NEC Grounding Electrode Conductor Table: Sizing, Rules, and FAQ

The grounding electrode conductor (GEC) is the critical link between your electrical service equipment and the earth. Sizing this wire incorrectly can result in catastrophic failure during a lightning strike or utility line-to-ground fault. The definitive reference for this is NEC Table 250.66, officially titled the Grounding Electrode Conductor for Alternating-Current Systems.

Unlike branch circuit sizing, which is based on the breaker rating and continuous load, the GEC is sized strictly based on the physical cross-sectional area of your largest ungrounded service-entrance conductor. Below is the complete reference data, followed by installation rules and common code misconceptions.

How to Read NEC Table 250.66

To use the grounding electrode conductor table correctly, you must first identify the size and material of the largest ungrounded (hot) service-entrance wire feeding your main panel. If you have parallel sets of wires, you must sum the circular mil (kcmil) area of the conductors in one phase to find your starting row.

Which Column Applies to Your Installation?

The table is divided into two primary sections: Copper and Aluminum.

• Left Columns (Service Entrance Size): Locate the size of your main feed wires here. The table provides paired rows (e.g., "2 or smaller" paired with "1/0"). The top value is for copper service wires; the bottom value is for aluminum service wires.
• Right Columns (GEC Size): Once you find your service wire row, read across to the right. The top value is the required copper GEC size; the bottom value is the required aluminum GEC size.

The "Derating" Misconception

A frequent question from apprentices and DIYers is: "How do the derating rows modify the base value for ambient temperature or conduit fill?" The direct answer is: They don't, because Table 250.66 does not use ampacity derating. Unlike standard current-carrying conductors governed by NEC Table 310.16, a GEC sits idle at 0 amps under normal operation. It only carries massive, brief current during a fault or surge. Because continuous thermal heating is not a factor, you do not apply 90°C column adjustments or ambient temperature correction factors to the GEC. The values in Table 250.66 are absolute minimums. The only modification to these base values occurs if you invoke specific electrode exceptions (detailed in the FAQ below) or if you switch from copper to aluminum.

Complete NEC Table 250.66 Data

Source Standard: NFPA 70 National Electrical Code (NEC), Article 250.66. For further reading on grounding topology, refer to the NFPA 70 National Electrical Code documentation or technical breakdowns in Electrical Contractor Magazine (ECM).

Quick-Jump Sizing for Common Residential Services

For bookmarking and quick field reference, here are the most queried residential and light-commercial service sizes mapped directly to their copper GEC requirements.

• 100A Service (Typically #3 Cu or #1 Al feed): Requires 8 AWG Copper or 6 AWG Aluminum GEC.
• 150A Service (Typically #1 Cu or 2/0 Al feed): Requires 6 AWG Copper or 4 AWG Aluminum GEC.
• 200A Service (Typically 2/0 Cu or 4/0 Al feed): Requires 4 AWG Copper or 2 AWG Aluminum GEC.
• 300A/320A Service (Typically 350 kcmil Cu feed): Requires 2/0 AWG Copper or 3/0 AWG Aluminum GEC.
• 400A Service (Typically 600 kcmil Cu feed): Requires 3/0 AWG Copper or 250 kcmil Aluminum GEC. (This hits the maximum cap of the table).

Frequently Asked Questions (FAQ)

Does the grounding electrode conductor table apply to subpanels?

No. This is the most common error in residential wiring. Table 250.66 applies only to the main service disconnect where the connection to the earth (grounding electrode system) is made. For subpanels, feeders, and branch circuits, you must use NEC Table 250.122, which sizes the Equipment Grounding Conductor (EGC) based on the rating of the overcurrent protective device (the breaker), not the wire size. Furthermore, subpanels require isolated neutral bars and do not connect to local ground rods via a GEC.

Why is there a 4 AWG maximum exception for concrete-encased electrodes (Ufer)?

While Table 250.66 might dictate a 2/0 AWG copper GEC for a 400A service, NEC 250.66(B) provides a specific exception. If your only connection to the grounding electrode system is a concrete-encased electrode (Ufer ground), the GEC is not required to be larger than 4 AWG copper. This is because the massive surface area and conductivity of the rebar in the concrete foundation provide an impedance path low enough that a larger wire yields no measurable safety benefit. Note: If you also have a ground rod or water pipe in the system, the full Table 250.66 value applies to the main GEC run before it splits at the grounding bus.

Can I use the grounding electrode conductor table for a standby generator?

It depends on the generator's role. If the generator is a Separately Derived System (SDS) with a 4-pole transfer switch that switches the neutral, it requires its own grounding electrode system. In this case, you size the generator's GEC using Table 250.66 based on the size of the ungrounded conductors feeding the first overcurrent device from the generator. However, if it is a non-SDS (3-pole switch, solid neutral), the generator relies on the main service's GEC, and you only need to run an Equipment Grounding Conductor (EGC) sized via Table 250.122 alongside the feeder wires.

Does the GEC need to be insulated, or can it be bare?

The NEC allows the GEC to be either insulated (typically green or bare with green stripe) or completely bare. In practice, 95% of installations use bare copper wire because it is cheaper, easier to terminate at the ground bus, and eliminates the need to strip back thick insulation inside tight panel gutters. However, if the GEC is run underground directly in the earth without a raceway, some local inspectors prefer insulated wire to prevent accelerated galvanic corrosion from soil acids, though bare copper is technically code-compliant.