For a standard 100-amp breaker, you need an 8 AWG copper or 6 AWG aluminum equipment grounding conductor (EGC). The NEC grounding wire size chart is dictated by NEC Table 250.122, which sizes the grounding wire based strictly on the rating of the overcurrent protective device (OCPD)—the breaker or fuse—not the actual expected load current. This ensures the ground wire can safely carry fault current long enough to trip the breaker without melting.
Below is the complete reference data for residential and light commercial installations, followed by the specific rules for adjusting these sizes when voltage drop or parallel conductors enter the picture.
How to Read the NEC Grounding Wire Size Chart
Before pulling wire, you need to understand how Table 250.122 is structured. Unlike standard ampacity tables (like NEC 310.16) that rely on temperature columns (60°C, 75°C, 90°C), the grounding chart ignores insulation temperature ratings. It looks only at the ampere rating of the breaker protecting the circuit.
Columns explained:
- Overcurrent Device Rating: The amp rating printed on the breaker or fuse handle (e.g., 20A, 50A, 100A).
- Copper Wire Size: The minimum AWG or kcmil for copper conductors (THHN, THWN-2, or bare).
- Aluminum Wire Size: The minimum AWG or kcmil for aluminum or copper-clad aluminum conductors.
Quick-Jump to Most Queried Values: 20A (Standard Outlets) | 50A (Ranges/Dryers) | 100A (Subpanels) | 200A (Main Service)
| Overcurrent Device Rating (Amps) | Copper Wire Size (AWG/kcmil) | Aluminum Wire Size (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 50 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 100 | 8 | 6 |
| 150 | 6 | 4 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
Which Column Applies and How Upsizing Modifies the Base Value
The most common question on the jobsite is whether to use the copper or aluminum column. The answer depends entirely on your conductor material, which is usually dictated by the installation type.
Choose the Copper Column when:
- Running branch circuits in EMT, PVC, or flexible conduit using THHN/THWN-2.
- Using NM-B (Romex) or MC cable (the internal ground is pre-sized copper).
- Wiring 120V/240V appliance receptacles (dryers, ranges, EV chargers).
Choose the Aluminum Column when:
- Pulling large feeder conductors to a subpanel using XHHW-2 or THWN-2 aluminum (e.g., a 100A or 200A subpanel feed). Aluminum is significantly cheaper for large gauges, but you must use an anti-oxidant paste like Noalox and torque the lugs to the manufacturer's spec, as aluminum creeps under pressure over time.
The "Derating" Misconception and the Proportional Upsizing Rule
Readers often ask how bundling derating (NEC 310.15) affects the grounding wire. Equipment grounding conductors are not derated for bundling. When you have nine current-carrying conductors in a raceway and must derate the hot and neutral wires by 70%, the EGC remains the size listed in Table 250.122. The EGC only carries current during a fault, so continuous heating from bundling is irrelevant.
However, NEC 250.122(B) introduces a mandatory modification: Proportional Upsizing for Voltage Drop. If your circuit run is long and you must upsize the ungrounded (hot) conductors to prevent voltage drop, you must increase the EGC proportionally.
A 100A breaker normally requires 3 AWG copper hot wires and an 8 AWG copper EGC.
At 400 feet, 3 AWG copper yields an unacceptable 4.5% voltage drop. You calculate that you need to upsize the hot wires to 1/0 AWG copper to keep the drop under 3%.
The Math:
1/0 AWG circular mils (105,600) ÷ 3 AWG circular mils (52,620) = 2.00 ratio.
You must multiply the base EGC circular mils by 2.00.
8 AWG EGC (16,510 cmil) × 2.00 = 33,020 cmil.
Looking at standard wire sizes, 4 AWG is 41,740 cmil. Therefore, your EGC must be upsized from 8 AWG to 4 AWG copper.
For deeper code interpretations on proportional sizing, resources like EC&M Code Corner provide excellent field-tested breakdowns of inspector expectations regarding 250.122(B).
What the Chart Cannot Tell You (Edge Cases & Site Rules)
While Table 250.122 covers 90% of residential and commercial wiring scenarios, it is a minimum baseline. It does not account for physical environment, extreme fault currents, or parallel installations. Here is what the chart leaves out.
1. High Available Fault Current
Table 250.122 assumes standard breaker clearing times (usually 1 to 3 cycles) under normal fault conditions. If you are working in an industrial setting or directly adjacent to a utility transformer where the available fault current exceeds 10,000 to 22,000 amps, the thermal stress on the EGC might melt a standard-sized wire before the breaker's magnetic trip engages. In these cases, an engineer must calculate the let-through current and specify a larger EGC or a current-limiting fuse.
2. Physical Damage and Exposure
The chart gives you the electrical size, but not the mechanical protection requirements. According to NEC 250.64(B), if you are running a ground wire smaller than 6 AWG (e.g., a 10 AWG ground for a 30A circuit) and it is exposed to physical damage (like running down the surface of a block wall in a garage), it must be protected by rigid metal conduit, intermediate metal conduit, rigid PVC, or EMT. If you use 6 AWG or larger, it can be run exposed and secured directly to the surface without a raceway.
3. Parallel Conductor Installations
For massive feeders (e.g., a 600A service using parallel sets of 350 kcmil wire), you do not just pull one giant ground wire. Under NEC 250.122(F), if ungrounded conductors are run in parallel across multiple raceways or cables, the EGC must also be run in parallel in each raceway. Furthermore, each parallel EGC must be sized based on the rating of the entire OCPD, not just the fraction of current that specific conduit carries. If you have a 600A breaker split across three PVC pipes, you must pull a 1/0 AWG copper EGC in each of the three pipes.
Always verify your final wire sizing against the official NFPA 70 National Electrical Code and consult your local Authority Having Jurisdiction (AHJ). Local inspectors may have specific amendments regarding aluminum grounding conductors or minimum ground sizes for specific appliance circuits that supersede the baseline table.
