For a standard 200-amp residential service using 2/0 AWG copper service entrance conductors, your grounding electrode conductor (GEC) must be 4 AWG copper. If you are using aluminum service conductors, the GEC sizing shifts based on equivalent circular mil area, requiring a 2 AWG copper GEC for a 4/0 AWG aluminum feeder. This guide provides the complete grounding electrode conductor size chart based on NEC Table 250.66, along with the exact decision path to finalize your wire pick without a second trip to the supply house.

How to Read the Grounding Electrode Conductor Size Chart

Unlike branch circuit sizing, which relies on 60°C, 75°C, or 90°C temperature rating columns to determine ampacity, GEC sizing ignores temperature columns entirely. A GEC only carries current during a fault event or lightning strike; it does not carry continuous load. Therefore, the NEC sizes the GEC strictly based on the circular mil area of the largest ungrounded service-entrance conductor (or the equivalent area of parallel conductor sets).

To use the chart below, identify the material (copper or aluminum) and the size of your largest ungrounded service feeder. Trace that row to the right to find the minimum required copper or aluminum GEC. If your service conductors fall between two rows, always round up to the next largest circular mil area.

Quick-Jump Reference for Common Services:
  • 100A Service (2 AWG Cu): Requires 8 AWG Cu GEC
  • 200A Service (2/0 AWG Cu): Requires 4 AWG Cu GEC
  • 400A Service (600 kcmil Cu): Requires 3/0 AWG Cu GEC

Complete NEC Table 250.66 Size Chart

The following data is sourced directly from NEC Table 250.66 (2023/2026 editions). This table dictates the minimum GEC size based on your service entrance conductors.

Largest Ungrounded Service Conductor (Copper) Largest Ungrounded Service Conductor (Aluminum) Minimum GEC Size (Copper) Minimum GEC Size (Aluminum)
2 AWG or smaller 1/0 AWG or smaller 8 AWG 6 AWG
1/0 AWG 3/0 AWG 6 AWG 4 AWG
2/0 AWG 4/0 AWG 4 AWG 2 AWG
3/0 AWG 250 kcmil 4 AWG 2 AWG
4/0 AWG 350 kcmil 2 AWG 1/0 AWG
250 kcmil 400 kcmil 1/0 AWG 2/0 AWG
350 kcmil 500 kcmil 1/0 AWG 2/0 AWG
400 kcmil 600 kcmil 2/0 AWG 3/0 AWG
500 kcmil 800 kcmil 3/0 AWG 4/0 AWG
600 kcmil 900 kcmil 3/0 AWG 4/0 AWG
800 kcmil 1000 kcmil 4/0 AWG 250 kcmil
1000 kcmil 1250 kcmil 4/0 AWG 250 kcmil
Over 1100 kcmil Over 1500 kcmil 250 kcmil 400 kcmil

Decision Path: Electrode Modifiers and 'Derating'

A common point of confusion is how derating rows modify the base GEC value. Standard ambient temperature and bundling derating factors (NEC 310.15) do not apply to GECs. You do not multiply the table value by a correction factor for a hot attic or conduit fill. Instead, the 'modifiers' that reduce your base table value are the electrode type maximum caps defined in NEC 250.66(A) and (B).

Use this decision tree to finalize your exact wire pick based on the specific grounding electrode you are terminating to:

Electrode Type Condition / Rule Final GEC Size Pick (Copper)
Metal Underground Water Pipe Must be fully sized per Table 250.66. No maximum cap applies. Use exact Table 250.66 value (e.g., 4 AWG for 200A)
Concrete-Encased (Ufer) NEC 250.66(B) caps the requirement. Larger wire provides no additional benefit. Maximum 4 AWG (even if table calls for 2/0)
Ground Rod (Driven) NEC 250.66(A) caps the requirement due to the high resistance of earth contact. Maximum 6 AWG (even if table calls for 2/0)
Building Steel Must be fully sized per Table 250.66. No maximum cap applies. Use exact Table 250.66 value
Multi-Electrode Systems: If your service is bonded to a ground rod AND a Ufer, the GEC must be sized for the electrode that requires the largest conductor. Since the Ufer requires a 4 AWG maximum and the ground rod requires a 6 AWG maximum, you must run a 4 AWG GEC to the Ufer, and you can then bond the ground rod with a 6 AWG bonding jumper.

What This Table Cannot Tell You

Table 250.66 only gives you the minimum wire gauge. It does not cover routing, physical protection, or termination mechanics. According to the Copper Development Association's grounding guidelines and NEC Article 250.64, you must account for the following field realities:

  • Physical Protection: If a GEC is exposed to physical damage (e.g., running down an exterior wall where it could be struck by a lawnmower or vehicle), it must be protected. A 6 AWG or larger GEC can be protected by schedule 80 PVC, RMC, or IMC. If the GEC is smaller than 6 AWG, it must be run in RMC, IMC, or EMT regardless of exposure.
  • Magnetic Enclosures: If you run an alternating-current GEC through a ferrous metal enclosure (like steel conduit), the magnetic field generated during a fault can choke the current, effectively turning the conduit into an inductor. You must bond the conduit to the GEC at both ends to cancel the magnetic field, or use non-magnetic PVC conduit.
  • Termination Hardware: The table assumes you are using listed mechanical clamps (like a brass or bronze acorn connector for ground rods) or exothermic welding (Cadweld). If you use an aluminum GEC, it cannot terminate directly to a ground rod or masonry; it must terminate to a copper wire via an irreversible compression connector or exothermic weld outside the earth.
  • Equipment Grounding Conductors (EGC): Do not confuse the GEC with the EGC. The GEC connects the panel to the earth. The EGC (sized per NEC Table 250.122) connects the panel to your appliances and outlets. They serve entirely different fault-clearing purposes.

Real-World Sizing Walkthroughs

Let's apply the chart and decision tree to two common jobsite scenarios to lock in the exact materials list.

Scenario 1: Standard 200A Residential Service

  • Service Conductors: 2/0 AWG Copper (THHN in conduit).
  • Base Table Value: Tracing 2/0 Cu on Table 250.66 yields a 4 AWG Copper GEC.
  • Electrode: Concrete-encased electrode (Ufer) and two 8-foot copper-bonded ground rods.
  • Modifier Check: The Ufer caps the requirement at 4 AWG. The ground rods cap at 6 AWG.
  • Final Pick: Pull a continuous 4 AWG bare copper wire from the panel ground bar to the Ufer rebar. From that same ground bar, run a 6 AWG bare copper wire to the two ground rods in series using listed acorn clamps.

Scenario 2: 400A Commercial Feeder

  • Service Conductors: 600 kcmil Aluminum (parallel sets, but we look at the largest single ungrounded conductor).
  • Base Table Value: Tracing 600 kcmil Al on Table 250.66 yields a 3/0 AWG Copper GEC (or 4/0 AWG Aluminum GEC).
  • Electrode: Metal underground water pipe and building steel.
  • Modifier Check: Neither water pipe nor building steel has a maximum cap. The full table value applies.
  • Final Pick: Use 3/0 AWG bare copper. Because it is terminating to a metal water pipe, the connection must be made within 5 feet of where the pipe enters the earth, and the water pipe must be bonded to the building steel per NEC 250.50.