For a 100 amp sub panel protected by a 100A breaker, the minimum ground wire (equipment grounding conductor) size is 8 AWG copper or 6 AWG aluminum. This assumes standard 75°C terminations and a 30°C ambient environment. Always verify local codes, as your local Authority Having Jurisdiction (AHJ) has final authority over all installations.
- Conductor Material: Copper (unless aluminum is explicitly stated)
- Termination Rating: 75°C column (standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Raceway (PVC/EMT conduit) or direct burial
- Insulation Type: THHN/THWN-2 or XHHW-2
- Scope: Equipment Grounding Conductor (EGC) routed with the feeder. This guide does not cover the Grounding Electrode Conductor (GEC) connecting to ground rods at a detached structure.
The NEC Baseline: Why 8 AWG Copper?
The size of your equipment grounding conductor (EGC) is not determined by the continuous load of the subpanel, but by the size of the overcurrent protection device (the breaker) feeding it. According to NEC Table 250.122, a 100-ampere breaker requires a minimum 8 AWG copper EGC.
Why not one smaller (like 10 AWG)?
The EGC has one job: provide a low-impedance fault current path to trip the breaker instantly during a short circuit. A 100A breaker relies on its magnetic trip mechanism to clear high-current faults, typically requiring 5 to 10 times the rated current (500A to 1000A) to trip in milliseconds. If you use a 10 AWG wire, its fusing current (the point where it melts) is roughly 300A to 400A over a short duration. If a high-impedance fault limits the short-circuit current to 350A, a 10 AWG wire will melt and open the ground path before the breaker trips. This leaves the subpanel chassis energized at line voltage—a lethal shock and fire hazard. 8 AWG copper has the thermal mass to survive the fault long enough for the breaker's magnetic latch to clear the circuit.
When the Answer Changes: Voltage Drop and Upsizing
The 8 AWG baseline assumes your ungrounded (hot) conductors are sized exactly to the breaker's ampacity. However, if your subpanel is located far from the main panel, you must upsized the hot wires to prevent voltage drop. When you upsize the hots, NEC 250.122(B) mandates that you must upsize the ground wire proportionally.
Let us look at a concrete jobsite example. You are feeding a 100A subpanel 150 feet away. To keep voltage drop under 3%, you upsize your copper hot wires from the baseline 3 AWG to 1 AWG.
| Parameter | Baseline (NEC 310.16) | Upsized for 150ft Run |
|---|---|---|
| Hot Conductor Size | 3 AWG (52,620 cmil) | 1 AWG (83,690 cmil) |
| Upsizing Ratio | 1.0 | 1.59 (83,690 / 52,620) |
| Base EGC (8 AWG) | 16,510 cmil | 16,510 cmil |
| Required EGC cmil | 16,510 cmil | 26,258 cmil (16,510 x 1.59) |
| Final EGC Size | 8 AWG | 4 AWG (See note below) |
Notice the required EGC is 26,258 circular mils (cmil). If you look at NEC Chapter 9, Table 8, a 6 AWG wire is 26,240 cmil. You are exactly 18 cmil short. While some lenient inspectors might pass 6 AWG, a strict AHJ will fail the inspection and force you to pull 4 AWG (41,740 cmil). Always round up to the next standard AWG size when doing proportional upsizing math.
Aluminum vs. Copper Feeder Rules
Aluminum and copper are never interchangeable at the same AWG size due to differences in conductivity and thermal expansion. If you are using aluminum feeder cables (like SER or USE-2) to save money on long runs, your minimum ground wire changes.
- Copper EGC: 8 AWG minimum.
- Aluminum EGC: 6 AWG minimum.
If you choose aluminum, you must apply an anti-oxidant compound (like Noalox) to the conductor ends before termination to prevent aluminum oxide buildup, which increases resistance and causes heat. Furthermore, aluminum expands and contracts more than copper under thermal cycling. You must use a calibrated torque screwdriver to tighten the panel lugs to the manufacturer's exact inch-pound specification to prevent the connection from loosening over time.
Complete 100A Subpanel Feeder Sizing Chart
To provide full context, here is the complete feeder specification for a standard 100A subpanel over a short distance (under 50 feet, where voltage drop is negligible). This references the 75°C column of NEC Table 310.16.
| Conductor Role | Copper AWG | Aluminum AWG | Color Code (Standard) |
|---|---|---|---|
| Ungrounded (Hot A) | 3 AWG | 1 AWG | Black |
| Ungrounded (Hot B) | 3 AWG | 1 AWG | Red |
| Grounded (Neutral) | 3 AWG | 1 AWG | White or Gray |
| Equipment Ground (EGC) | 8 AWG | 6 AWG | Bare or Green |
Note: If your calculated continuous load is strictly under 80A, some electricians use 4 AWG copper hots (rated 85A at 75°C), but the ground wire remains dictated by the physical breaker size installed in the main panel.
Frequently Asked Questions
Can I use a 10 AWG ground wire for a 100 amp sub panel if the run is very short?
No. NEC Table 250.122 establishes the absolute minimum EGC size based strictly on the rating of the overcurrent device (the breaker), regardless of the physical length of the run. Even if the subpanel is mounted directly next to the main panel and requires only 2 feet of wire, a 100A breaker demands the fault-current capacity of an 8 AWG copper wire. Using 10 AWG is a code violation and a safety hazard.
Does the ground wire for a 100 amp sub panel need to be in the same conduit as the hots?
Yes. NEC 300.3(B) and 250.134 require the equipment grounding conductor to be routed in the same raceway, cable, or trench as the ungrounded and grounded (neutral) conductors. Keeping all current-carrying conductors and the EGC together ensures their magnetic fields cancel each other out during normal operation. If the ground is routed separately, the alternating magnetic field will induce eddy currents in surrounding metal, causing severe inductive heating and potentially starting a fire. It also keeps the fault loop impedance as low as possible.
What size ground wire do I need for a 100 amp sub panel 200 feet away?
For a 200-foot run, voltage drop becomes a major factor. Using a standard voltage drop calculator, feeding a full 100A load at 240V over 200 feet requires upsizing your copper hot wires to at least 1/0 AWG to stay under a 3% drop. Because 1/0 AWG (105,600 cmil) is roughly double the cross-sectional area of the baseline 3 AWG (52,620 cmil), you must apply the proportional upsizing rule. Your 8 AWG ground must also be doubled in area, requiring you to pull a 3 AWG or 2 AWG copper ground wire alongside your 1/0 AWG hots. Always run the exact math based on your actual calculated load, not just the breaker size.
When must a licensed engineer or the AHJ confirm my ground wire size?
You must involve a licensed electrical engineer or schedule a special consultation with your local AHJ inspector under the following conditions:
- Parallel Feeders: If you are running multiple sets of conductors in parallel to achieve 100A (rare for this size, but common for 400A+), each raceway requires its own EGC sized to the individual breaker, not the total system.
- High Fault Current Availability: If your utility transformer supplies massive available fault current (e.g., >42,000 amps), standard thermal limits in Table 250.122 might be insufficient, requiring engineered upsizing to prevent the wire from vaporizing before the breaker clears.
- Detached Structures: If the subpanel is in a detached building, you need a Grounding Electrode Conductor (GEC) to connect to local ground rods. While the EGC is 8 AWG, the GEC sizing falls under NEC 250.66 and depends on the size of the largest ungrounded feeder conductor, often capping out at 8 AWG copper for a 100A feeder, but local soil resistivity may require an engineered grounding array.
