The correct ground size for 100 amp feeders and branch circuits is 8 AWG copper or 6 AWG aluminum. This is the absolute minimum equipment grounding conductor (EGC) size required to safely clear a fault on a 100A overcurrent device, based on NEC Table 250.122. If you are running a standard 100A subpanel feeder using 3 AWG copper or 1 AWG aluminum hot wires, 8 AWG copper is your default ground wire.
However, simply pulling an 8 AWG wire and calling it a day can be dangerous if your feeder run is long enough to require upsized hot wires for voltage drop. The National Electrical Code (NEC) provides baseline guidance, but your local Authority Having Jurisdiction (AHJ) has the final say on compliance. Below is the complete technical breakdown of why this specific gauge is required, the physics of what happens when you get it wrong, and the exact decision path for sizing your ground.
The Baseline: Minimum Ground Size for 100 Amp
For a standard installation where the ungrounded (hot) conductors are sized exactly to the 75°C ampacity column of NEC Table 310.16, the EGC sizing is straightforward. The ground wire does not carry current during normal operation; it exists solely as an emergency fault path.
| Overcurrent Device Rating | Minimum Copper EGC | Minimum Aluminum EGC | Typical Hot Wire Size (75°C) |
|---|---|---|---|
| 100 Amps | 8 AWG | 6 AWG | 3 AWG Cu / 1 AWG Al |
The Hazard: What Happens When the EGC is Undersized?
To understand why 8 AWG is the hard floor for a 100A circuit, you have to look at the physics of a ground fault and the magnetic trip curve of a circuit breaker. The hazard an undersized ground prevents is chassis energization and subsequent lethal shock or electrical fire.
Imagine a scenario in your subpanel where a loose 120V hot wire falls and touches the metal chassis of the panel. The breaker must trip instantly to protect anyone who might touch the panel cover. A standard 100A thermal-magnetic breaker relies on its magnetic trip mechanism to clear a short circuit in under 0.1 seconds. To trigger this magnetic trip, the fault current must typically reach 5 to 10 times the breaker rating—meaning you need 500A to 1000A of instantaneous current flow.
Using Ohm’s Law ($I = V / R$), we can see why wire gauge matters:
- With an 8 AWG Copper Ground: The resistance of the fault loop is low enough to allow 800A+ of current to rush back to the source. The breaker trips in milliseconds. The chassis is safe.
- With an Undersized 14 AWG Copper Ground: The resistance is significantly higher. The fault current might only reach 200A. This is enough to heat the 14 AWG wire to its melting point, but not enough to trigger the 100A breaker's instantaneous magnetic trip. The breaker holds, the ground wire vaporizes inside the conduit, and the metal panel chassis remains energized at 120V indefinitely.
This is why the NEC scales ground wire sizes with the overcurrent device. A 100A breaker requires a massive fault current to trip, which demands a thick, low-resistance emergency highway (the 8 AWG EGC) to deliver that current.
Ground vs. Neutral vs. Bond: Clearing the Confusion
Misidentifying the neutral and the ground is one of the most common and dangerous mistakes in DIY subpanel wiring. They are fundamentally different conductors with different jobs.
- Neutral (Grounded Conductor): This is a current-carrying conductor. It completes the 120V circuit and carries the unbalanced return current back to the transformer during normal operation. It is insulated (white or gray) and must be sized to carry the maximum unbalanced load.
- Ground (Equipment Grounding Conductor / EGC): This is a non-current-carrying conductor under normal conditions. It connects all non-current-carrying metal parts (panel chassis, tool housings, appliance frames) back to the source. It only carries current during a fault.
- Bonding: This is the physical connection between the neutral and the ground. In a residential system, the neutral and ground are bonded only at the main service disconnect. In a 100A subpanel, the neutral bar and ground bar must be physically isolated. If you bond them at the subpanel, normal neutral return current will flow on the ground wire, energizing the chassis of every appliance on that subpanel.
The Voltage Drop Exception: When 8 AWG Isn't Enough
The 8 AWG copper rule assumes your hot wires are sized exactly for 100A. But if you are running a 100A feeder to a detached garage 150 feet away, you must upsize the hot wires to prevent excessive voltage drop. When you upsize the hot wires, NEC 250.122(B) requires you to proportionally upsize the ground wire.
