When sizing a ground wire for a 400-amp service, the correct wire gauge depends entirely on whether you are installing an Equipment Grounding Conductor (EGC) or a Grounding Electrode Conductor (GEC). Confusing the two is the most common mistake on high-amperage residential and light commercial jobs.

The Direct Answer: Per NEC Table 250.122, a 400A Equipment Grounding Conductor (EGC) running to a subpanel requires 3 AWG Copper or 1 AWG Aluminum. Per NEC Table 250.66, a 400A service Grounding Electrode Conductor (GEC) connecting to earth (assuming standard 600 kcmil Cu hot wires) requires 1/0 AWG Copper.

Below is the complete reference data to ensure your installation passes inspection and safely clears fault currents.

The 400 Amp Ground Wire Size Chart (NEC 250.122 & 250.66)

This table maps the NFPA 70 National Electrical Code (NEC) requirements for both grounding systems across common high-amperage service ratings. The 400-amp row is highlighted for quick reference.

Breaker Size (OCPD) Typical Hot Wire (Cu, 75°C) EGC Size (Copper) EGC Size (Aluminum) GEC Size (Copper)*
200 Amps 2/0 AWG 6 AWG 4 AWG 4 AWG
300 Amps 350 kcmil 4 AWG 2 AWG 2 AWG
400 Amps 600 kcmil 3 AWG 1 AWG 1/0 AWG
500 Amps 800 kcmil 2 AWG 1/0 AWG 2/0 AWG
600 Amps 1000 kcmil 1 AWG 2/0 AWG 2/0 AWG
How to Read This Table: The EGC columns are derived directly from the Overcurrent Protective Device (OCPD) rating (your main breaker). The GEC column, however, is derived from the circular mil area of your ungrounded (hot) conductors. The GEC sizes above assume you are using standard 75°C rated copper hot wires sized exactly to the breaker's ampacity. If your hot wires are aluminum, or if you are running parallel sets, your GEC size will change (explained below).

Which Column Applies to Your Installation?

To use the chart correctly, you must identify which physical wire you are pulling. The NEC treats these two conductors as entirely different systems with different jobs.

Equipment Grounding Conductor (EGC) - Table 250.122

The EGC runs with your hot and neutral wires inside the conduit or cable assembly from the main panel to a subpanel, or from a breaker to a heavy appliance. Its sole purpose is to provide a low-impedance fault-current path back to the source to trip the breaker during a short circuit. If you are wiring a 400A subpanel feeder, you need the 3 AWG Copper EGC.

Grounding Electrode Conductor (GEC) - Table 250.66

The GEC connects your service panel's neutral/ground bus to the physical earth (ground rods, metal water pipes, or a concrete-encased Ufer ground). It does not carry fault current from internal short circuits; it stabilizes voltage during lightning strikes and line surges. If you are bonding a 400A meter main to a ground rod, you need the 1/0 AWG Copper GEC.

Temperature Column Rule: Per NEC 110.14(C), even if you pull 90°C THHN wire in conduit, you must size your conductors based on the 75°C ampacity column because the lugs inside standard 400A panels and disconnects are only rated for 75°C. Always use the 75°C column for your baseline hot wire sizing, which in turn dictates your GEC size.

How Derating and Parallel Runs Modify the Base Value

The chart above provides baseline values, but real-world jobsite conditions frequently force you to upsize your wires. When you upsize your current-carrying conductors, the NEC requires specific adjustments to your grounding wires.

Proportional EGC Upsizing (NEC 250.122(B))

If your 400A feeder run is long enough that you must upsize the hot wires to mitigate voltage drop, you must proportionally upsize the EGC. The EGC is not automatically protected by the breaker's trip curve if the wire resistance is too high.

Worked Example:
Let's say your 400A feeder requires 600 kcmil Cu (600,000 circular mils) and a 3 AWG Cu EGC (26,240 circular mils). Due to a 250-foot run, you upsize the hot wires to 750 kcmil Cu (750,000 circular mils) to keep voltage drop under 3%.

  • Calculate the multiplier: 750,000 / 600,000 = 1.25
  • Multiply the base EGC area: 26,240 cmil × 1.25 = 32,800 cmil
  • Select the next standard size up: 2 AWG Copper (33,620 cmil)

In this scenario, your 400A EGC upgrades from 3 AWG to 2 AWG.

Parallel Conductor GEC Sizing (NEC 250.66)

For a 400A service, bending 600 kcmil solid copper is a nightmare. Most electricians use parallel runs (e.g., two sets of 3/0 AWG or 4/0 AWG copper). To find the GEC size for parallel feeds, you must add the circular mils of the parallel conductors in each phase together before consulting Table 250.66.

If you use two parallel sets of 3/0 AWG Cu per phase (167,800 cmil × 2 = 335,600 cmil), you fall into the 'Over 250 kcmil through 350 kcmil' row on Table 250.66, requiring a 1 AWG Copper GEC. If you use two sets of 4/0 AWG Cu (211,600 × 2 = 423,200 cmil), you jump to the 'Over 350 kcmil through 600 kcmil' row, requiring the standard 1/0 AWG Copper GEC.

What the Table Cannot Tell You (Edge Cases & Code Caveats)

Reference charts assume standard installations. According to EC&M's National Electrical Code analysis, local Authorities Having Jurisdiction (AHJ) and specific electrode types introduce critical exceptions that override the baseline table values.

Safety & Code Caveat: The NEC is a baseline standard. Your local electrical inspector (AHJ) has final authority. Always verify local amendments before purchasing expensive 1/0 or 2/0 copper grounding wire, as some municipalities mandate a flat 2/0 Cu GEC for all 400A services regardless of hot wire sizing.

The Ufer Ground Exception (NEC 250.66(B))

If your 400A service is grounding to a Concrete-Encased Electrode (commonly called a Ufer ground—a bare copper wire buried in the concrete footing), the NEC caps the required GEC size. Regardless of how massive your hot wires are, the GEC connecting to a Ufer ground never needs to be larger than 4 AWG Copper. If an inspector tries to fail you for using 4 AWG on a 400A Ufer connection, politely point them to 250.66(B).

The Ground Rod Exception (NEC 250.66(A))

Similarly, if you are connecting exclusively to a ground rod or a ground ring, the GEC is capped at 6 AWG Copper. However, a 400A service almost always requires a Ufer or metal water pipe bond as the primary electrode, with ground rods used as supplementary. The GEC must be sized for the largest electrode requirement present, which usually defaults back to the 1/0 AWG requirement for the water pipe or Ufer (if the Ufer exception isn't utilized for the main bonding jumper).

Aluminum GEC Restrictions

While Table 250.66 permits aluminum GECs (which would be 3/0 AWG for a 400A service with 600 kcmil hot wires), NEC 250.64(A) strictly forbids using aluminum or copper-clad aluminum GECs within 18 inches of the earth, in masonry, or where subject to corrosive conditions. Because the GEC ultimately terminates at a ground rod or water pipe in the dirt, always use bare copper for the GEC to avoid inspection failures and galvanic corrosion.