When sizing ground wire—technically known as the Equipment Grounding Conductor (EGC)—the rule is counterintuitive to many DIYers: the ground wire size is dictated by the rating of the overcurrent protective device (the breaker), not the expected load current. A 20-amp circuit requires a minimum 12 AWG copper ground wire, even if the connected appliance only draws 5 amps. This sizing ensures that during a catastrophic short circuit, the ground wire can safely carry massive fault current long enough to trip the breaker instantly.
The following guide uses NEC-style guidance (specifically Article 250) to break down the exact wire sizes, the physics of fault loops, and how to verify your work. Note: The National Electrical Code (NEC) is a model code; your local Authority Having Jurisdiction (AHJ) or local inspector has final legal authority over all installations.
The Physics of the Fault: Why Ground Wire Sizing Matters
To understand why we size the EGC based on the breaker, you have to understand what happens during a ground fault. If a frayed hot wire inside a metal-cased appliance touches the chassis, the case becomes energized at 120V. When you touch it, you become the path to ground.
The EGC exists to provide a low-impedance parallel path back to the panel. Because electricity takes all available paths (with the majority flowing through the lowest resistance), a properly sized, low-resistance ground wire will draw hundreds of amps of instantaneous fault current. This massive surge forces the magnetic trip mechanism inside the breaker to snap open in milliseconds, clearing the fault before it can cause a shock or start a fire.
Ground vs. Neutral vs. Bond: Clearing the Confusion
Misidentifying these three conductors is the most common cause of failed inspections and dangerous shock hazards in subpanels.
- Neutral (Grounded Conductor): The white or gray wire. It carries the normal, unbalanced return current back to the source during everyday operation. It is a current-carrying conductor.
- Ground (Equipment Grounding Conductor / EGC): The bare or green wire. It carries zero current during normal operation. It only carries current during a fault event to trip the breaker.
- Bond: This is not a wire; it is a physical connection. The main bonding jumper ties the neutral bus and the ground bus together only at the main service disconnect. This bond is what creates the low-impedance loop required for the breaker to trip. In subpanels, neutral and ground must remain strictly isolated.
Sizing Ground Wire Using NEC Table 250.122
NEC Table 250.122 provides the absolute minimum sizes for EGCs based on the breaker rating. Below is the reference chart for standard copper and aluminum conductors.
| Breaker / Fuse Rating | Minimum Copper EGC (AWG) | Minimum Aluminum EGC (AWG) |
|---|---|---|
| 15 Amps | 14 | 12 |
| 20 Amps | 12 | 10 |
| 30 Amps | 10 | 8 |
| 40 Amps | 10 | 8 |
| 60 Amps | 10 | 8 |
| 100 Amps | 8 | 6 |
| 150 Amps | 6 | 4 |
| 200 Amps | 6 | 4 |
The Proportional Upsizing Rule (NEC 250.122(B))
Here is where many DIYers fail inspections. If your circuit run is exceptionally long, voltage drop may force you to upsize the ungrounded (hot) conductors. If you upsize the hot wires, you must proportionally upsize the ground wire.
Worked Example: You are running a 40A circuit to a detached garage 150 feet away.
• Standard 40A hot wire: 8 AWG Copper (16,510 circular mils).
• Standard 40A ground wire: 10 AWG Copper (10,380 circular mils).
• To keep voltage drop under 3%, you upsize the hot wires to 6 AWG Copper (26,240 circular mils).
• The ratio of increase is 26,240 / 16,510 = 1.58.
• Multiply the standard ground size by 1.58: 10,380 cmil × 1.58 = 16,400 cmil.
• The next standard wire size up is 8 AWG (16,510 cmil). Therefore, your ground wire must be upsized from 10 AWG to 8 AWG.
Step-by-Step Verification and Testing
Never assume a ground path is valid just because the wire is connected at both ends. Corrosion, loose terminal screws, or a broken strand can introduce high impedance, preventing the breaker from tripping during a fault.
- De-energize and Lock Out: Turn off the main breaker for the circuit you are testing. Verify it is dead using a non-contact voltage tester (like the Fluke 1AC-II) and a multimeter.
- Set Multimeter to Low Impedance: Use a high-quality multimeter (e.g., Fluke 117 or Klein Tools MM400). Standard continuity modes often inject too little current to detect high-resistance faults. Set the dial to Ohms (Ω).
- Probe the Fault Path: Place one probe on the ground slot of the receptacle (or the metal appliance chassis) and the other probe directly on the ground bus bar in the originating panel.
- Evaluate the Reading: A healthy, properly sized EGC should read less than 1.0 ohm (ideally under 0.5 ohms). If you read OL (Open Loop) or a fluctuating high resistance, the ground path is broken or compromised.
- Energize and Verify Polarity: Once the panel is closed and the breaker is on, plug in a receptacle tester like the Klein Tools RT210. The lights should indicate "Correct" wiring. If it shows "Open Ground," the EGC is disconnected somewhere in the run.
When to Call a Licensed Electrician
While running branch circuit EGCs and wiring subpanels is manageable for competent DIYers, certain grounding tasks carry severe arc-flash and electrocution risks and require a licensed professional:
- Service Entrance Grounding: Sizing and installing the Grounding Electrode Conductor (GEC) that connects the main panel to ground rods or the municipal water pipe (NEC Article 250 Part III). This conductor is sized via Table 250.66 based on the service entrance conductors, not the breaker.
- Main Bonding Jumper Installation: Installing the bond between the neutral and ground bus in a new main service panel. An error here can energize the entire grounding system of the house.
- Upgrading Service Panels: Moving from a 100A to a 200A service requires recalculating the entire grounding electrode system and upgrading the physical ground wire to the grounding rods.
Ground Wire Sizing FAQ
Can I use a smaller ground wire if the run is very short?
No. The NEC does not allow you to downsize the EGC based on a short physical distance. The breaker's magnetic trip mechanism requires a specific volume of fault current to activate within milliseconds. A smaller wire has higher resistance, which limits fault current and delays tripping, regardless of whether the run is 5 feet or 50 feet. Always follow the minimums in Table 250.122.
Do I need to upsize the ground wire if I upsized the hot wires for voltage drop?
Yes. Under NEC 250.122(B), if you increase the size of the ungrounded (hot) conductors to compensate for voltage drop on long runs, you must increase the size of the EGC proportionally based on the circular mil area of the conductors. Failing to do so creates a bottleneck in the fault-current path, negating the safety of the upsized hot wires.
What color should the ground wire be in a conduit run?
When pulling individual THHN/THWN wires through conduit, the EGC must be bare, green, or green with one or more yellow stripes. While bare copper is perfectly legal and common, many professional electricians prefer green-insulated wire in conduit because bare copper can sometimes be mistaken for a neutral if it becomes coated in drywall dust or paint, and insulation prevents accidental contact with other live conductors in crowded junction boxes.
Does a 240V-only circuit (like a baseboard heater) need a ground wire?
Yes. Even though a pure 240V circuit uses two hot wires and no neutral, it still requires an Equipment Grounding Conductor. The metal chassis of the heater or the metal junction box must be grounded so that if one of the hot wires shorts to the case, the 2-pole breaker will trip. Size the ground wire based on the 2-pole breaker rating using Table 250.122.
