Grounding Electrode Conductor Sizing

Size the Grounding Electrode Conductor (GEC) from the largest service entrance conductor per NEC §250.66. Applies ground rod, concrete-encased (Ufer), and ground ring electrode caps.

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

The Grounding Electrode Conductor (GEC) connects the grounded electrical service conductor (neutral) and equipment grounding system directly to the earth via one or more grounding electrodes (NEC §250.50).

Critical Sizing Rules & Maximum Caps (§250.66):

  • Table 250.66 Base Sizing: Sized based on the cross-sectional area of the largest ungrounded service-entrance conductor (or equivalent area for parallel sets).
  • Ground Rod / Pipe / Plate Cap (§250.66(A)): Where the sole connection is to a driven ground rod, pipe, or plate electrode, the GEC is never required to be larger than #6 AWG Copper or #4 AWG Aluminum!
  • Concrete-Encased Electrode / Ufer Cap (§250.66(B)): Where the sole connection is to a concrete-encased electrode (rebar/copper in foundation footing), the GEC is never required to be larger than #4 AWG Copper.
  • Ground Ring Cap (§250.66(C)): Where the sole connection is to an encircling ground ring, the GEC is not required to exceed #2 AWG Copper.
  • Metal Underground Water Pipe & Structural Steel: Requires full Table 250.66 size (no cap applies).

Formulas & equations

Base GEC = Table 250.66 [Largest Service Conductor kcmil Area] Final GEC = MIN(Base GEC, Electrode Specific Cap per §250.66(A–C))
Electrode Caps Summary:
• Rod / Pipe / Plate (§250.66(A)): Max #6 Copper / #4 Aluminum
• Concrete-Encased Ufer (§250.66(B)): Max #4 Copper
• Ground Ring (§250.66(C)): Max #2 Copper

NEC reference table

Service Conductor Size (Copper) Service Conductor Size (Aluminum) Table 250.66 Copper GEC Table 250.66 Aluminum GEC
#2 AWG or smaller#1/0 AWG or smaller#8 AWG#6 AWG
#1 or #1/0 AWG#2/0 or #3/0 AWG#6 AWG#4 AWG
#2/0 or #3/0 AWG#4/0 AWG or 250 kcmil#4 AWG#2 AWG
Over 3/0 through 350 kcmilOver 250 through 500 kcmil#2 AWG#1/0 AWG
Over 350 through 600 kcmilOver 500 through 900 kcmil#1/0 AWG#3/0 AWG
Over 600 through 1100 kcmilOver 900 through 1750 kcmil#2/0 AWG#4/0 AWG
Over 1100 kcmilOver 1750 kcmil#3/0 AWG250 kcmil

*Source: NEC Table 250.66 (Grounding Electrode Conductor for Alternating-Current Systems).

Worked example, step by step

Example Calculation: 200A Service with Ground Rod & Ufer Electrode

Scenario: You are installing a 200 Amp residential service with #2/0 AWG Copper service entrance conductors connected to both two ground rods and a foundation Ufer rebar electrode.

  • Step 1 (Table 250.66 Lookup): For #2/0 Cu service entrance conductors, Table 250.66 dictates a base size of #4 AWG Copper.
  • Step 2 (Ground Rod GEC): Per §250.66(A), the conductor connecting to the ground rods is capped at #6 AWG Copper.
  • Step 3 (Ufer Electrode GEC): Per §250.66(B), the conductor connecting to the concrete-encased Ufer electrode is capped at #4 AWG Copper.

Frequently asked questions

Under NEC §250.53(A)(2), a single ground rod electrode must have an electrical resistance to earth of 25 ohms or less. If it does not (or if resistance is untested), a second supplemental ground rod must be installed at least 6 feet apart. In modern practice, electricians almost universally install two rods to avoid costly resistance testing.

Under NEC §250.64(C), a grounding electrode conductor must be installed in one continuous length without a splice, unless spliced by irreversible compression fittings (e.g. exothermic welding / Cadweld or listed irreversible hydraulic crimps) or connected to a copper grounding busbar.

Yes! Under NEC §250.52(A)(3), a concrete-encased electrode (Ufer ground) consisting of at least 20 feet of conductive bare copper wire (#4 AWG or larger) or steel reinforcing bar (rebar, 1/2" diameter minimum) encased in at least 2 inches of concrete in the footing is a required and premier grounding electrode.

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