The correct 100 amp ground wire size for an equipment grounding conductor (EGC) is 8 AWG copper or 6 AWG aluminum. This sizing applies to feeders and branch circuits protected by a 100-amp overcurrent device, such as a subpanel feeder or a heavy-duty workshop circuit.

⚠️ The Hazard of Undersized Grounding: The ground wire does not carry current during normal operation. Its sole job is to survive a massive, sudden surge of fault current long enough for the breaker to trip. If you use a smaller wire (like 12 AWG) on a 100A circuit and a hot wire shorts to the metal panel chassis, the 12 AWG wire will instantly vaporize from the heat before the 100A breaker trips. The metal panel will remain energized at 120V/240V, creating a lethal shock hazard for anyone who touches it while standing on the ground.

Sizing the Equipment Grounding Conductor (EGC)

When electricians and DIYers ask about ground wire sizing, they are almost always referring to the Equipment Grounding Conductor (EGC) that runs alongside the hot and neutral wires in a conduit or cable assembly. The National Electrical Code (NEC) dictates EGC sizing based on the rating of the overcurrent protective device (the breaker), not the actual load current.

Below is the critical sizing data extracted from NEC Table 250.122. Keep this table handy whenever you are pulling wire for subpanels, hot tubs, or heavy machinery.

Overcurrent Device Rating Copper EGC Size (AWG/kcmil) Aluminum EGC Size (AWG/kcmil)
15 Amps 14 AWG 12 AWG
20 Amps 12 AWG 10 AWG
60 Amps 10 AWG 8 AWG
100 Amps 8 AWG 6 AWG
200 Amps 6 AWG 4 AWG

Source guidance: Sizing aligns with NFPA 70 (NEC) Table 250.122. Always defer to your local Authority Having Jurisdiction (AHJ) or inspector, as local amendments may require larger conductors in specific environments.

The Voltage Drop Exception: Proportional Upsizing

There is a major edge case that catches many builders off guard. If your 100-amp feeder run is exceptionally long (e.g., a detached garage 150 feet away), you will likely upsize your ungrounded (hot) conductors from 3 AWG to 1 AWG or 1/0 AWG to mitigate voltage drop.

Per NEC 250.122(B), if you upsize the hot wires for voltage drop, you must proportionally upsize the ground wire. If you increase the cross-sectional area of your hot wires by 60%, your 8 AWG copper ground wire must also be increased in area by 60%, pushing it up to a 4 AWG or 3 AWG copper EGC. Failing to do this means the ground wire's resistance might be too high to allow sufficient fault current to trip the breaker quickly at the far end of the circuit.

Ground vs. Neutral vs. Bond: Clearing the Confusion

To safely install a 100-amp feeder, you must understand the distinct roles of the grounding system. Mixing these up is the most common cause of failed electrical inspections and dangerous shock hazards.

  • Neutral (Grounded Conductor): Typically white or gray insulation. This wire carries the normal, unbalanced return current back to the transformer during everyday operation. It is a current-carrying conductor.
  • Ground (Equipment Grounding Conductor / EGC): Bare copper or green insulation. This wire carries zero current under normal conditions. It only provides a low-impedance path back to the source during a fault (short circuit) to trip the breaker.
  • Bond: This is not a wire; it is a physical connection. The 'main bonding jumper' connects the neutral bus bar to the ground bus bar and the metal panel chassis.
💡 The Subpanel Rule: The main bonding jumper is only installed at the main service disconnect (your primary utility panel). If you are running a 100-amp feeder to a subpanel, the neutral and ground bus bars must remain strictly isolated (floating neutral). If you bond the neutral to the ground in a subpanel, normal return current will travel back along the bare ground wire, energizing the panel chassis and any connected metal framing or plumbing.

For a deeper dive into the physics of grounding and bonding fault-current paths, resources like EC&M's grounding fundamentals provide excellent visual breakdowns of how fault current returns to the source transformer.

Verifying Your Ground and When to Call a Pro

Once your 8 AWG copper or 6 AWG aluminum ground wire is pulled and terminated, you must verify the integrity of the path before energizing the circuit. Relying on visual inspection alone is insufficient; a loose lug can create a high-resistance fault path that won't trip the breaker.

Testing the Grounding Path

  1. Visual and Torque Check: Ensure the EGC is stripped correctly (no nicked strands) and torqued to the manufacturer's specification on the ground bus bar. For an 8 AWG copper wire on a standard square-head lug, this is typically around 35 to 45 inch-pounds, but always check the panel label.
  2. Continuity Test (De-energized): With the main breaker OFF and the circuit completely dead, use a multimeter set to the lowest Ohms (Ω) setting. Place one probe on the subpanel's ground bus and the other on the main panel's ground bus. You should read less than 1.0 ohm (ideally 0.1 to 0.3 ohms). Anything higher indicates a poor termination.
  3. Voltage Test (Energized): Turn the power on. Set your multimeter to AC Volts. Measure Hot-to-Ground (should read ~120V). Measure Neutral-to-Ground (should read 0V to 1.5V). If Neutral-to-Ground reads high (e.g., 5V+), you likely have a loose neutral connection or an improper bond.
  4. Clamp Meter Check: Under normal load conditions, clamp an AC current meter around the bare EGC. It must read 0.0 Amps. If it reads current, neutral return current is leaking onto the ground path, indicating an illegal neutral-to-ground bond downstream.

When a Licensed Electrician is Required

While pulling a 100-amp feeder to a detached workshop subpanel is a common advanced DIY project, certain boundaries legally and safely require a licensed professional:

  • Service Entrance Upgrades: If you are upgrading your main home service from 100A to 200A, or working on the conductors between the utility meter and the main panel, this is strictly licensed work. The utility company must pull the meter, and local codes almost universally mandate a master electrician for service entrance work.
  • Grounding Electrode System (GEC): Do not confuse the EGC (feeder ground) with the Grounding Electrode Conductor (the wire connecting your main panel to the ground rods or ufer ground). Sizing the GEC involves NEC Table 250.66 and requires understanding soil resistivity and local grounding electrode requirements. Driving ground rods and bonding to municipal water lines should be handled by a pro to ensure the main service can safely dissipate a lightning strike or utility line surge.
  • AHJ Inspections: Any new subpanel or 100-amp circuit requires a permit. Your local inspector will check your wire sizing, torque marks, and grounding electrode connections. If you are unsure about your local amendments regarding grounding rods for detached structures, hire an electrician to pull the permit and perform the final terminations.

Getting the 100 amp ground wire size right is non-negotiable. By using 8 AWG copper (or upsizing proportionally for voltage drop), keeping your neutrals and grounds isolated at the subpanel, and verifying the path with a meter, you ensure that a fault results in a tripped breaker—not a tragedy.