For a standard 100-amp service, you need an 8 AWG copper or 6 AWG aluminum equipment grounding conductor (EGC), protected by a 100A breaker. This baseline assumes 75°C terminations, 30°C ambient temperature, and THHN/THWN-2 insulation in a single raceway.
Baseline Assumptions for This Sizing
- Conductor Material: Copper (Aluminum alternatives noted separately)
- Temperature Rating: 75°C column (Standard for 100A breakers and panel lugs)
- Ambient Temperature: 30°C (86°F)
- Insulation Type: THHN/THWN-2 or XHHW
- Installation Method: Single conduit/raceway, maximum 3 current-carrying conductors
- Terminations: Standard 75°C rated lugs (NEC 110.14(C))
The Core Sizing Rule: NEC Table 250.122
When hobbyists ask about the 'ground wire,' they are referring to the Equipment Grounding Conductor (EGC). This is the bare copper or green-insulated wire that provides a low-impedance fault path back to the source. It does not carry current under normal operation; it only carries current during a short circuit or ground fault.
The National Electrical Code (NEC) dictates EGC sizing based on the rating of the overcurrent protective device (the breaker), not the actual load current. According to NFPA 70 (NEC) Table 250.122, the minimum EGC sizes for common residential and light commercial breakers are as follows:
| Overcurrent Device Rating (Amps) | Minimum Copper EGC (AWG) | Minimum Aluminum EGC (AWG) |
|---|---|---|
| 60A | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
| 125A | 6 AWG | 4 AWG |
| 200A | 6 AWG | 4 AWG |
| 250A | 4 AWG | 2 AWG |
Why 8 AWG and Not One Size Smaller?
It is tempting to think that because a 10 AWG wire can handle 30A to 40A of continuous load, it should be fine for a ground wire that normally carries zero amps. However, during a dead-bolt fault (e.g., a hot wire touches the metal panel chassis), thousands of amps will attempt to flow through the EGC to trip the 100A breaker.
A 10 AWG wire lacks the thermal mass to survive this massive, instantaneous current surge. It could melt or vaporize before the breaker's magnetic trip mechanism clears the fault (which takes roughly 1 to 2 cycles, or 16 to 33 milliseconds). An 8 AWG copper wire provides the necessary cross-sectional area to safely conduct the let-through current of a 100A breaker without failing, keeping the metal enclosure safe to touch.
The Voltage Drop Trap: When 8 AWG Fails
Here is where most DIYers and junior apprentices make a critical mistake. Ground wires do not carry continuous load, so you do not calculate voltage drop for the ground wire itself. However, if your feeder run is long, you must upsize your ungrounded (hot) conductors to mitigate voltage drop. When you upsize the hot wires, NEC 250.122(B) requires you to proportionally upsize the EGC.
Let us look at the feeder ampacity requirements first. Based on NEC Table 310.16 (75°C column, THHN insulation), the standard ungrounded feeder wires for a 100A service are:
| Conductor Size | Material | Ampacity (75°C Column) | Use Case |
|---|---|---|---|
| 3 AWG | Copper | 100A | Standard short runs (< 50 ft) |
| 2 AWG | Copper | 115A | Moderate runs or high ambient |
| 1 AWG | Copper | 130A | Long runs requiring VD mitigation |
| 1/0 AWG | Aluminum | 100A | Standard short runs (Aluminum) |
Worked Numeric Example: The 150-Foot Run
Imagine you are feeding a 100A subpanel in a detached workshop 150 feet away. Your calculated continuous load is 80A.
- Standard Feeder: 3 AWG Copper (Circular Mils = 26,240)
- Voltage Drop Formula: VD = (2 x K x I x D) / CM
- Calculation: (2 x 12.9 x 80A x 150ft) / 26,240 = 11.79 Volts
- Percentage: 11.79V / 240V = 4.9%
A 4.9% voltage drop exceeds the NEC recommended maximum of 3% for branch circuits and feeders combined. To fix this, you must upsize your hot conductors to 1 AWG Copper (CM = 83,690), which drops the VD to a safe 1.5%.
Material and Bundling Variables
The physical environment and material choices will alter your installation method, though the base AWG requirements remain tied to the breaker size unless the conditions force an upsize.
Copper vs. Aluminum
You can absolutely use aluminum for your EGC to save money on long runs, provided you use 6 AWG Aluminum for a 100A breaker. However, you must never treat copper and aluminum interchangeably at the termination points. Aluminum requires specific anti-oxidant paste (like Noalox) and lugs explicitly rated for AL/CU. If your 100A breaker or panel bus bar is only rated for copper, you must use copper pigtails or stick to a copper EGC.
Conduit Bundling and Derating
Does bundling multiple circuits in one conduit change your ground wire size? No, not directly. Under NEC 310.15(C)(1), ampacity derating factors apply only to current-carrying conductors. The EGC is not a current-carrying conductor under normal conditions. Therefore, pulling three separate 100A circuits through a single large PVC conduit does not require you to derate the 8 AWG ground wires.
However, bundling will force you to derate your hot and neutral wires. If that derating forces you to upsize your hot wires to maintain 100A ampacity, the proportional upsizing rule (NEC 250.122(B)) kicks in, and your ground wire must grow with them.
When an Engineer or AHJ Must Confirm
While NEC Table 250.122 covers 95% of residential and light commercial 100A services, edge cases exist where standard sizing falls short. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- High Available Fault Current: If your 100A service is fed directly from a massive utility transformer (e.g., a network protector in a dense urban grid), the available fault current might exceed 10,000 amps. Standard 100A breakers have an interrupting rating (AIC) of 10kA. If the fault current exceeds the breaker's AIC, the breaker could catastrophically fail before clearing. An engineer may specify current-limiting fuses and a correspondingly larger EGC to handle the thermal stress.
- Parallel Feeders: While rare for a 100A service (NEC 250.122(F) generally applies to 1/0 AWG and larger), if local code allows paralleled conductors for a 100A feed due to specific physical constraints, each raceway must contain its own EGC, and the sizing math changes.
- Local Amendments: Some municipalities adopt stricter grounding rules than the baseline NEC. For example, certain AHJs require a minimum 6 AWG copper EGC for all outdoor underground feeder runs regardless of breaker size, to provide extra mechanical protection against digging and soil corrosion. Always check local amendments via your city's building department or reference resources like EC&M's NEC analysis archives.
Sizing a ground wire is not just about matching a number on a chart; it is about ensuring the fault path survives the worst-case scenario. Stick to 8 AWG copper for standard short runs, remember the proportional upsizing rule for long distances, and always verify your termination ratings before pulling wire.
