The ground wire (Equipment Grounding Conductor) provides a low-impedance fault path to trip the breaker during a short circuit. For a standard 20A breaker, you must use a 12 AWG copper ground wire. It does not carry normal load current, but its sizing is strictly tied to the overcurrent protective device rating.
- Material: Copper conductors (unless aluminum is explicitly stated).
- Temperature: 75°C termination ratings (standard for modern breakers and receptacles).
- Ambient: 30°C (86°F) baseline environment.
- Raceway: EMT (metal) or PVC conduit, or standard NM-B cable assemblies.
The Core Purpose: Fault Current vs. Load Current
To understand what the ground wire is for, you have to separate normal operation from failure modes. The ungrounded (hot) and grounded (neutral) conductors carry your everyday load current. The Equipment Grounding Conductor (EGC) sits idle 99.9% of the time. Its sole job is to provide a dedicated, ultra-low-impedance highway for fault current if a hot wire touches a metal appliance chassis or junction box.
Think of it like a pressure relief valve on a boiler. The valve doesn't handle normal steam flow; it only opens when pressure exceeds safe limits to prevent an explosion. In electrical terms, when a hot wire shorts to a metal case, the ground wire channels that massive surge of current back to the panel. This sudden spike in current (often hundreds of amps) forces the breaker's magnetic trip mechanism to snap open in milliseconds, clearing the fault before the metal case becomes lethal to anyone who touches it.
If the ground wire is too small, its impedance will be too high. High impedance restricts fault current, meaning the breaker might not trip fast enough—or at all—leaving the appliance chassis energized at 120V or 240V. This is why we never guess on ground wire sizing.
Sizing the Equipment Grounding Conductor (EGC)
Unlike hot and neutral wires, which are sized based on the continuous load and ampacity tables, the minimum ground wire size is dictated entirely by the rating of the overcurrent protective device (the breaker or fuse) upstream. This is codified in NEC Article 250.122.
Before looking at the ground table, we must verify the circuit conductors. According to NEC Table 310.16, a 12 AWG THHN copper conductor has an ampacity of 30A in the 90°C column. However, because standard breakers and receptacles are rated for 75°C terminations, we are limited to the 75°C column, which restricts 12 AWG copper to 20A. Therefore, a 20A breaker protects a 12 AWG circuit.
| Breaker / Fuse Rating (Amps) | Minimum Copper EGC Size (AWG) | Minimum Aluminum EGC Size (AWG) |
|---|---|---|
| 15A | 14 AWG | 12 AWG |
| 20A | 12 AWG | 10 AWG |
| 30A | 10 AWG | 8 AWG |
| 40A | 10 AWG | 8 AWG |
| 60A | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
Why this size and not one smaller? A 12 AWG copper wire has enough physical mass and low enough resistance to allow the massive magnetic inrush of a short circuit to pass through it without melting before the breaker trips. If you used a 14 AWG ground on a 20A breaker, the wire could literally vaporize inside the wall during a dead short before the breaker's mechanical parts could physically separate, resulting in an arc fire.
The Voltage Drop Trap: Proportional Upsizing
Here is where even experienced DIYers and some journeymen make a critical error. The ground wire does not carry normal load, so it doesn't suffer from voltage drop. However, the hot and neutral wires do. If you have to upsize your hot and neutral wires to mitigate voltage drop, you must proportionally upsize the ground wire as well per NEC 250.122(B).
Worked Example: Let's run a 120V, 20A circuit to a workshop outlet 150 feet away from the panel.
- Standard Setup: 12 AWG copper hot, neutral, and ground.
- Voltage Drop Check: Using the standard formula (VD = 2 × K × I × D / CM), a 150-foot run at 20A on 12 AWG copper yields a voltage drop of roughly 3.6%. This exceeds the NEC-recommended 3% maximum for branch circuits.
- The Fix: You upsize the hot and neutral to 8 AWG copper to bring the voltage drop down to an acceptable 1.4%.
- The Grounding Rule: Because you increased the ungrounded conductor from 12 AWG (6,530 circular mils) to 8 AWG (16,510 circular mils)—an increase of roughly 2.5 times—you must increase the ground wire by the same ratio. The standard ground for 20A is 12 AWG. Upsizing it proportionally means your new ground wire must be 8 AWG copper.
If you leave a 12 AWG ground in that conduit with 8 AWG hots, a fault at the far end of the 150-foot run will see too much resistance in the ground wire. The fault current will be choked, the breaker might delay tripping, and the metal chassis of your table saw could remain energized long enough to be fatal.
Decision Tree: Picking the Right Ground Wire AWG
Use this decision path to lock in your exact ground wire size for any standard residential or commercial branch circuit.
| Condition / Scenario | Action Required | Final Concrete Pick (for 20A Circuit) |
|---|---|---|
| Standard run (< 50 ft), no voltage drop issues, copper wire. | Match ground to NEC 250.122 minimum for breaker size. | 12 AWG Bare Copper |
| Long run (> 75 ft), hot/neutral upsized by one AWG step for voltage drop. | Upsize ground wire by one AWG step proportionally. | 10 AWG Bare Copper |
| Long run (> 125 ft), hot/neutral upsized by two AWG steps. | Upsize ground wire by two AWG steps proportionally. | 8 AWG Bare Copper |
| Using Aluminum wire for the entire circuit (hot, neutral, ground). | Use the Aluminum column in NEC 250.122 (requires larger diameter). | 10 AWG Aluminum |
Edge Cases: Aluminum, Bundling, and AHJ Sign-Off
While copper is the standard for branch circuits, aluminum changes the math. Aluminum has higher resistance than copper. If you are pulling aluminum SER cable for a subpanel feeder or using aluminum branch circuit wire (rare but legal), you must use the aluminum column in Table 250.122. For a 20A breaker, the minimum aluminum ground is 10 AWG, not 12 AWG. Never interchange copper and aluminum sizing charts; doing so will result in an undersized fault path.
What about bundling and conduit fill? If you are pulling multiple circuits through a single conduit, you must apply NEC 310.15(C)(1) derating factors to your current-carrying conductors (hots and neutrals). If bundling forces you to upsize your hot wires from 12 AWG to 10 AWG just to maintain a 20A ampacity after derating, you do not need to upsize the ground wire. The ground only upsizes for voltage drop mitigation, not for thermal ampacity derating, because the ground does not carry continuous load and does not generate heat under normal operation.
When an Engineer or the AHJ Must Confirm
You can confidently use the tables above for standard residential and light commercial branch circuits. However, you must defer to a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- Parallel Feeders: If you are running parallel sets of conductors for a large feeder (e.g., 400A service), each parallel conduit must contain its own ground wire, sized based on the main overcurrent device, not the individual conductor ampacity.
- Adjustable Trip Breakers: If your breaker has an adjustable trip setting (common in industrial panels), the ground wire must be sized based on the highest possible trip setting of the breaker, not the setting it is currently dialed to.
- Ground Fault Protection of Equipment (GFPE): In systems with GFPE, the clearing time is different, and engineers often specify larger ground wires to ensure the sensor can read the fault current accurately before the system trips.
For further reading on the physics of grounding and bonding, the OSHA wiring design standards provide excellent baseline safety requirements for equipment grounding in workplace environments. Always remember: the ground wire is your silent bodyguard. Size it strictly by the book, respect the voltage drop upsizing rule, and it will do its job when you need it most.
