, product listings, and the authority having jurisdiction.