For a 100 amp subpanel, the minimum ground wire (equipment grounding conductor) size is 8 AWG copper or 6 AWG aluminum, protected by a 100A 2-pole breaker. This assumes standard 75°C terminations and 30°C ambient temperature. Always size your ungrounded (hot) conductors first, as long feeder runs may force you to upsize the ground.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C (standard for modern residential breakers and lugs)
- Ambient Temperature: 30°C (86°F) baseline; no extreme heat derating applied
- Conduit Type: PVC Schedule 80 or EMT; no more than 3 current-carrying conductors in the raceway
- Code Reference: NEC 2023/2026 Article 250 (Grounding and Bonding) and Article 310 (Conductors)
The Baseline Sizing: NEC Table 250.122 Explained
The National Electrical Code (NEC) does not size the equipment grounding conductor (EGC) based on the continuous load of the circuit. Instead, it sizes the ground based on the rating of the overcurrent protective device (the breaker). The ground wire’s sole job is to carry fault current—thousands of amps for a fraction of a second—long enough for the breaker to trip without melting.
According to NFPA 70 (NEC) Table 250.122, the minimum EGC sizes for common residential and light commercial subpanel feeders are as follows:
| Overcurrent Device Rating (Breaker) | Copper EGC (Minimum) | Aluminum EGC (Minimum) | Standard Hot Conductor Size (75°C) |
|---|---|---|---|
| 60 Amps | 10 AWG | 8 AWG | 6 AWG Cu / 4 AWG Al |
| 100 Amps (Target) | 8 AWG | 6 AWG | 3 AWG Cu / 1 AWG Al |
| 125 Amps | 6 AWG | 4 AWG | 1 AWG Cu / 1/0 AWG Al |
| 200 Amps | 6 AWG | 4 AWG | 2/0 AWG Cu / 4/0 AWG Al |
Why 8 AWG and Not One Size Smaller?
You might wonder why a 100A circuit requires 8 AWG for the ground, while a 60A circuit only needs 10 AWG. The answer lies in let-through current and thermal mass. During a dead-bolt short circuit, a 100A breaker will allow roughly 10,000 to 14,000 amps of fault current to flow for the 1 to 2 cycles (16-32 milliseconds) it takes for the magnetic trip mechanism to physically open the contacts. A 10 AWG wire subjected to that specific energy spike would vaporize or melt its insulation before the breaker clears the fault, leaving the subpanel chassis energized at line voltage. An 8 AWG copper wire possesses the exact cross-sectional mass required to absorb that thermal spike and maintain circuit integrity until the breaker trips.
In a subpanel, the grounded conductor (neutral) and the equipment grounding conductor (ground) must remain physically isolated. Do not bond the neutral busbar to the panel chassis. Only the main service disconnect panel gets a main bonding jumper. If you bond neutral and ground at a subpanel, normal neutral return current will flow on your 8 AWG ground wire, conduit, and plumbing, creating a severe shock hazard.
When to Upsize: Voltage Drop and Proportional Grounding
The 8 AWG copper baseline only applies if your ungrounded (hot) conductors are sized at their standard ampacity (3 AWG copper for 100A). However, NEC 250.122(B) introduces a critical exception: Proportional Upsizing.
If your subpanel is located far from the main panel, you must upsize the hot conductors to prevent excessive voltage drop (NEC recommends keeping feeder drop under 2%). When you increase the circular mil area of the hot wires to compensate for distance, you must increase the ground wire proportionally.
The Proportional Upsizing Math (150-Foot Run Example)
Let’s say you are feeding a 100A subpanel in a detached garage 150 feet away. Standard 3 AWG copper hots will result in a 3.8% voltage drop at full 80A continuous load, which is too high. You decide to upsize the hots to 1 AWG copper to bring the drop down to 1.5%.
