If you are pulling wire for a 100-amp feeder or branch circuit, the minimum equipment grounding conductor (EGC) size is 8 AWG copper or 6 AWG aluminum. This baseline is dictated by NEC Table 250.122, which sizes the ground wire based on the rating of the overcurrent protective device (the breaker), not the size of the current-carrying conductors.

The Hazard of Undersized Grounds: The EGC does not carry current during normal operation; it exists solely to provide a low-impedance fault path. If you use an undersized ground (like 12 AWG) on a 100A circuit, a dead short to the metal chassis will push thousands of amps through that wire. The breaker takes milliseconds to trip, but the I²t (current squared × time) let-through energy will vaporize a 12 AWG wire before the breaker clears. The fault path burns open, the breaker fails to trip, and the metal enclosure remains fully energized at 120V/240V—a lethal shock hazard for the next person who touches it.

NEC Table 250.122: Sizing the Equipment Grounding Conductor

The National Electrical Code (NEC) sizes the EGC to ensure it can survive the thermal and magnetic stresses of a short circuit long enough for the breaker to trip. The table below outlines the minimum sizes for common breaker ratings. Note: These values represent NEC-style guidance for standard installations; your local Authority Having Jurisdiction (AHJ) or local inspector always has the final legal authority on code compliance in your area.

Breaker Rating (Amps) Min. Copper EGC (AWG) Min. Aluminum EGC (AWG) Typical Application
15A 14 12 Standard lighting and receptacle branch circuits
20A 12 10 Kitchen small appliance, bathroom, and garage circuits
30A 10 8 Dryers, water heaters, and small HVAC condensers
60A 10 8 EV chargers (Level 2), subpanel feeders, large ranges
100A 8 6 Main subpanel feeders, large workshop equipment, HVAC
150A 6 4 Large residential subpanels, light commercial feeders
200A 6 4 Standard residential main service entrance panels

The Voltage Drop Upsizing Rule (NEC 250.122(B))

Here is a critical edge case that catches many DIYers and even some journeymen off guard. If you have to upsize your ungrounded (hot) conductors to compensate for voltage drop over a long distance, you must proportionally upsize your ground wire.

Worked Example: You are running a 100A feeder to a detached garage 150 feet away. To keep voltage drop under 3%, you calculate that you need to upsize the hot wires from the standard 1 AWG copper to 3 AWG copper (two AWG steps larger). Because you increased the hot wires by two steps, NEC 250.122(B) requires you to increase the EGC by the exact same ratio. Your ground wire must be upsized two steps from 8 AWG to 4 AWG copper. If you pull 3 AWG hots but only run an 8 AWG ground, you will fail inspection and compromise the fault-clearing impedance of the circuit.

Ground vs. Neutral vs. Bonding: Clearing the Confusion

To understand why the 8 AWG ground wire is so vital, you must understand how it differs from the neutral and the bonding jumper. Mixing these up is the most common cause of stray voltage and shocked homeowners.

  • Neutral (Grounded Conductor): This is the white or gray wire. It is a current-carrying conductor designed to handle the unbalanced return current of the circuit back to the transformer. It is sized exactly the same as the hot wires (or calculated based on the maximum unbalanced load).
  • Ground (Equipment Grounding Conductor / EGC): This is the bare or green wire. It carries zero current during normal operation. Its only job is to provide a low-impedance path back to the source to facilitate the immediate tripping of the breaker during a ground fault.
  • Bonding: This is the physical, permanent connection between the neutral bus bar and the ground bus bar (and the metal panel enclosure).
Subpanel Rule: The neutral and ground must be bonded only at the main service disconnect (the first point the power enters the building). In any downstream subpanel fed by your 100-amp breaker, the neutral bus and ground bus must be physically isolated. If you bond neutral and ground at a subpanel, normal return current will split and travel back along the bare 8 AWG ground wire, energizing the metal conduit and panel enclosures along the way.

How to Verify Your Grounding Path

Simply pulling an 8 AWG wire and landing it on a bus bar does not guarantee a safe fault path. A loose lug, a painted panel enclosure, or a missing bonding screw can render the ground useless. According to best practices outlined by testing authorities like Fluke, verifying the ground requires specific testing protocols.

Step 1: De-Energized Continuity and Torque Check

  1. De-energize and Lockout: Turn off the 100A feeder breaker at the main panel. Verify the subpanel or load side is dead using a non-contact voltage tester and a multimeter.
  2. Torque Verification: Use a calibrated torque screwdriver to check the lug securing the 8 AWG ground wire to the ground bus bar. A loose connection adds resistance, which limits fault current and delays breaker tripping.
  3. Milliohm Testing: Use a low-resistance ohmmeter (not a standard multimeter, which lacks the resolution) to measure the resistance between the subpanel ground bus and the main panel ground bus. You are looking for a reading in the milliohm range (typically < 0.5 ohms). High resistance indicates a poor mechanical connection or a compromised wire.

Step 2: Energized Impedance and Voltage Drop

For existing installations where de-energizing is not immediately possible, you can perform a voltage drop test under load. Measure the voltage between the hot bus bar and the ground bus bar at the subpanel with no load. Then, turn on a known heavy load (like a 2kW space heater or a bank of incandescent lights). If the voltage between hot and ground drops significantly, or if you measure more than 1 to 2 volts between the neutral bus and the ground bus under load, your grounding path has high impedance and needs immediate investigation.

When a Licensed Electrician is Required

While running a feeder and landing wires in a subpanel is a common advanced DIY project, certain aspects of the grounding system cross the line into work that legally and practically requires a licensed electrician. According to the National Fire Protection Association (NFPA) and local building codes, you must call a professional for:

  • Service Entrance Upgrades: Any work on the conductors between the utility meter and the main service disconnect. The utility owns the meter, and the fault current available on the line side of the main breaker can exceed 10,000 amps. An arc flash here is fatal.
  • Grounding Electrode System (GES): Driving ground rods, connecting to metal underground water pipes, or installing ufer (concrete-encased) grounds. The sizing of the Grounding Electrode Conductor (GEC) is governed by a different code section (NEC 250.66) and requires specialized knowledge of soil resistivity and local utility requirements.
  • Main Bonding Jumper Installation: Installing or modifying the main bonding jumper inside the main service panel. If this is done incorrectly, the entire neighborhood's neutral current could seek a path back through your home's plumbing and gas lines.

Getting the ground wire size right for a 100-amp circuit is non-negotiable. Stick to 8 AWG copper (or upsize proportionally for voltage drop), keep your neutrals and grounds separated at subpanels, and always verify the mechanical integrity of your terminations. When in doubt, or when dealing with the service entrance, hand the tools to a licensed professional.