The correct 100 amp sub panel ground wire size is #8 AWG copper or #6 AWG aluminum, protected by a 100-amp, 2-pole breaker. This is dictated by NEC Table 250.122, which sizes the equipment grounding conductor (EGC) based on the overcurrent device rating, not the ungrounded (hot) conductor size.
Baseline Assumptions for This Guide
- Material: Copper (unless Aluminum is explicitly stated)
- Insulation: THHN/THWN-2 (90°C rated wire, but terminated at 75°C limits)
- Termination Rating: 75°C column (standard for 100A breakers like Square D QO2100 or Siemens Q2100 and subpanel lugs)
- Ambient Temperature: 30°C (86°F)
- Installation: Standard PVC Schedule 80 or EMT conduit, indoors or buried, with no more than 3 current-carrying conductors in the raceway.
The Baseline Sizing Rule: Why #8 AWG and Not #10?
A common mistake on the jobsite is looking at NEC Table 310.16 for the hot wires and assuming the ground wire scales identically. It does not. The ungrounded (hot) conductors for a 100A feeder are sized to carry continuous load current without overheating. For 100A, you need #3 AWG copper or #1 AWG aluminum based on the 75°C column of Table 310.16.
The equipment grounding conductor (EGC), however, has a completely different job. It sits dormant carrying zero current under normal conditions. Its only purpose is to carry a massive, instantaneous fault current (a dead short) long enough to trip the breaker. According to NFPA 70 (NEC) Article 250.122, the EGC must be sized to prevent the wire from melting before the breaker's magnetic trip mechanism clears the fault.
For a 100A breaker, the magnetic trip engages almost instantly at roughly 5 to 10 times the rated current (500A to 1000A). A #10 AWG copper wire would melt or fuse under that thermal stress before the breaker fully clears, leaving the panel enclosure energized and creating a lethal shock hazard. A #8 AWG copper wire has the exact thermal mass required to survive that fault clearing time. Therefore, #10 AWG is strictly forbidden for a 100A feeder, even though it might physically fit under the panel's ground lug.
The Voltage Drop Upsizing Trap (Distance Matters)
The #8 AWG copper rule holds true only if your hot wires are exactly #3 AWG copper. If your subpanel is located far from the main service, voltage drop forces you to upsize the hot wires. When you upsize the hots, NEC 250.122(B) requires you to proportionally upsize the ground wire.
Let’s run the math on a 150-foot run to a detached garage subpanel:
Here is how the proportional upsizing works using circular mils (cmil):
- Base Hot Size: #3 AWG = 52,620 cmil
- Upsized Hot Size: #1 AWG = 83,690 cmil
- Upsize Ratio: 83,690 / 52,620 = 1.59
- Base Ground Size: #8 AWG = 16,510 cmil
- Required Upsized Ground: 16,510 cmil × 1.59 = 26,250 cmil
Looking at standard wire tables, #6 AWG copper is 26,240 cmil. Because we are slightly short of the exact mathematical requirement, we must round up to the next standard size. Therefore, for a 150-foot run where hots are upsized to #1 AWG, your 100 amp sub panel ground wire size becomes #6 AWG copper.
Decision Tree: Material, Conduit, and Bundling
Wire sizing is never one-size-fits-all. Use this decision matrix to verify your specific installation parameters before pulling wire.