Here is how you calculate the new ground size based on EC&M's interpretation of NEC 250.122(B):
- Original Hot Size: 3 AWG Cu = 52,620 circular mils (cmil)
- Original Ground Size: 8 AWG Cu = 16,510 cmil
- Ratio: 16,510 / 52,620 = 0.313
- New Hot Size: 1 AWG Cu = 83,690 cmil
- New Ground Requirement: 83,690 × 0.313 = 26,194 cmil
Looking at NEC Chapter 9, Table 8, the next standard wire size that meets or exceeds 26,194 cmil is 6 AWG copper (26,240 cmil). Therefore, your long-distance 100A feeder requires 1 AWG hots, 1 AWG neutral, and a 6 AWG ground.
| Feeder Scenario | Hot Wire Size (Cu) | Required Ground (Cu) | Triggering Factor |
|---|---|---|---|
| Standard Run (< 75 ft) | 3 AWG | 8 AWG | Baseline Table 250.122 |
| Medium Run (75 - 110 ft) | 2 AWG | 8 AWG | VD mitigation (upsizing one step doesn't always trigger ground jump) |
| Long Run (110 - 160 ft) | 1 AWG | 6 AWG | Proportional upsizing per 250.122(B) |
| 4+ Conductors in Conduit | 2 AWG or 1 AWG | 8 AWG or 6 AWG | Derating applies to hots, not ground; but ground follows hot upsizing |
Material Swaps, Bundling, and AHJ Triggers
While copper is the default for most residential subpanel feeders, aluminum is frequently used for cost savings on longer runs. If you switch to aluminum, the rules shift slightly, and you must be hyper-aware of termination torque and oxidation.
Aluminum vs. Copper Grounding Rules
Never present aluminum and copper interchangeably. Aluminum has lower conductivity and a higher coefficient of thermal expansion. For a 100A breaker, Table 250.122 mandates a 6 AWG aluminum ground. Furthermore, if you are running aluminum feeders (like 1-1-1-2 SER cable), the ground wire inside that cable assembly is already sized correctly by the manufacturer. Do not attempt to splice a copper ground wire to an aluminum ground wire inside the panel without using a proper bi-metallic lug or split bolt with anti-oxidant paste, as galvanic corrosion will increase resistance and defeat the fault-clearing path.
Does Conduit Bundling Affect the Ground?
A common bench question is whether conduit fill and bundling derating (NEC 310.15) affects the ground wire. The short answer is no. The equipment grounding conductor is not counted as a current-carrying conductor when applying derating factors for conduit fill, because it carries zero current under normal operation. However, if bundling forces you to upsize your hot conductors to maintain ampacity, you are back to the proportional upsizing rule (250.122(B))—meaning the ground must grow in proportion to the new hot wire size.
When an Engineer or AHJ Must Confirm
While the rules above cover 95% of residential and light commercial 100A subpanel installations, you must defer to a licensed professional engineer or your local Authority Having Jurisdiction (AHJ) in the following edge cases:
- High Available Fault Current: If your utility transformer supplies more than 10,000 Amps of available short-circuit current at the service entrance, standard Table 250.122 sizing may be insufficient to handle the let-through energy before the breaker trips. An engineer must perform a fault current study.
- Parallel Feeders: If you are running multiple sets of conductors in parallel to achieve 100A (rare for 100A, common for 400A+), NEC 250.122(F) requires a full-sized equipment grounding conductor in every parallel raceway, not just one.
- Metal Conduit as EGC: If you are using rigid metal conduit (RMC) or EMT as the actual equipment grounding conductor (per NEC 250.118), the AHJ must verify that all couplings and set-screw fittings are properly tightened to ensure a low-impedance fault path. Many local jurisdictions now mandate a separate wire EGC inside the conduit regardless of the raceway type.
By starting with the 8 AWG copper baseline, verifying your voltage drop over the specific run distance, and applying proportional upsizing where required, you ensure your 100A subpanel will clear faults safely and pass inspection on the first attempt.
