What Ground Size for 100 Amp Subpanel Feeds? (NEC Sizing Guide)

The minimum ground size for a 100 amp subpanel is 8 AWG copper or 6 AWG aluminum, protected by a 100A breaker. This is based on NEC Table 250.122. Always verify local codes, as your Authority Having Jurisdiction (AHJ) has final say.

The Core Sizing Matrix: NEC Tables 250.122 & 310.16

When pulling a feeder to a subpanel, you are managing two distinct sizing rules. The ungrounded (hot) and grounded (neutral) conductors are sized by NEC Table 310.16 based on continuous ampacity. The equipment grounding conductor (EGC) is sized by NEC Table 250.122 based on the rating of the upstream overcurrent protective device (the breaker).

Below is the reference matrix for common subpanel feeds. Notice how the ground wire size does not scale linearly with the hot wires; it scales with the breaker's fault-clearing threshold.

Source: NFPA 70 National Electrical Code, Tables 250.122 and 310.16 (75°C column).

Row Notes: For the 100A row, 3 AWG copper is rated for 100A in the 75°C column. Do not use the 90°C column for breaker sizing unless the breaker is explicitly rated for 90°C (most residential breakers are not). The 8 AWG copper ground is the absolute minimum baseline before distance or bundling adjustments.

Why 8 AWG and Not 10 AWG? The Physics of Fault Clearing

A common mistake on the jobsite is assuming the ground wire only needs to handle minor leakage currents. Under normal operation, the EGC carries exactly zero amps. Its entire purpose is to survive a catastrophic dead short long enough for the breaker to trip.

This is governed by let-through current (I²t). If a hot busbar inside the subpanel faults to the metal chassis, thousands of amps will rush back toward the source through the ground wire. A 100A breaker takes roughly one AC cycle (16.6 milliseconds) to clear a high-level magnetic fault. During that 16.6ms window, the ground wire must absorb massive thermal energy without melting.

The Fusing Current Danger: The fusing current (the point where a wire melts and vaporizes) for 10 AWG copper is roughly 300A in a fraction of a second. If you use a 10 AWG ground on a 100A breaker, a severe fault could vaporize the ground wire before the breaker's mechanical latch releases. The circuit remains open, but the subpanel chassis is now energized at 120V/240V waiting for the next person to touch it. 8 AWG copper has the thermal mass to survive the I²t let-through energy of a standard 100A molded-case breaker.

What Changes the Answer: Length, Bundling, and Material

The 8 AWG baseline assumes a standard, short run. Real-world conditions frequently force you to upsize the ground wire. Here is the decision tree for when the baseline changes.

The Voltage Drop Upsize Rule (NEC 250.122(B))

Ground wires do not carry continuous load, so they do not suffer from voltage drop. However, if your hot wires are long enough to require upsizing for voltage drop, NEC 250.122(B) mandates that the ground wire must be upsized proportionally based on circular mil area.

Worked Example: You are feeding a 100A subpanel 200 feet away.
1. Baseline hot wire: 3 AWG Copper (52,620 circular mils).
2. Voltage drop at 200 ft: ~4.08% (exceeds the 3% NEC recommendation).
3. Upsize hot wire to 2 AWG Copper (66,360 circular mils) to drop voltage to ~3.2%.
4. Calculate upsize ratio: 66,360 / 52,620 = 1.26.
5. Apply ratio to baseline 8 AWG ground (16,510 circular mils): 16,510 × 1.26 = 20,802 circular mils.
6. 8 AWG is too small. You must step up to 6 AWG Copper (26,240 circular mils) for the ground.

Bundling and Derating

When you pull more than three current-carrying conductors in a single conduit, NEC Table 310.15(C)(1) requires you to derate the ampacity of the hot and neutral wires. The ground wire is not a current-carrying conductor, so it does not derate. However, if you are forced to upsize your hot wires to compensate for bundling derating, the proportional upsizing rule from NEC 250.122(B) applies to the ground wire exactly as it does for voltage drop.

Switching to Aluminum

If you are using aluminum feeder wire to save money on a long run, your baseline ground size jumps to 6 AWG Aluminum. Aluminum has lower conductivity and a higher thermal expansion coefficient than copper. When terminating aluminum ground wires to a steel or copper busbar, you must apply an anti-oxidant compound (like Noalox) to prevent galvanic corrosion, and you must use a calibrated torque screwdriver. Aluminum creeps under pressure; a hand-tightened lug will loosen over a year of thermal cycling, creating a high-resistance fault path.

When an Engineer or AHJ Must Confirm

While NEC Table 250.122 covers 95% of residential and light commercial subpanel feeds, there are edge cases where standard tables fail and an electrical engineer or your local AHJ must review the design.

• High Available Fault Current (AFC): If your utility transformer is located right next to your main service and can deliver 40kA or more of fault current, the magnetic and thermal stresses during a short circuit are immense. Standard 8 AWG wire and 10kA-rated breakers may violently fail. An engineer must calculate the specific I²t let-through and may require a larger EGC or current-limiting fuses.
• Parallel Feeds: If your 100A load requires parallel sets of wire (rare for 100A, common for 400A+), NEC 310.10 requires a full-sized EGC in every parallel raceway. You cannot just run one ground wire in one of the conduits.
• Separate Structures: If the subpanel is in a detached garage or shed, NEC 250.32 requires a grounding electrode system (ground rods) at the second building, in addition to the EGC pulled from the main panel. The EGC and the ground rod serve entirely different purposes; one clears faults, the other stabilizes voltage to earth.

For further reading on grounding electrode systems versus equipment grounding conductors, refer to the Southwire Technical Resources guides on NEC Article 250. Always pull a permit and have your local inspector verify your specific conduit fill and termination torques before energizing the panel.