If you fail to upsize the ground, the increased resistance of the longer run will choke the fault current, preventing the breaker from tripping instantly.
Decision Path: Sizing the Ground for Long Runs
Use this decision tree to determine your final ground size based on your hot wire selection.
| Scenario | Hot Wire Used | Circular Mil Ratio | Required Ground Size |
|---|---|---|---|
| Standard Run (< 50 ft) | 1 AWG Aluminum (66,360 cmil) | 1.0 (Baseline) | 6 AWG Aluminum |
| Long Run (~150 ft) | 1/0 Aluminum (105,600 cmil) | 105,600 / 66,360 = 1.59 | 2 AWG Aluminum (26,240 cmil x 1.59 = 41,721 cmil; next size up) |
| Very Long Run (~250 ft) | 2/0 Aluminum (133,100 cmil) | 133,100 / 66,360 = 2.00 | 1/0 Aluminum (26,240 cmil x 2.0 = 52,480 cmil; next size up) |
Note: Always calculate the ratio using the circular mil area of the conductors, not the AWG gauge numbers. Refer to NEC Chapter 9, Table 8 for exact circular mil values.
How to Verify Your Grounding Path
You cannot assume a ground is effective just because the wire is connected to the bar. A loose lug or a broken strand can increase resistance enough to defeat the fault-clearing mechanism. Follow these numbered steps to verify the 100A feeder ground.
- De-energize and Lock Out: Turn off the main breaker feeding the 100A circuit. Verify the panel is dead using a non-contact voltage tester and a multimeter set to AC voltage across the hot bus bars.
- Visual Gauge Inspection: Check the printing on the THHN/THWN-2 jacket of the EGC. Confirm it reads '8 AWG' (or your upsized equivalent). Look for nicks or cuts in the insulation that might expose the wire to corrosion.
- Torque Verification: This is the most skipped step. Use a calibrated torque screwdriver (like the Klein Tools 60175) to tighten the ground bar set screws. For a standard Square D Homeline or QO ground bar, the required torque is typically 20 in-lbs for 14-4 AWG conductors. Check the panel label for the exact specification. Under-torqued lugs loosen under thermal cycling, creating high-resistance faults.
- Continuity Testing: With the circuit still de-energized, set your multimeter to the continuity or low-ohms setting. Place one probe on the subpanel's ground bar and the other on the ground pin of a downstream 15A or 20A receptacle. You should read less than 1.0 ohm. A reading of 'OL' (open loop) means the ground path is broken.
- Branch Circuit Verification: Once re-energized, use a plug-in receptacle tester with a GFCI test button (such as the Klein Tools RT250) on all downstream outlets to verify the ground is present and the hot/neutral are not reversed.
When a Licensed Electrician is Required
While wiring a subpanel feeder is a common advanced DIY project, certain boundaries require a licensed professional. You must hire a licensed electrician and pull a permit if your 100A circuit involves:
- Service Entrance Conductors: If this 100A feed is coming directly from the utility meter base to your main service panel, it is governed by utility rules and strict service entrance codes. The utility company owns the meter, and tampering with the service mast or meter seal is illegal and highly dangerous.
- Grounding Electrode System (GES): Driving ground rods, connecting to a ufer ground (concrete-encased electrode), or bonding to the municipal water pipe requires specific sizing (typically 4 AWG bare copper for a 100A service per NEC Table 250.66) and must be inspected by the AHJ.
- AHJ Permit Requirements: Most municipalities require a permit and rough-in inspection for any new feeder over 60A. The inspector will verify your voltage drop calculations, your proportional ground upsizing, and your torque markings.
For further reading on the physics of grounding and bonding, refer to the educational resources provided by Mike Holt Enterprises and the official NFPA 70 National Electrical Code documentation. Always defer to your local inspector's interpretation of the code over general internet guidance.
Final Verdict: For a standard 100A subpanel feeder, pull 8 AWG bare or green-insulated copper alongside your hot and neutral wires. If your run exceeds 50-75 feet and you upsize your hot wires to mitigate voltage drop, use the circular mil ratio to upsize your ground proportionally. Torque every lug to the manufacturer's spec, isolate the neutral bar in the subpanel, and verify continuity before energizing.
