An Equipment Grounding Conductor (EGC) is a dedicated safety wire that connects the non-current-carrying metal enclosures of electrical equipment back to the service panel to ensure a breaker trips instantly during a ground fault. It does not carry current during normal operation; it sits idle until a hot wire escapes its insulation and touches a metal chassis, at which point it provides a low-impedance path back to the source to force the overcurrent protective device (OCPD) to open.
The Core Function: What an EGC Actually Changes in a Circuit
To understand the EGC, you have to look at the math of a ground fault. If a 120V hot wire inside a metal-cased drill press frays and touches the chassis, the metal casing becomes energized at 120V nominal. If there is no EGC, and you touch the casing while standing on a concrete floor, your body and the earth become the fault path. Human skin resistance (when wet or sweaty) can drop to 1,000 ohms, and earth resistance is highly variable. At 1,000 ohms, the fault current is roughly 120mA (0.12A). This is well below the 20A breaker's trip threshold, meaning the breaker stays closed, the chassis remains lethal, and the shock is sustained.
When you install a properly sized EGC, the physics change entirely. The EGC creates a dedicated, extremely low-impedance loop back to the transformer. Assuming a total loop impedance (hot wire + EGC + connections) of just 0.1 ohms, Ohm's Law dictates the fault current:
The Math of a Cleared Fault:
I = E / R
I = 120V / 0.1Ω = 1,200 Amps
A standard 20A thermal-magnetic breaker requires roughly 5x to 10x its rated current (100A to 200A) to trigger the instantaneous magnetic trip. At 1,200A, the breaker's magnetic latch engages in under 0.02 seconds, clearing the fault before it can cause fatal ventricular fibrillation or start a fire. The EGC turns a sustained shock hazard into a harmless, momentary spark and a tripped breaker.
NEC Table 250.122: Minimum EGC Sizing by Breaker Rating
The National Electrical Code (NEC) does not size the EGC based on the actual load current of the appliance; it sizes the EGC based on the rating of the overcurrent device (breaker or fuse) protecting the circuit. This ensures the wire can handle the maximum available fault current long enough for the breaker to trip without melting. Below are the minimum sizes extracted from NEC Table 250.122.
| Overcurrent Device Rating (Amps) | Minimum Copper EGC (AWG) | Minimum Aluminum EGC (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 |
| 200A | 6 AWG | 4 AWG |
Note: These are minimums. If you are pulling wire over long distances, voltage drop considerations may force you to upsize the EGC as well (covered below).
The Proportional Upsizing Rule (NEC 250.122(B))
A massive point of failure for DIYers and even some journeymen is ignoring the proportional upsizing rule. If your circuit run is so long that you must upsize the ungrounded (hot) conductors to mitigate voltage drop, you must upsize the EGC proportionally.
Imagine you are wiring a 60A subpanel feeder 200 feet away. To keep voltage drop under 3%, you upsize your hot wires from the standard 6 AWG copper to 4 AWG copper. According to Table 250.122, a 60A breaker normally only requires a 10 AWG copper EGC. However, because you increased the hot wire cross-sectional area by two AWG steps (6 to 4), you must increase the EGC by the exact same ratio. The EGC must be bumped from 10 AWG to 8 AWG. If you leave the EGC at 10 AWG, its higher resistance will artificially limit the fault current on that long run, potentially delaying the breaker's magnetic trip and creating a severe fire and shock hazard.
Where You Meet the EGC in Practice
You interact with the EGC in almost every wiring method on a modern jobsite, though it takes different physical forms depending on the installation:
- NM-B Cable (Romex): The bare copper wire inside the sheath. It is uninsulated because it is protected by the outer PVC jacket. It terminates on the green grounding screw in receptacles and switches, and bonds to metal boxes via a grounding clip or screw.
- THHN/THWN in Raceway: Typically an insulated wire with green (or green with a yellow stripe) insulation. While the NEC allows bare wires in raceways, insulated green is the industry standard for commercial work to prevent accidental contact with energized parts inside crowded conduits.
- Metal Conduit (EMT/Rigid): Under NEC 250.118, the metal raceway itself is approved as an EGC. However, relying on setscrew couplings for a fault path is risky; vibration and corrosion can increase impedance over time. Best practice for critical or high-vibration circuits is to pull a separate green insulated EGC wire inside the metal conduit.
- Appliance Cords: The third, round prong on a NEMA 5-15P plug is the EGC connection, routing the fault path through the cord back to the receptacle's ground terminal.
Common Confusions: EGC vs. Neutral vs. GEC
Mixing up grounding and bonding terminology is the fastest way to fail an electrical inspection or create a dangerous parallel neutral path. Here is how the EGC differs from the other 'ground' wires in your panel:
| Conductor Type | NEC Name | Function | Carries Normal Current? |
|---|---|---|---|
| EGC | Equipment Grounding Conductor | Clears ground faults by tripping the breaker; bonds metal enclosures. | Never (only during a fault) |
| Neutral | Grounded Conductor | Provides the intentional return path for 120V circuit current back to the transformer. | Yes (carries the same current as the hot wire) |
| GEC | Grounding Electrode Conductor | Connects the service panel neutral bar to the earth (ground rods, ufer) to stabilize voltage against lightning and surges. | Rarely (only during a surge/lightning strike) |
Safety Caveat: Never connect the EGC to the neutral bus bar anywhere except at the main service disconnect. If you bond neutral and ground at a subpanel, normal return current will split and flow back on the EGC, energizing metal appliance chassis and conduit with stray voltage.
Frequently Asked Questions
Can I use the bare copper wire in NM-B cable as a neutral?
No. The bare wire is strictly the EGC. It is not insulated to handle continuous current, and using it as a neutral will cause the entire metal grounding system of your house to carry return current, creating a shock hazard and tripping GFCI/AFCI breakers.
Does the EGC need to be insulated?
Inside a cable assembly like NM-B, no. Inside a conduit with THHN wires, the NEC allows bare, covered, or insulated EGCs. However, if the EGC is insulated, it must be colored green, green with yellow stripes, or bare copper. Using a white or black wire as an EGC inside a conduit is a code violation.
What happens if my EGC is too small?
During a short circuit, an undersized EGC will act like a fuse. It can melt or vaporize before the breaker has time to trip. Once the EGC burns open, the fault current stops, the breaker resets (or stays closed), and the metal appliance casing remains fully energized at line voltage, waiting for someone to touch it.
