The correct size of ground wire for 400 amp service is 3 AWG copper for the Equipment Grounding Conductor (EGC) paired with a 400A breaker. If you are routing a Grounding Electrode Conductor (GEC) to a ground rod or water pipe, you need 1/0 AWG copper. This assumes standard 75°C terminations, 30°C ambient temperature, and no voltage-drop upsizing on the feeder.
The Baseline: NEC Sizing and Core Assumptions
When electricians talk about the 'ground wire' for a service, they are usually referring to two distinct conductors governed by different tables in the National Electrical Code (NEC). Confusing the two is a common jobsite error.
• EGC (Equipment Grounding Conductor): Runs with your feeder wires to the subpanel. Clears faults by tripping the breaker.
• GEC (Grounding Electrode Conductor): Connects your panel's ground bus to the earth (ground rods, Ufer, water pipe). Stabilizes voltage to earth.
For the EGC, we look at NEC Table 250.122, which sizes the wire based on the rating of the Overcurrent Protective Device (OCPD)—in this case, your 400A main breaker.
| Parameter | Copper (Cu) | Aluminum (Al) |
|---|---|---|
| Minimum EGC Size | 3 AWG | 1 AWG |
| Temperature Column | 75°C (Standard for panel terminations) | |
| Insulation Type | THHN/THWN-2 (or bare copper) | |
Stated Assumptions for This Baseline
- Material: Copper (Aluminum is addressed in the variables section below).
- Termination Rating: 75°C column. Even if you use 90°C THHN wire, panel lugs are almost universally rated for 75°C per NEC 110.14(C).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit Type: Single set in PVC, RMC, or EMT. No more than 3 current-carrying conductors in the raceway.
The Voltage Drop Trap: Proportional Upsizing
The 3 AWG baseline holds true only if your ungrounded (hot) feeder conductors are the minimum size required for 400A. But what if your panel is 250 feet away from the utility transformer or upstream disconnect?
At 250 feet, a standard 600 kcmil copper feeder will experience roughly 3.5% voltage drop at full load. To keep the drop under the NEC-recommended 3% (per Informational Note 210.19(A)), you must upsize your hot conductors to 800 kcmil.
The Math for a 250-Foot Run:
- Base ungrounded conductor: 600 kcmil (600,000 circular mils).
- Upsized ungrounded conductor: 800 kcmil (800,000 circular mils).
- Ratio of increase: 800,000 / 600,000 = 1.333.
- Base 3 AWG EGC area (NEC Chapter 9, Table 8): 26,240 circular mils.
- Required upsized area: 26,240 × 1.333 = 34,977 circular mils.
Looking back at Chapter 9, Table 8, a 2 AWG wire is only 33,090 circular mils (too small). You must step up to 1 AWG copper (41,740 circular mils) to satisfy the code for this 250-foot run.
Decision Tree: Sizing Your 400A Ground Wire
Use this decision path to lock in your exact material pick. Do not deviate from the final column.
| Scenario Condition | NEC Rule Applied | Final EGC Pick (Buy This) |
|---|---|---|
| Run is under 100 ft; standard 600 kcmil hot wires. | Table 250.122 baseline. | 3 AWG Bare or Green THHN Copper |
| Run is 150–300 ft; hot wires upsized to 800 kcmil for voltage drop. | 250.122(B) proportional increase (33% bump). | 1 AWG Bare or Green THHN Copper |
| Run is over 300 ft; hot wires upsized to parallel sets (e.g., two sets of 350 kcmil). | 250.122(B) applied to equivalent circular mil area of parallel sets. | 1/0 AWG Copper (Verify math with AHJ) |
| Routing the GEC to a ground rod / Ufer ground (not the subpanel feeder). | Table 250.66 (based on largest ungrounded conductor). | 1/0 AWG Bare Copper (assuming 600 kcmil+ feeder) |
Why 3 AWG? The Physics of Fault Clearing
A common question from apprentices is: 'Why can't I just use 6 AWG or 8 AWG for the ground? It doesn't carry normal load current anyway.'
The ground wire doesn't carry load; it carries fault current. If a hot wire shorts to the metal panel chassis, the ground wire must carry hundreds or thousands of amps back to the source to create a magnetic field strong enough to instantly trip the 400A breaker's magnetic trip mechanism.
This is governed by the physics of let-through energy, known as I²t (Current squared × time). A 400A breaker might take a few milliseconds to clear a 20,000-amp dead-bolt fault. If you use a wire that is too small (like 6 AWG), the immense heat generated by the fault current will melt (fuse) the copper wire before the breaker trips.
If the ground wire vaporizes, the fault path is broken. The panel chassis remains energized at 240V, waiting for the next person to touch it and complete the circuit to earth. 3 AWG copper has the thermal mass to survive the fault long enough for the breaker to clear it safely.
Variables That Change the Sizing
The baseline assumes ideal conditions. Here is what forces you to change your wire size or material:
1. Aluminum Conductors
If you are using aluminum for your ungrounded feeders to save money, Table 250.122 allows a 1 AWG Aluminum EGC for a 400A breaker. However, do not mix aluminum EGCs in wet or corrosive environments. Aluminum is highly susceptible to galvanic corrosion when terminated to copper lugs or steel enclosures without proper anti-oxidant paste (like Noalox) and bi-metallic lugs. For ground wires, the minor cost savings of aluminum are rarely worth the termination risk; stick to copper.
2. High Ambient Temperatures
If your conduit runs through an attic in Arizona where ambient temperatures exceed 30°C (86°F), the insulation on your THHN ground wire must be derated per NEC Table 310.15(B)(1)(1). While the ground wire doesn't carry continuous load, its insulation must not melt during a fault. If the attic hits 50°C (122°F), you may need to upsize to 2 AWG copper simply to maintain the physical integrity of the insulation jacket under fault conditions, though the copper mass of 3 AWG is technically sufficient for the I²t calculation.
3. Bundling and Conduit Fill
Adjustment factors for more than three current-carrying conductors in a raceway (NEC 310.15(C)(1)) do not apply to the Equipment Grounding Conductor, because it carries zero current under normal operation. You do not need to upsize the EGC for conduit bundling derating.
When the AHJ or an Engineer Must Step In
While the rules above cover 95% of residential and light commercial 400A services, you must pull the permit and consult your local Authority Having Jurisdiction (AHJ) or a licensed Professional Engineer (PE) in these specific scenarios:
- Parallel Feeder Runs: If your 400A service is split into two parallel conduit runs (e.g., two sets of 3/0 AWG copper), NEC 250.122 requires an EGC in each raceway. The size of each EGC is based on the OCPD (400A), meaning you need a 3 AWG copper EGC in both pipes, not one 3 AWG split between them.
- Utility-Specific GEC Rules: Some local utility companies require a 2/0 AWG or 4/0 AWG bare copper Grounding Electrode Conductor for the service entrance, regardless of what NEC Table 250.66 dictates. The utility's service manual always supersedes baseline NEC for the line-side connection.
- High Fault Current Availability: If the utility transformer is pad-mounted directly next to your building, the available fault current might exceed the 10,000A or 22,000A interrupt rating of standard breakers. An engineer must calculate the specific I²t let-through to verify if 3 AWG is sufficient or if a larger EGC is required to handle the extreme thermal stress.
For a standard 400A service with a run under 100 feet, pull a spool of 3 AWG bare copper (or green THHN if local code requires insulated grounds in conduit), torque your lugs to the manufacturer's spec, and you will pass inspection.
