For a standard 20A, 120V branch circuit, the breaker is 20A and the minimum copper ground wire (Equipment Grounding Conductor or EGC) is 12 AWG. The ground wire provides a low-impedance fault current path to trip the breaker during a short circuit, preventing the metal chassis of an appliance from becoming energized and electrocuting a user.
- Material: Copper (THHN/THWN-2 insulation)
- Temperature Column: 75°C (standard for most residential/commercial terminations)
- Ambient Temperature: 30°C (86°F)
- Conduit: EMT or PVC, maximum 3 current-carrying conductors (no bundling derating applied to the EGC)
The Core Purpose: What Is a Ground Wire For?
There is a persistent myth among hobbyists and novice DIYers that the ground wire exists to "drain static electricity into the earth" or to "give stray voltage a safe place to go." In reality, the Equipment Grounding Conductor (EGC) has one primary job: to create a highly conductive, low-impedance fault current path back to the electrical panel.
When a live (ungrounded) wire touches the metal casing of a drill press or a refrigerator, the casing becomes energized at 120V. If you touch it, your body completes the circuit to ground. However, if a properly sized EGC is bonded to that casing, the fault current takes the path of least resistance—the copper wire. Because the impedance of the EGC is extremely low, the fault current spikes massively and instantaneously. This massive spike triggers the magnetic trip mechanism inside your 20A breaker, cutting the power in milliseconds before a lethal shock can occur.
The earth itself (grounding rods) is irrelevant to clearing a branch circuit fault. According to NFPA 70 (National Electrical Code), the EGC must be sized specifically to handle this momentary fault current surge without melting or vaporizing before the breaker clears the fault.
Sizing the EGC: The NEC 250.122 Baseline
The NEC dictates EGC sizing based on the rating of the overcurrent protective device (the breaker or fuse), not the actual load of the appliance. For our 20A circuit baseline, we look at NEC Table 250.122.
| Breaker Rating | Min. Copper EGC | Min. Aluminum EGC | Wire Area (cmil) |
|---|---|---|---|
| 15 Amps | 14 AWG | 12 AWG | 4,110 |
| 20 Amps | 12 AWG | 10 AWG | 6,530 |
| 30 Amps | 10 AWG | 8 AWG | 10,380 |
| 60 Amps | 10 AWG | 8 AWG | 10,380 |
Why 12 AWG and not one size smaller? While a 14 AWG copper wire has an ampacity of 15A (and 25A in the 75°C column for THHN), a 20A breaker can sustain currents up to 20A indefinitely without tripping. If a fault occurs, the 12 AWG wire (rated 25A at 75°C) provides the necessary thermal mass and conductivity to survive the fault current surge long enough for the breaker's magnetic trip to engage. Using 14 AWG on a 20A circuit risks the EGC melting inside the wall before the breaker trips.
The Voltage Drop Trap: Upsizing the Ground Wire
This is where even experienced installers fail inspections. If your circuit run is long enough that you must upsize the ungrounded (hot) and grounded (neutral) conductors to mitigate voltage drop, you must proportionally upsize the EGC per NEC 250.122(B).
Let's run the math on a 20A circuit powering a continuous 16A load (like a commercial heater) located 120 feet from the panel.
Using standard 12 AWG copper (6,530 cmil), the voltage drop is calculated as:
VD = (2 × K × I × D) / cmil
VD = (2 × 12.9 × 16 × 120) / 6,530 = 7.58V (6.3% drop).
This exceeds the NEC recommended 3% maximum for branch circuits. We must upsize the ungrounded conductors to 8 AWG (16,510 cmil), which drops the VD to 2.99V (2.49%).
Because we upsized the ungrounded conductors from 12 AWG to 8 AWG, we must apply the proportional increase to the EGC:
- Calculate the ratio: 8 AWG area (16,510 cmil) ÷ 12 AWG area (6,530 cmil) = 2.528
- Apply ratio to base EGC: Base EGC for 20A is 12 AWG (6,530 cmil). 6,530 × 2.528 = 16,455 cmil
- Select new EGC: The next standard wire size equal to or greater than 16,455 cmil is 8 AWG (16,510 cmil).
If you pull 8 AWG hots but leave a 12 AWG ground in that conduit, the fault current impedance will be too high relative to the circuit length, potentially delaying the breaker trip. Your EGC must be 8 AWG.
Decision Tree: Picking Your Exact EGC Size
Use this decision matrix to lock in your final ground wire size. Do not mix aluminum and copper assumptions; aluminum requires a larger cross-sectional area due to its higher resistivity and lower thermal melting point during faults.
| Condition / Scenario | Action Required | Final 20A Circuit Pick (Copper) |
|---|---|---|
| Standard run (< 50 ft), no bundling derating | Use NEC Table 250.122 baseline | 12 AWG |
| Long run (> 50 ft) requiring ungrounded upsize for <3% VD | Apply NEC 250.122(B) proportional increase | 8 AWG (if hots upsized to 8 AWG) |
| More than 3 current-carrying conductors in conduit (bundling) | Upsize ungrounded for ampacity derating; EGC does NOT require derating, but upsize proportionally if ungrounded changes size | 12 AWG (unless ungrounded was upsized) |
| Using Aluminum conductors instead of Copper | Reference Aluminum column in Table 250.122 | 10 AWG Aluminum (Minimum) |
The Concrete Pick: For a standard residential 20A receptacle circuit running 45 feet in 1/2" EMT with no voltage drop concerns, your final, non-negotiable pick is 12 AWG bare or green-insulated copper THHN.
When to Call an Engineer or the AHJ
While Table 250.122 covers 95% of residential and light commercial branch circuits, certain scenarios require formal fault current calculations and sign-off from a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ). According to OSHA electrical safety guidelines and NEC Article 110, you must escalate the design when:
- High Available Fault Current: If your facility has a massive utility transformer nearby and the available fault current at the panel exceeds the standard 10,000 AIC (Ampere Interrupting Capacity) rating of standard residential breakers. The EGC must be sized to withstand the specific let-through energy of the upstream protective device.
- Parallel Conductor Runs: When you are running parallel sets of ungrounded conductors (e.g., two sets of 500 kcmil for a 800A feeder). NEC 300.20 requires specific EGC routing and sizing rules to prevent inductive heating in metallic enclosures.
- Service Entrance Grounding: If you are sizing the Grounding Electrode Conductor (GEC) that connects the main panel to the earth grounding rod or water pipe. This is governed by NEC Table 250.66, not 250.122, and the physics of fault clearing differ entirely from branch circuit EGCs.
For standard branch circuits, stick to the proportional math, respect the 75°C termination limits, and always pull a ground wire that matches the physical robustness of your current-carrying conductors.
