For a standard 100-amp circuit, such as a subpanel feeder or a large appliance branch circuit, the minimum equipment grounding conductor (EGC) size is 8 AWG copper or 6 AWG aluminum. This baseline is derived directly from NEC Table 250.122. However, simply pulling the minimum wire from a spool without considering voltage drop, conductor upsizing, and termination physics is a common jobsite mistake that compromises safety.
The Hazard: What Happens When Ground Wire is Undersized?
To understand why the 100 amp ground size is strictly regulated, you have to look at the physics of a fault condition. When a 120V ungrounded (hot) conductor comes loose and touches the metal enclosure of a subpanel, a short circuit occurs. The breaker must trip in a fraction of a second to prevent a fire or fatal shock.
A standard 100-amp thermal-magnetic breaker requires roughly 500 to 1,000 amps of instantaneous current to trigger its magnetic trip mechanism and open the circuit in under 0.1 seconds. The equipment grounding conductor provides the low-impedance path back to the source to allow this massive surge of current to flow.
If you mistakenly use a 14 AWG or 12 AWG ground wire because 'it is just a ground,' the high resistance of that thin wire chokes the fault current. Instead of 800 amps, only 30 or 40 amps might flow. The breaker's thermal strip will see this as a mild overload, not a dead short, and it may take several minutes to trip—or it may never trip at all. During that time, the entire metal subpanel enclosure sits at 120V relative to the earth. According to NFPA Electrical Safety guidelines, this delayed clearing time is a primary cause of electrical fires and fatal shocks in residential wiring.
Ground, Neutral, and Bond: Clearing Up the Confusion
Before terminating your 100A feeder, you must clearly separate three distinct concepts that DIYers frequently conflate:
- Neutral (Grounded Conductor): The white or gray wire. It carries the unbalanced return current back to the source during normal operation. It is a current-carrying conductor and must be sized based on the load (typically 3 AWG copper or 1 AWG aluminum for 100A).
- Ground (Equipment Grounding Conductor / EGC): The bare or green wire. It carries zero current during normal operation. Its only job is to clear faults by providing a low-impedance path back to the main panel's grounding bus.
- Bonding: The physical connection between the neutral and ground systems. In a main service panel, neutral and ground are bonded together (via a green bonding screw or strap). In a 100A subpanel, they must remain strictly isolated. If you bond neutral and ground at a subpanel, normal return current will travel back on the ground wire, energizing metal enclosures and defeating the safety system.
Sizing Decision Tree: Copper vs. Aluminum for 100A
While NEC Table 250.122 provides the baseline, real-world installations require a decision path based on your feeder conductor material and run length. Below is the decision matrix for selecting your exact 100 amp ground size.
| Scenario / Condition | Copper EGC Size | Aluminum EGC Size | Notes & Edge Cases |
|---|---|---|---|
| Standard run (< 50 ft), no voltage drop upsizing | 8 AWG | 6 AWG | Baseline per NEC 250.122. Assumes standard 75°C terminations. |
| Long run (> 50 ft), ungrounded conductors upsized to 2 AWG Cu / 1/0 Al for voltage drop | 6 AWG | 4 AWG | NEC 250.122(B) requires the EGC to be increased proportionally when hot wires are upsized. |
| Run in PVC conduit with parallel feeders | 8 AWG per conduit | 6 AWG per conduit | Each parallel raceway must contain its own full-sized EGC. You cannot share one ground wire between conduits. |
For a deeper look at how proportional upsizing works when you increase your hot wire gauge to mitigate voltage drop, reference the Electrical Technology grounding charts, which break down the math for extended feeder runs.
Step-by-Step Installation and Verification
Pulling the correct 100 amp ground size is only half the job; verifying the integrity of the path ensures the system will actually clear a fault. Follow these steps to terminate and test your EGC.
- De-energize and Verify: Turn off the main 100A breaker feeding the circuit. Use a non-contact voltage tester, followed by a digital multimeter set to AC voltage, to confirm 0V between the hot legs and the ground bus.
- Strip and Terminate: Strip the 8 AWG copper ground wire. If using a lug-type grounding bar, ensure the wire is inserted fully so no bare strands are exposed outside the lug. Torque the set screw to the manufacturer's specification (typically 20 to 30 in-lbs for small lugs, but check the panel label).
- Isolate the Subpanel Neutral: Verify that the subpanel's neutral busbar is floating (not bonded to the metal enclosure). The ground busbar must be bonded directly to the metal enclosure.
- Continuity Test (De-energized): Set your multimeter to the 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. If you read infinite (OL) or high resistance, your ground path is broken or poorly terminated.
- Voltage Verification (Energized): Restore power. Set the multimeter to AC Voltage. Measure Hot-to-Ground at the subpanel; it should read nominally 120V (114V-126V range). Measure Neutral-to-Ground; it should read less than 2.0V. If Neutral-to-Ground reads high (e.g., 5V+), you likely have a loose neutral connection upstream or an illegal neutral-ground bond at the subpanel pushing return current onto the ground wire.
When to Stop and Call a Licensed Electrician
While sizing and pulling a ground wire for a 100A subpanel feeder is a standard task for advanced DIYers, the boundaries of legal and safe electrical work are strictly defined by your local Authority Having Jurisdiction (AHJ). The NEC provides the baseline physics and safety framework, but local code amendments always hold final legal authority.
You must hire a licensed, insured electrician if your project involves any of the following:
- Service Entrance Work: If your 100A feed originates directly from the utility meter or the main service disconnect, you are working on the service entrance conductors. Utility companies and AHJs strictly prohibit unlicensed personnel from breaking the meter seal or working on the line side of the main disconnect.
- Main Bonding Jumper Installation: Sizing and installing the main bonding jumper inside the primary service panel requires precise adherence to NEC 250.28. An error here compromises the grounding electrode system for the entire structure.
- Upgrading the Grounding Electrode System: If your home relies on an outdated grounding electrode (like a single driven rod or a metallic water pipe without a supplemental electrode), adding a 100A subpanel may trigger a code requirement to upgrade the entire premises grounding system to include two 8-foot ground rods or a UFER (concrete-encased) ground.
Always pull a permit for a 100-amp subpanel installation. The inspector's primary focus will be on your grounding and bonding methodology, as this is the most frequent point of failure in residential electrical upgrades. By selecting the correct 8 AWG copper or 6 AWG aluminum EGC, proportionally upsizing for long runs, and keeping your neutral and ground strictly isolated at the subpanel, you ensure the system will safely clear faults and protect the occupants.
