What Size Ground Wire for 100 Amp Service: NEC Sizing & Upsizing Rules

For a standard 100-amp service protected by a 100A breaker, the minimum equipment grounding conductor (EGC) size is 8 AWG copper or 6 AWG aluminum. This is dictated by NEC Table 250.122. However, if your feeder run exceeds 50 feet, voltage drop rules will force you to upsize both the hot wires and this ground wire.

The Baseline: NEC Table 250.122 and the 8 AWG Rule

Many DIYers assume that because a ground wire carries zero current during normal operation, they can use a thin 10 AWG or 12 AWG wire to save money. This is a critical misunderstanding of circuit physics. The equipment grounding conductor (EGC) does not carry load; it carries fault current.

If a hot wire breaks loose and touches the metal chassis of your panel or an appliance, the ground wire must provide a low-impedance path back to the source. This massive, instantaneous surge of current (often thousands of amps) is what triggers the magnetic trip mechanism inside your 100A breaker, snapping it open in milliseconds.

Why 8 AWG and not 10 AWG? A 10 AWG copper wire is only rated as an EGC for breakers up to 60A. On a 100A breaker, the magnetic trip threshold is higher, and the available fault energy is greater. If you use 10 AWG on a 100A service, the wire's impedance might be high enough to limit the fault current, delaying the breaker's trip. Worse, the 10 AWG wire could melt or vaporize before the breaker clears, leaving the panel chassis energized at 120V/240V. According to NFPA 70 (NEC) Table 250.122, 8 AWG copper is the absolute minimum to safely clear a fault on a 100-amp overcurrent device.

The Voltage Drop Trap: When 8 AWG Becomes Illegal

The most common mistake in feeder sizing is ignoring NEC 250.122(B). This rule states that if you increase the size of your ungrounded (hot) conductors to compensate for voltage drop, you must proportionately increase the size of your equipment grounding conductor.

Let’s run a real-world scenario. You are feeding a 100A subpanel located 200 feet from the main service. Your calculated continuous load is 80A.

The Math: Proportional Upsizing

Using the base 3 AWG copper hot wire (rated 100A at 75°C), the voltage drop over 200 feet at 80A is roughly 3.26% on a 240V circuit. Because the NEC recommends keeping feeder voltage drop under 3%, we must upsize the hot wires to 2 AWG copper, which drops the VD to a safe 2.58%.

Now, we apply the 250.122(B) ratio:

1. Base Hot Wire Area (3 AWG): 52,620 circular mils (cmil)
2. Upsized Hot Wire Area (2 AWG): 66,360 cmil
3. Upsize Ratio: 66,360 / 52,620 = 1.26
4. Base Ground Wire Area (8 AWG): 16,510 cmil
5. Required Ground Area: 16,510 × 1.26 = 20,802 cmil

Since 20,802 cmil is larger than 8 AWG (16,510) but smaller than 6 AWG (26,240), the code mandates you step up to the next standard size. Therefore, for a 200-foot run, your ground wire must be 6 AWG copper. Failing to upsize the ground wire when you upsize the hots is a frequent reason for failed electrical inspections.

Material Swaps, Bundling, and Derating Edge Cases

The baseline assumptions change the moment you alter the physical installation environment or the conductor material. Here is how those variables impact your ground wire sizing.

Aluminum vs. Copper

If you are using aluminum feeder wires (such as 1 AWG Al for the hots), your baseline EGC must be 6 AWG aluminum. You cannot mix aluminum and copper in the same raceway without specific precautions, and you must never terminate an aluminum ground wire directly to a copper bus bar without using bimetallic lugs and an anti-oxidant compound like Noalox. Aluminum creeps and oxidizes, which increases resistance at the termination point—exactly what you want to avoid on a fault-clearing path.

The Bundling Derating Domino Effect

Under NEC 310.15(C)(1), the equipment grounding conductor is not counted as a current-carrying conductor (CCC) when applying adjustment factors for bundled wires in a conduit. If you pull four hots and a neutral through a single conduit, the ground wire itself does not require derating.

However, the hot wires do. If bundling forces you to derate the ampacity of your hot wires, you will be forced to upsize the hot wires to maintain your 100A rating. And as we proved in the voltage drop section, upsizing the hot wires triggers NEC 250.122(B), forcing you to upsize the ground wire proportionately. The ground wire doesn't derate, but it still gets bigger because of the hots.

High Fault Current and AHJ Sign-Off

There is one scenario where NEC Table 250.122 is not enough: high available fault current at the service entrance. Standard residential breakers have an Ampere Interrupting Capacity (AIC) of 10,000 amps (10kAIC). If your utility company has recently upgraded the neighborhood transformer, the available fault current at your meter might be 22,000 or even 42,000 amps.

In a high-fault environment, a standard 100A breaker might let through enough thermal energy ($I^2t$) during the milliseconds it takes to trip to melt an 8 AWG or even 6 AWG ground wire. When the available fault current exceeds 10kA, you must install breakers with higher AIC ratings (e.g., 22kAIC or 42kAIC) and potentially parallel your grounding conductors or upsize the EGC beyond standard table requirements to handle the thermal let-through.