The size of your equipment grounding conductor (EGC) is dictated entirely by the rating of the overcurrent protective device (breaker or fuse), not the size of the circuit wires. If you are looking for the exact grounding conductor size table, you need NEC Table 250.122. A 20A breaker requires a minimum 12 AWG copper ground, while a 60A breaker requires 10 AWG copper. Sizing the ground wire based on the hot wire's ampacity is a common DIY mistake that leads to failed inspections and unsafe fault-clearing times.
The Master Grounding Conductor Size Table (NEC 250.122)
This table defines the minimum size for the Equipment Grounding Conductor (EGC)—the bare copper or green-insulated wire that runs alongside your hot and neutral wires to clear faults. Note: This is not the Grounding Electrode Conductor (GEC) that connects your panel to a ground rod; that is covered under NEC 250.66.
| Overcurrent Device Rating (Amps) | Copper (AWG/kcmil) | Aluminum (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 110 | 8 | 6 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
How to Read the Table and Pick Your Column
The most common point of confusion for DIYers and junior apprentices is looking for a temperature column (60°C, 75°C, 90°C) like you would when sizing hot conductors under NEC 310.16. There is no temperature column for EGC sizing. The grounding conductor does not carry current under normal operation; it only carries fault current for a fraction of a second until the breaker trips. Therefore, its size is based strictly on the magnetic/thermal trip rating of the breaker protecting the circuit.
Which material column applies to you?
- Copper Column: Use this for 99% of residential and commercial branch circuits. If you are pulling NM-B (Romex), THHN in conduit, or MC cable, you are using copper.
- Aluminum Column: Use this only if you are specifically pulling aluminum SER cable, XHHW-2 aluminum feeders, or using aluminum busbars where the EGC is explicitly pulled as a separate aluminum wire. Never mix copper and aluminum in the same grounding path without listed bimetallic lugs.
Decision Path: Sizing Your EGC in 4 Steps
Use this decision tree to arrive at your exact wire size without guessing.
| Step | Condition | Action / Result |
|---|---|---|
| 1 | Is this a branch circuit or feeder protected by a single breaker? | YES: Go to Step 2. NO (Parallel runs): See "When the Table Fails" below. |
| 2 | What is the breaker rating? (e.g., 50A) | Locate 50A (falls under 60A row). Base size is 10 AWG Cu. |
| 3 | Did you upsize the hot wires for voltage drop? | NO: Use base size (10 AWG). YES: Go to Step 4. |
| 4 | Calculate proportional increase (Circular Mil ratio). | Upsize ground by the exact same percentage. (e.g., Hot wire doubled = Ground wire doubles). |
When the Table Fails: Derating, Upsizing, and Parallel Runs
The NEC 250.122 table is robust, but it cannot tell you everything. Here is where the base table requires manual modification based on jobsite conditions.
1. Ambient Temperature Derating Does NOT Apply
Because the EGC is not a current-carrying conductor under normal load, you do not apply the ambient temperature correction factors from NEC 310.15(B) to the ground wire. If you are running THHN through a 110°F attic, your hot wires might need to be upsized, but your ground wire stays exactly the size listed in Table 250.122.
2. Voltage Drop Upsizing (The Proportional Rule)
This is the most frequently botched calculation in feeder runs. If your hot wires are upsized to compensate for voltage drop over a long distance, NEC 250.122(B) mandates that the EGC must be upsized proportionally based on circular mil area.
Worked Example: You are running a 100A subpanel feeder 200 feet. To keep voltage drop under 3%, you upsize your hot wires from 3 AWG Copper (41,740 cmil) to 1/0 AWG Copper (105,600 cmil).
The ratio is 105,600 / 41,740 = 2.53.
The base ground for 100A is 8 AWG (16,510 cmil).
16,510 x 2.53 = 41,770 cmil.
Looking at Chapter 9 Table 8, you must pull a 3 AWG Copper ground wire, not the standard 8 AWG.
3. What the Table Cannot Tell You: GEC vs. EGC
If you are sizing the wire that connects your main service panel to a ground rod, water pipe, or ufer ground, stop. You are looking for a Grounding Electrode Conductor (GEC), which is sized using NEC Table 250.66, not 250.122. Table 250.122 is strictly for the equipment ground that travels inside the conduit or cable jacket to your outlets, disconnects, and appliances.
Quick-Jump Reference for the Most Common Circuits
Bookmark this section for your most frequent residential and light-commercial pulls. These assume standard copper THHN or NM-B wire with no voltage drop upsizing.
- 15A Lighting/Receptacle Circuit: 14 AWG Cu (Standard 14/2 NM-B handles this natively).
- 20A Kitchen/Bath/Appliance Circuit: 12 AWG Cu (Standard 12/2 NM-B handles this natively).
- 30A Dryer / RV Receptacle: 10 AWG Cu (10/3 NM-B or 3x 10 AWG THHN + 10 AWG ground).
- 40A Oven / Cooktop: 10 AWG Cu (Do not use 12 AWG, even though the hot wires are 8 AWG).
- 50A Hot Tub / Welder / Range: 10 AWG Cu.
- 60A EV Charger / Subpanel Feeder: 10 AWG Cu.
- 100A Subpanel Feeder: 8 AWG Cu.
- 200A Main Service / Subpanel: 6 AWG Cu (or 4 AWG if using aluminum SER cable for the whole run).
For deeper reading on grounding and bonding mechanics, the Electrical Contractor Magazine (ECM) NEC archives provide excellent field-tested scenarios on fault-current clearing times. Ultimately, when in doubt on a standard residential branch circuit under 60A, 10 AWG copper is your universal baseline that covers almost every 120V/240V appliance run without requiring proportional math or special ordering.
