The minimum ground wire size for a 100 amp sub panel is #8 AWG copper or #6 AWG aluminum, protected by a 100A 2-pole breaker. This assumes standard THHN/THWN-2 insulation, 75°C terminations, and a run under 100 feet. If your ungrounded conductors are upsized for voltage drop, the ground must scale proportionally.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C (standard for modern breakers and lugs)
- Ambient Temperature: 30°C (86°F) baseline
- Conduit Type: Schedule 80 PVC or EMT, 3 current-carrying conductors + 1 EGC
- Code Reference: NEC-style guidance (NFPA 70); your local AHJ has final authority
The Baseline Spec Sheet: 100A Feeder & Grounding
When sizing a subpanel feeder, you are actually sizing two different things: the current-carrying conductors (hots and neutral) and the Equipment Grounding Conductor (EGC). The EGC does not carry current during normal operation; it exists solely to provide a low-impedance fault path back to the source to trip the breaker during a short circuit.
According to NFPA 70 (NEC) Table 250.122, the minimum EGC size is dictated strictly by the rating of the overcurrent protective device (OCPD)—in this case, your 100A breaker. It is not dictated by the size of the hot wires, unless those hot wires are upsized.
| Conductor Role | NEC Table | Copper Size | Aluminum Size | Notes |
|---|---|---|---|---|
| Ungrounded (Hots) | 310.16 (75°C) | #3 AWG | #1 AWG | Sized for 100A continuous/non-continuous |
| Grounded (Neutral) | 220.61 / 310.16 | #3 AWG | #1 AWG | Must match hots unless load calc proves otherwise |
| Equipment Ground (EGC) | 250.122 | #8 AWG | #6 AWG | Sized based on 100A OCPD rating |
Decision Path: Why #8 AWG and When to Change It
A common mistake on the jobsite is assuming the ground wire should be the same size as the hot wires, or worse, sizing it down to save copper.
Why this size and not one smaller?
The #8 AWG requirement is rooted in fault-current physics and the let-through energy of a 100A molded case circuit breaker (MCCB). During a dead short, fault current can spike to 2,000A or more. The breaker’s magnetic trip mechanism takes roughly 1 cycle (16 milliseconds) to clear this fault. A #10 AWG or #12 AWG wire lacks the thermal mass to survive that 16ms spike; it can literally vaporize before the breaker trips. If the EGC burns open, your subpanel chassis remains energized at 120V/240V, creating a lethal shock hazard. #8 AWG copper has the exact cross-sectional area required to absorb that thermal stress and keep the fault path intact until the breaker clears.
| Condition | Action | Final Ground Wire Pick |
|---|---|---|
| Run is under 100 ft, standard 30°C ambient, copper wire | Use baseline Table 250.122 | #8 AWG Copper |
| Run is under 100 ft, using aluminum wire (XHHW-2) | Use baseline Table 250.122 | #6 AWG Aluminum |
| Run exceeds 100 ft, requiring hot wire upsizing for voltage drop | Apply proportional upsizing (250.122(B)) | #6 AWG Copper (if hots go to #2) |
| More than 3 current-carrying conductors in conduit (bundling) | Derate hots; ground does not derate but hots may upsize | Recalculate based on new hot wire size |
The Voltage Drop Trap: Proportional Upsizing
What changes the answer? Distance. If your subpanel is located 150 feet from the main service, a standard #3 AWG copper feeder will suffer unacceptable voltage drop under heavy continuous loads.
Let’s run the math for a 150-foot run carrying an 80A continuous load (a realistic scenario for a workshop subpanel running a compressor, welder, and lighting simultaneously):
- Formula: VD = (2 × K × I × D) / Circular Mils
- K (Copper): 12.9
- I (Current): 80A
- D (Distance): 150 ft
- CM (#3 AWG): 52,620
- Result: 5.88V drop.
On a 240V circuit, 5.88V is a 2.45% drop, which is acceptable. But if that subpanel feeds 120V branch circuits, that same 5.88V drop represents a 4.9% drop on the 120V leg. NEC informational note 210.19(A) recommends keeping feeder drop under 3%.
To fix this, you upsize the hot and neutral conductors to #2 AWG copper (CM = 66,360), bringing the 120V drop down to 3.8%. But here is where DIYers fail the inspection: NEC 250.122(B) requires the equipment grounding conductor to be increased in size proportionally.
If you increase the ungrounded (hot) conductors by one AWG step (from #3 to #2), you must increase the EGC by one AWG step. Your ground wire must upgrade from #8 AWG to #6 AWG copper. You cannot leave the ground at #8 AWG just because the breaker is still 100A. The increased impedance of the longer wire run requires a thicker ground to ensure the breaker still trips magnetically during a fault at the far end of the circuit.
Aluminum, Bundling, and Ambient Heat
Copper is the benchmark, but many installers use aluminum (specifically XHHW-2) for subpanel feeders to save money. If you choose aluminum, your baseline ground wire shifts to #6 AWG aluminum per Table 250.122. Never mix copper and aluminum in the same conduit run without accounting for the different expansion rates and termination requirements (use Al9CU-rated lugs and anti-oxidant paste).
What about conduit bundling? If you pull two separate 100A feeders through the same conduit (6 current-carrying conductors total), NEC Table 310.15(C)(1) requires an 80% derating factor for the hot wires. Your #3 AWG copper (100A at 75°C) derates to 80A, meaning you must upsize the hots to #2 AWG just to maintain your 100A breaker rating. Consequently, the ground wire must also step up to #6 AWG copper to match the new hot wire cross-section ratio.
Subpanel Isolation: The Ground vs. Neutral Rule
When discussing the "ground wire" for a subpanel, we must explicitly separate the Equipment Grounding Conductor (bare or green) from the Grounded Conductor (the white neutral).
In a main service panel, the neutral and ground bars are bonded together. In a subpanel, they must be strictly isolated. The neutral bar must float (isolated from the metal enclosure), while the ground bar must be bonded directly to the enclosure. The #8 AWG ground wire terminates on the bonded ground bar. The #3 AWG neutral terminates on the floating neutral bar. If you fail to remove the green bonding screw or strap in the subpanel, normal neutral return current will travel back to the main panel via both the neutral wire and the ground wire, energizing the conduit and creating a shock hazard.
When an Engineer or AHJ Must Confirm
While the rules above cover 95% of residential and light-commercial subpanel installations, there are edge cases where you must defer to a licensed professional engineer or your local Authority Having Jurisdiction (AHJ):
- High Available Fault Current: If your main service is fed by a large utility transformer with extremely low impedance, the available fault current might exceed the 10,000A or 22,000A interrupting rating of standard residential breakers. This requires specialized current-limiting breakers and potentially larger ground busbars.
- Parallel Feeds: If you are feeding a 400A+ panel using parallel sets of 100A-equivalent wires, NEC 250.122(F) has strict rules about routing the EGC in every conduit and sizing it based on the main OCPD, not the individual parallel conductors.
- Extreme Distances (>500 ft): At very long distances, the impedance of the ground wire itself can prevent the magnetic trip curve of the breaker from engaging, forcing reliance on the slower thermal trip. An engineer must calculate the exact fault loop impedance to ensure safe clearing times.
For standard runs under 150 feet, stick to the decision tree: #8 AWG copper baseline, upsize to #6 AWG copper if voltage drop forces your hots to #2 AWG, and always verify dead with a tested meter before terminating.
