The size of your grounding wire is determined strictly by the rating of the overcurrent protective device (the breaker or fuse), not the actual calculated load of the circuit. For a standard residential 100-amp feeder using copper, you need an 8 AWG equipment grounding conductor (EGC). For a 200-amp copper feeder, you need a 4 AWG EGC. This grounding wire size table provides the exact, code-compliant AWG picks for both copper and aluminum installations based on NEC Article 250.122.
How to Read the NEC Grounding Wire Size Table
Before pulling wire, you must understand the three columns in the master table below. The first column lists the standard breaker or fuse ampere ratings (15A through 400A). The second column provides the minimum AWG or kcmil size for copper conductors, and the third column provides the minimum size for aluminum or copper-clad aluminum conductors.
The most critical rule to internalize: this table sizes the Equipment Grounding Conductor (EGC)—the wire that runs alongside your hot and neutral wires back to the panel to clear faults. It does not size the Grounding Electrode Conductor (GEC), which is the wire that connects your main panel to the physical earth (ground rods or ufers). Sizing the GEC requires a completely different chart (NEC Table 250.66).
The Master Grounding Wire Size Table (NEC 250.122)
Below is the complete reference chart derived directly from the National Fire Protection Association's National Electrical Code (NFPA 70), specifically Table 250.122. Use the bookmark-friendly anchors for the most common residential and light-commercial breaker sizes.
| Breaker/Fuse Rating (Amps) | Copper EGC (AWG/kcmil) | Aluminum EGC (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 50 | 10 | 8 |
| 60 | 10 | 8 |
| 70 | 8 | 6 |
| 80 | 8 | 6 |
| 90 | 8 | 6 |
| 100 (Quick Jump) | 8 | 6 |
| 110 | 6 | 4 |
| 125 | 6 | 4 |
| 150 | 6 | 4 |
| 175 | 4 | 2 |
| 200 (Quick Jump) | 4 | 2 |
| 225 | 3 | 1 |
| 250 | 3 | 1 |
| 300 | 2 | 1/0 |
| 350 | 1 | 2/0 |
| 400 | 1/0 | 3/0 |
Decision Path: Sizing Your Equipment Grounding Conductor (EGC)
Follow this decision tree to lock in your exact wire size without second-guessing the math. This path terminates in a concrete pick for standard branch circuits and feeders.
| Step | Condition / Question | Action / Result |
|---|---|---|
| 1 | What is the rating of the breaker protecting this circuit? | Identify the row in the master table above (e.g., 60A). |
| 2 | Are your ungrounded (hot) conductors Copper or Aluminum? | Select the corresponding column. (e.g., 60A Copper = 10 AWG). |
| 3 | Did you up-size the hot conductors to compensate for voltage drop over a long distance? | If NO: Stop here. Buy the wire from Step 2. If YES: Proceed to Step 4. |
| 4 | Calculate the circular mil ratio of your up-sized hot wire vs. the minimum required hot wire. | Multiply the base EGC circular mils by this ratio. Round up to the next standard AWG. (See math example below). |
Derating, Up-Sizing, and Parallel Runs
A common jobsite mistake is up-sizing the hot and neutral wires for a long feeder run to mitigate voltage drop, but leaving the grounding wire at the base table size. NEC 250.122(B) explicitly forbids this. If you increase the size of your ungrounded conductors, you must proportionally increase the size of the equipment grounding conductor based on the ratio of their circular mil areas.
Worked Numeric Example:
You are running a 100-amp copper feeder to a detached garage 200 feet away. To keep voltage drop under 3%, you up-size your hot conductors from the minimum 3 AWG (required for 100A at 75°C) to 1 AWG copper.
- Base EGC (from table): 8 AWG copper (16,510 circular mils).
- Hot Wire Ratio: 1 AWG (83,690 cmil) ÷ 3 AWG (52,620 cmil) = 1.59.
- Required EGC cmil: 16,510 × 1.59 = 26,250 circular mils.
- Final Pick: 6 AWG copper has 26,240 cmil (slightly under, so we must step up) → Use 4 AWG copper (41,740 cmil) for your EGC.
For parallel conductor runs (typically 400A and above), NEC 250.122(F) requires an EGC in every parallel raceway or cable tray, and each EGC must be sized based on the breaker rating, not divided among the runs.
What This Table Cannot Tell You (Edge Cases & Limits)
While Table 250.122 covers 95% of residential and commercial wiring scenarios, it has strict boundaries. Do not use this table for the following situations:
- Grounding Electrode Conductors (GEC): If you are connecting your main service panel to a ground rod, metal water pipe, or ufer ground, you are sizing a GEC, not an EGC. You must use NEC Table 250.66, which sizes the wire based on the largest ungrounded service entrance conductor, not the breaker.
- Motor Circuits: Motors have high inrush currents and use specialized overcurrent protection (like inverse-time breakers sized at 250% of full load amps). For motor circuits, the EGC is sized based on the motor's full-load current and the specific overload protection device, governed by NEC Article 430, not the standard breaker table.
- Aluminum to Copper Terminations: If you are using aluminum feeder wire but terminating onto a copper ground bar, ensure you are using an anti-oxidant compound (like Noalox) and torque the lug to the manufacturer's exact inch-pound specification. Aluminum creeps under pressure and will loosen over time if not torqued correctly.
Final Recommendation: When your calculations fall between two AWG sizes, or when dealing with high-ambient temperature derating, always default to the next standard AWG size up. Buy a spool of 6 AWG bare copper for general 60A-100A subpanel feeds and 4 AWG for 200A services to ensure you never fail an inspection due to an undersized fault-clearing path.
