To correctly size ground wire for 100 amp service or a 100-amp feeder, you need an 8 AWG copper Equipment Grounding Conductor (EGC). If pulling aluminum, step up to 6 AWG. For the main service Grounding Electrode Conductor (GEC) tied to #3 AWG copper hots, the minimum is also 8 AWG copper.
Baseline Assumptions for This Guide
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Rating: 75°C column (standard for modern breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) baseline
- Installation Method: Up to 3 current-carrying conductors in PVC conduit or standard NM-B cable
The NEC Tables Behind the 8 AWG Rule
When DIYers and apprentices ask about the 'ground wire,' they are usually conflating two entirely different conductors. The National Electrical Code (NEC) treats the wire that clears faults on your subpanel feeder differently than the wire that bonds your main service to the earth. Understanding this distinction prevents failed inspections.
The NFPA 70 (NEC) dictates grounding sizes based on the rating of the overcurrent protection device (the breaker) or the size of the ungrounded (hot) service entrance conductors. Here is the spec sheet for a standard 100-amp setup:
| Conductor Type | NEC Reference | Sizing Basis | Min. Copper | Min. Aluminum |
|---|---|---|---|---|
| Equipment Grounding Conductor (EGC) | Table 250.122 | Breaker Size (100A) | 8 AWG | 6 AWG |
| Grounding Electrode Conductor (GEC) | Table 250.66 | Hot Wire Size (#3 Cu / #1 Al) | 8 AWG | 6 AWG |
For a 100-amp subpanel feeder, you are pulling an EGC. The breaker size alone dictates the minimum gauge. For the main service panel tied to a ground rod or water pipe, you are sizing a GEC based on the physical cross-sectional area of your incoming hot wires.
Why 8 AWG and Not 10 AWG? (The Physics of Fault Clearing)
I have seen guys try to save money by pulling 10 AWG ground on a 100-amp subpanel feed because 'it is just a ground and carries no current.' This is a dangerous misunderstanding of let-through energy.
The ground wire does not carry current during normal operation. It exists solely to provide a low-impedance fault path during a catastrophic short circuit. If a hot wire shorts to the metal subpanel enclosure, the ground wire must safely carry thousands of amps for the milliseconds it takes the breaker's magnetic trip to engage.
Under a 5,000-amp bolted fault, a 10 AWG copper wire will reach its melting point and vaporize before a standard 100-amp thermal-magnetic breaker fully clears the arc. If the ground wire melts, the panel enclosure remains energized at 120V/240V, waiting for a human to touch it and complete the circuit to earth. The 8 AWG minimum in NEC Table 250.122 is calculated specifically to survive the $I^2t$ (let-through energy) of a 100A breaker.
What Changes the Answer? (Length, Bundling, and Voltage Drop)
The 8 AWG rule is the absolute minimum for a standard run. Three specific conditions will force you to upsize that ground wire.
1. Voltage Drop and Proportional Upsizing
If your 100-amp feeder run exceeds 100 feet, you will likely upsize the hot conductors to mitigate voltage drop. NEC 250.122(B) mandates that if you increase the size of the ungrounded (hot) conductors for any reason, you must proportionally increase the size of the equipment grounding conductor.
Worked Example (Aluminum Feeder at 150 feet):
- Standard 100A Hot: #1 AWG Aluminum (83,690 circular mils)
- Upsized for Voltage Drop: #1/0 AWG Aluminum (105,600 circular mils)
- Upsize Ratio: 105,600 / 83,690 = 1.26
- Standard 100A Ground: #6 AWG Aluminum (26,240 circular mils)
- Required Ground: 26,240 x 1.26 = 33,062 circular mils
The next standard wire size above 33,062 circular mils is #4 AWG Aluminum (41,740 cmil). If you pull #1/0 hots but only pull a #6 ground, you will fail inspection. For more on feeder calculations, reference the EC&M codes and standards archive for detailed voltage drop matrices.
2. Ambient Temperature and Bundling Derating
If you are running your feeder through an attic in a hot climate (e.g., 115°F ambient) or bundling multiple circuits in a single conduit, you must apply NEC Table 310.16 derating factors. If derating forces you to upsize your hot wires from #3 AWG to #2 AWG copper to maintain 100A ampacity, your ground wire must again be upsized proportionally based on the circular mil ratio.
3. Aluminum vs. Copper
Never treat aluminum and copper interchangeably. Aluminum has higher resistance and lower melting point characteristics per volume. If your main service entrance uses #1 AWG aluminum hots, your GEC to the ground rod must be #6 AWG aluminum (or #8 AWG copper). Always use antioxidant paste (like Noalox) on aluminum ground terminations to prevent galvanic corrosion and high-resistance faults over time.
When an Engineer or the AHJ Must Confirm
While the NEC tables cover 95% of residential and light commercial jobs, you must defer to a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- High Available Fault Current (AFC): If your utility transformer is close to the service drop, the available short-circuit current might exceed 10,000 amps (10kA). Standard residential breakers are rated for 10kA. If the AFC is 22kA, your breaker will explode before tripping. An engineer must spec higher AIC-rated breakers and potentially larger ground busbars and conductors to handle the magnetic forces.
- Parallel Conductor Runs: If you are running parallel sets of wire for a massive service (e.g., 400A split into two 200A feeds), NEC 250.122(C)(2) requires a full-sized ground wire in each raceway, not just one ground for the whole run.
- Local Amendments: Some municipalities in high-lightning or high-soil-resistivity areas mandate larger GECs (e.g., #4 AWG bare copper minimum) regardless of the service size to ensure adequate earth bonding.
Frequently Asked Questions
Can I use a 10 AWG ground wire for a 100 amp subpanel if the run is very short?
No. NEC Table 250.122 does not offer distance exceptions for the minimum EGC size. The 8 AWG copper minimum is based on the thermal withstand capacity required to survive a fault cleared by a 100-amp breaker, not on the length of the run or voltage drop. Even for a 5-foot run through a wall, 8 AWG is the legal and physical minimum.
Does the equipment ground wire need to be insulated for a 100 amp feeder?
No. For a feeder running in conduit or as part of a cable assembly, a bare copper ground is perfectly legal and standard practice. However, if you are pulling individual THHN wires through a raceway and prefer insulated wire for easy identification, it must be wrapped in green or green-with-yellow-stripes insulation. Never use white or gray for a ground, as those are reserved for the neutral (grounded conductor).
What size ground bar do I need in a 100 amp subpanel?
The physical ground bar must be rated for the panel and accommodate the wire size. Most 100-amp subpanels (like the Eaton BR100 or Siemens 100A main lug) come with a pre-installed or included ground bar rated for up to #4 AWG or #2 AWG wire. Ensure the terminal screw is torqued to the manufacturer's specification (usually around 25 to 40 in-lbs for an 8 AWG wire) using a calibrated torque screwdriver. A loose ground lug creates a high-impedance fault path, rendering the 8 AWG wire useless during a short circuit.
