If you are pulling wire for a standard residential or light commercial feed, the direct answer to "what size ground for 100 amp service" is **8 AWG copper** or **6 AWG aluminum** for the Equipment Grounding Conductor (EGC). If you are running the Grounding Electrode Conductor (GEC) from the main panel to the earth ground rod, the minimum is also **8 AWG copper**. However, slapping an 8 AWG wire on a lug and calling it a day ignores the physics of fault currents and the strict definitions of the National Electrical Code (NEC). Getting this wrong doesn't just fail inspection; it creates a lethal shock hazard.

The Hazard: What Happens When Your Ground is Undersized?

To understand why wire size matters, you have to understand what a ground wire actually does. Under normal conditions, the ground wire carries exactly zero current. It is an emergency highway reserved exclusively for ground faults—like when a frayed hot wire touches the metal chassis of your table saw or water heater. When that fault occurs, the ground wire must carry a massive surge of current back to the panel. This surge is what creates the magnetic trip inside the breaker, shutting off the power in milliseconds.
CRITICAL HAZARD: If your ground wire is undersized (for example, using 12 AWG instead of the required 8 AWG for a 100A feed), it acts as a high-resistance bottleneck. During a dead-bolt fault, the undersized wire will rapidly heat up and potentially vaporize before the breaker trips. This leaves the metal enclosure of the appliance energized at 120V or 240V, turning a simple short circuit into an electrocution risk or an arc-flash fire.
The NEC sizes grounding conductors based on the ampere rating of the overcurrent protective device (the breaker) to ensure the wire can survive the thermal and magnetic stress of the fault long enough for the breaker to clear it.

Ground vs. Neutral vs. Bond: Clearing the Confusion

Before you cut your wire, you must separate three terms that DIYers and even some apprentice electricians confuse:
  • Neutral (Grounded Conductor): The intentional return path for normal operating current. It is a current-carrying conductor and must be sized to carry the full load of the circuit (e.g., 3 AWG copper for 100A).
  • Ground (Equipment Grounding Conductor / EGC): The non-current-carrying emergency path for fault current. It only sees current when something has gone wrong.
  • Bond: The physical, intentional connection between the neutral and the ground. In a main service panel, the neutral bus and ground bus are bonded together. This bond is what allows fault current traveling on the ground wire to cross over to the neutral path and trip the main breaker.

Analogy: Think of the neutral as the regular highway lanes carrying daily traffic. The ground is the emergency shoulder, completely empty until a car breaks down. The bond is the on-ramp at the very beginning of the highway that connects the shoulder back to the main lanes so the tow truck (the breaker) can be dispatched.

Sizing the Ground: EGC vs. GEC for 100A Services

When planning a 100 amp service, you are actually dealing with two different grounding paths. The NEC treats them in separate tables. The following data is based on NEC-style guidance (specifically Articles 250.122 and 250.66); however, your local Authority Having Jurisdiction (AHJ) or utility company always has final legal authority and may require larger sizes.
Conductor Type Purpose NEC Reference Min. Copper Size (100A) Min. Aluminum Size (100A)
Equipment Grounding Conductor (EGC) Runs with circuit wires to subpanels or appliances to clear faults. Table 250.122 8 AWG 6 AWG
Grounding Electrode Conductor (GEC) Connects the main panel to the earth (ground rod, ufer, water pipe). Table 250.66 8 AWG 6 AWG

Pro-Tip on the GEC: While 8 AWG copper is the NEC minimum for a 100A service GEC, many utility companies and local inspectors mandate a baseline of 6 AWG bare copper for all service entrance grounding electrode conductors, regardless of service size. Always call your local building department before pulling the wire. You can review standard code practices via the NFPA NEC portal or industry breakdowns on EC&M's NEC section.

How to Verify Your Grounding Path in the Field

You cannot verify a ground just by looking at the wire gauge. A properly sized 8 AWG wire is useless if the termination is loose or the path is broken. Here is how to verify the integrity of your grounding system.
  1. Visual and Mechanical Inspection: With the panel de-energized (main breaker off, verified dead with a non-contact voltage tester and a multimeter), check the ground bus bar. Ensure the 8 AWG wire is stripped cleanly, seated fully under the lug, and torqued to the manufacturer's specification (usually 20-30 in-lbs for small lugs). Look for green oxidation on aluminum, which indicates a failing connection.
  2. Neutral-to-Ground Voltage Test: Restore power. Under a heavy load (e.g., running a space heater or hair dryer on the circuit), measure the AC voltage between the neutral slot and the ground slot at a downstream receptacle using a digital multimeter. A healthy ground path will read less than 1.0V to 1.5V. If you read 3V or higher, your ground path has high impedance (a loose connection or undersized wire).
  3. Ground Impedance Testing: For the main service GEC (the wire going to the earth rod), standard multimeters are useless. You need a dedicated ground impedance tester (like a Fluke 1625-2) which uses stake-less testing or fall-of-potential methods to measure the actual resistance of the earth connection. The NEC generally requires a single ground rod to have a resistance to ground of 25 ohms or less; if it fails, you must drive a second rod 6 feet away.
WHEN TO CALL A PRO: If your testing reveals a fault at the service entrance, meter base, or main panel lugs, stop immediately. Working on the line-side of the main breaker involves exposed, unfused utility voltage that can cause lethal arc flashes. This work requires a licensed electrician and a utility disconnect.

Frequently Asked Questions

Can I use a 10 AWG ground wire for a 100 amp subpanel?

No. A 100-amp feeder requires an 8 AWG copper EGC. While 10 AWG is sufficient for 30A to 60A circuits, putting it on a 100A breaker violates NEC Table 250.122. If a fault occurs on the subpanel bus, the 10 AWG wire could melt before the 100A main breaker trips, leaving the subpanel chassis energized.

Does the ground wire need to be the same size as the hot wires?

No. For a 100-amp service, your ungrounded (hot) conductors are typically 3 AWG copper or 1 AWG aluminum. The ground wire (EGC) is sized based on the breaker rating, not the hot wire size. Therefore, an 8 AWG copper ground is perfectly legal and safe to run alongside 3 AWG copper hots, provided it is in the same raceway or cable assembly.

What size ground rod do I need for a 100 amp service?

The physical size of the ground rod does not change based on your amperage. The standard requirement is a 5/8-inch diameter, 8-foot long copper-bonded steel ground rod. It must be driven completely into the earth so the top is flush with or below grade. The 8 AWG (or 6 AWG) GEC wire is then clamped to the top of this rod using an acorn-style grounding clamp listed for direct burial.

Can I use bare copper for the 100 amp service ground?

Yes, and it is actually preferred. Bare copper is the standard for both the Grounding Electrode Conductor (running to the earth rod) and the Equipment Grounding Conductor (running in conduit to subpanels). The only time you must use insulated wire (colored green or green with a yellow stripe) for an EGC is when running it inside a multi-conductor cable assembly (like NM-B/Romex) or when local AHJ rules specifically mandate it to prevent accidental contact with other systems in shared cable trays.