For a 100-amp breaker, the minimum copper wire for earthing is 8 AWG. For a 20-amp breaker, use 12 AWG. This assumes copper conductors, 75°C terminations, 30°C ambient temperature, and standard conduit routing, sized per NEC Table 250.122 to safely clear fault currents without overheating.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C (Standard for modern breakers and terminals rated 100A and below)
- Ambient Temperature: 30°C (86°F) baseline; no high-heat ambient derating applied
- Conduit Type: Standard PVC or EMT; not bundled in high-density thermal insulation
- Conductor Type: THHN/THWN-2 (90°C insulation, but terminated at 75°C limits)
The Master Sizing Chart: NEC Table 250.122
When electricians refer to the "wire for earthing" in a branch circuit or feeder, they are talking about the Equipment Grounding Conductor (EGC). This is the bare or green wire that runs alongside your hot and neutral wires to provide a low-impedance fault path back to the panel. The National Electrical Code (NEC) sizes this wire based on the rating of the overcurrent protective device (the breaker), not the load itself.
Below is the essential data from NEC Table 250.122, which dictates the absolute minimum sizes required to keep the fault path intact while the breaker trips.
| Breaker / Fuse Size | Min. Copper EGC | Min. Aluminum EGC | Common Application |
|---|---|---|---|
| 15 Amps | 14 AWG | 12 AWG | Standard lighting / receptacles |
| 20 Amps | 12 AWG | 10 AWG | Kitchen / bathroom small appliance |
| 30 Amps | 10 AWG | 8 AWG | Dryers, water heaters, A/C |
| 60 Amps | 10 AWG | 8 AWG | EV chargers, subpanel feeders |
| 100 Amps | 8 AWG | 6 AWG | Standard 100A subpanel feeder |
| 200 Amps | 6 AWG | 4 AWG | Residential main service / large feeder |
| 400 Amps | 3 AWG | 1 AWG | Commercial service entrance |
Critical Row Note: Notice the 60A row. The minimum copper EGC is 10 AWG, the same as the 30A row. This is because 10 AWG is the smallest wire the NEC generally permits for mechanical durability in larger conduit runs, and it possesses more than enough thermal mass to handle a 60A fault clearing event. Always verify your specific breaker's termination temperature rating; if you are using older 60°C rated equipment, you must size the ungrounded conductors from the 60°C column, though the EGC table remains independent of the wire's insulation temperature rating.
Why This Size and Not One Smaller?
It is tempting to think that because an equipment grounding conductor carries zero current during normal operation, you could use a tiny wire to save money. This is a catastrophic misunderstanding of let-through current and thermal limits.
The EGC only carries current during a fault (a short circuit). When a hot wire touches a metal appliance chassis, the current spikes instantly to thousands of amps. A standard 100A thermal-magnetic breaker takes roughly 16 milliseconds (one AC cycle at 60Hz) to trip under a massive short-circuit condition. During those 16 milliseconds, the EGC must absorb immense thermal energy—calculated as $I^2t$ (current squared multiplied by time).
If you use a 10 AWG wire on a 100A breaker instead of the required 8 AWG, the 10 AWG wire may reach temperatures exceeding 250°C before the breaker opens. While the copper itself won't melt (melting point is 1083°C), the THHN insulation will vaporize, melt, or catch fire. This destroys the dielectric barrier inside the conduit, potentially causing a secondary phase-to-phase arc flash that the breaker cannot safely interrupt. The sizes in Table 250.122 are engineered specifically to keep the conductor temperature below the insulation failure threshold during the maximum clearing time of the breaker.
What Changes the Answer: Length, Bundling, and Aluminum
Table 250.122 gives you the minimum baseline. However, three real-world jobsite conditions will force you to upsize your wire for earthing.
1. The Voltage Drop Catch (NEC 250.122(B))
Standard voltage drop calculators (like the excellent Southwire Voltage Drop Calculator) apply only to current-carrying conductors. The EGC does not carry continuous load, so standard 3% voltage drop limits do not apply to it. However, if your circuit is so long that you must upsize the hot wires to prevent voltage drop, you must proportionally upsize the EGC.
Worked Example: You are running a 30A circuit 120 feet to a detached garage. Standard 10 AWG copper yields a voltage drop over 4%. To fix this, you upsize the ungrounded conductors to 8 AWG.
According to NEC Chapter 9, Table 8:
• 10 AWG = 10,380 circular mils (cmil)
• 8 AWG = 16,510 cmil
• Upsizing Ratio = 16,510 / 10,380 = 1.59
You must multiply the standard EGC (10 AWG / 10,380 cmil) by 1.59.
10,380 × 1.59 = 16,504 cmil.
The next standard wire size that meets or exceeds 16,504 cmil is 8 AWG (16,510 cmil). Therefore, your EGC must be upsized from 10 AWG to 8 AWG to maintain the same low-impedance fault path ratio.
2. Bundling and Conduit Fill
Unlike hot and neutral wires, a single EGC does not count toward the ampacity derating calculations for conduit fill (NEC 310.15(C)(1)). You do not need to upsize the EGC just because you have 15 other wires in the same PVC pipe. However, if you are running parallel feeders (e.g., two sets of 500 kcmil wires for a 800A service), you must run a full-sized EGC in each parallel conduit. You cannot run one massive EGC in just one of the pipes; the fault current must have parallel, low-impedance paths to prevent magnetic choke effects in the individual conduits.
3. Aluminum vs. Copper
Never present aluminum and copper interchangeably. Aluminum has higher electrical resistance and lower thermal mass than copper. As shown in the master table, if you are using aluminum conductors for a 100A feeder, the minimum wire for earthing jumps from 8 AWG (copper) to 6 AWG (aluminum). Furthermore, if you use an aluminum EGC, it must be terminated with anti-oxidant compound (like Noalox) and torqued to the exact inch-pound specification on the breaker lug to prevent high-resistance heating over time.
When an Engineer or the AHJ Must Confirm
While Table 250.122 covers 95% of residential and light commercial work, you must pull in a licensed professional engineer (PE) or consult your local Authority Having Jurisdiction (AHJ) under these specific conditions:
- High Available Fault Current: If your utility transformer can deliver 42,000 Amps or more of fault current, standard thermal-magnetic breakers may take longer to clear, or the magnetic forces may physically rip a minimally-sized EGC from its lug. An engineer must verify the $I^2t$ rating of the wire against the specific breaker's let-through energy curve.
- Grounding Electrode Conductors (GEC): Do not confuse the EGC with the GEC. The GEC is the wire that connects your panel's neutral bar to the actual ground rods or ufer ground in the earth. The GEC is sized by NEC Table 250.66 based on the size of the largest ungrounded service entrance conductor, not the breaker. Using Table 250.122 for a ground rod connection is a common, dangerous mistake.
- Specialized Medical or Industrial Equipment: Equipment requiring "isolated ground" (IG) receptacles or impedance-grounded systems (like high-resistance grounding in industrial plants) requires custom engineering that bypasses standard NEC tables entirely.
Getting the wire for earthing right is not about passing a basic inspection; it is about ensuring that when a catastrophic short circuit occurs, the fault path holds together just long enough for the breaker to do its job. Always default to the 75°C column, check your run lengths for proportional upsizing, and never substitute aluminum sizing logic for copper.