| Installation Variable | Copper Configuration | Aluminum Configuration | Code Reference & Notes |
|---|---|---|---|
| Standard Run (< 100 ft) | Hot: #3 AWG Ground: #8 AWG |
Hot: #1 AWG Ground: #6 AWG |
NEC 310.16 & 250.122. Baseline 75°C terminations. |
| Long Run (100 - 150 ft) | Hot: #1 AWG Ground: #6 AWG |
Hot: #1/0 AWG Ground: #4 AWG |
NEC 250.122(B). Proportional increase for voltage drop mitigation. |
| Conduit Fill (>3 current-carrying wires) | Hot: Upsize as needed Ground: Proportional upsize |
Hot: Upsize as needed Ground: Proportional upsize |
NEC 310.15(C)(1). Derating applies to hots, not the EGC, but conduit fill limits still apply. |
| High Ambient Temp (40°C+) | Hot: #2 AWG or #1 AWG Ground: #8 AWG (or proportional) |
Hot: #1/0 AWG Ground: #6 AWG (or proportional) |
NEC 310.15(B). Ambient correction factors reduce hot wire ampacity, forcing an upsize. Ground stays baseline unless hots are upsized. |
Note: Never mix copper and aluminum wire in the same splice or lug without using specifically rated bi-metal connectors (like AlumiConn) or applying anti-oxidant compound (Noalox) on aluminum. Standard 100A panel lugs are typically rated for aluminum (ALR), which automatically accepts copper, but always verify the lug stamping.
FAQ: Common 100 Amp Sub Panel Ground Wire Questions
Can I use a bare ground wire for a 100 amp sub panel?
Yes, a bare copper wire is fully compliant and standard practice for the equipment grounding conductor in a feeder circuit, provided it is the correct size (#8 AWG or larger for 100A). Bare wire is actually preferred by many electricians for ground wires because it eliminates the need to strip insulation at the lugs, ensuring maximum surface contact and reducing the chance of nicking the conductor. However, if you are pulling through long runs of PVC conduit with multiple tight bends, a green-insulated THHN wire will pull significantly easier due to the slick outer jacket.
Does the 100 amp sub panel ground wire size change if I use PVC conduit instead of EMT?
The wire size itself does not change based on the conduit material. Whether you use Schedule 80 PVC, rigid metal conduit (RMC), or electrical metallic tubing (EMT), the copper ground wire remains #8 AWG for a standard run. However, there is a critical distinction in how the grounding path works. If you use metal EMT or RMC with proper threaded fittings or listed grounding bushings, the metal raceway itself can serve as the EGC per NEC 250.118. If you use PVC, the PVC is an insulator, meaning you must pull a dedicated wire EGC inside the pipe. Most inspectors and best-practice guides, including those from ECM Magazine, recommend always pulling a dedicated wire ground regardless of conduit type to guarantee a low-impedance fault path.
Why is the ground wire smaller than the hot wires for a 100 amp subpanel?
This comes down to thermal mass and duty cycle. The hot wires (#3 AWG) must carry up to 100 amps continuously without the insulation degrading from heat. The ground wire carries exactly zero amps during normal operation. It only carries current during a fault (a short circuit). Because the 100A breaker will trip magnetically in milliseconds during a dead short, the #8 AWG ground wire only needs enough mass to survive that fraction of a second without vaporizing. Sizing the ground wire to match the hot wires for a standard run would be a waste of expensive copper. For deeper reading on conductor thermal properties, the Copper Development Association provides extensive data on fault-current withstand ratings for various AWG sizes.
When must an engineer or AHJ confirm my 100 amp ground wire size?
You must defer to a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ/inspector) under the following conditions:
- High Available Fault Current: If your main service has an available fault current exceeding 10,000 amps (common in newer commercial or high-density residential builds), standard breaker trip curves change, and the #8 AWG wire might not clear the fault fast enough. An engineer must calculate the specific let-through energy (I²t).
- Parallel Feeder Runs: If you are running two parallel sets of 1/0 AWG copper to achieve your 100A capacity (rare for 100A, but possible for larger panels), NEC 250.122(F) requires a full-sized EGC in each parallel raceway, not just one shared ground.
- Local Amendments: Some municipalities adopt local amendments that require all feeder grounds to be insulated (green) or mandate a minimum #6 AWG ground for all subpanels regardless of breaker size to prevent future upgrade errors. Always check your local AHJ's specific wiring bulletins before rough-in.
