When DIYers and apprentices search for a "grounding chart," they are almost always looking for one of two distinct tables in the National Electrical Code (NEC). Sizing a ground wire is not a one-size-fits-all calculation; it depends entirely on whether you are bonding a circuit back to the panel (Equipment Grounding Conductor) or bonding the main service panel to the earth (Grounding Electrode Conductor).
The direct answer: For standard 15A and 20A branch circuits, your minimum Equipment Grounding Conductor (EGC) is 14 AWG and 12 AWG copper, respectively. For a standard 200A residential service, your Grounding Electrode Conductor (GEC) to the ground rods is 4 AWG bare copper. Below is the complete reference data, the decision path to ensure you are using the right table, and the specific edge cases that cause failed inspections.
The Core Grounding Chart: NEC Table 250.122 (EGC Sizing)
This table dictates the minimum size for the Equipment Grounding Conductor (EGC)—the bare or green wire that runs inside your conduit or Romex alongside your hot and neutral wires to clear a fault.
| Overcurrent Device Rating (Amps) | Minimum Copper EGC (AWG/kcmil) | Minimum Aluminum EGC (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 110 | 6 | 4 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
Quick-Jump Rows: Standard Residential & Feeder Sizes
Bookmark this section for the most common residential and light-commercial runs. These values assume standard copper conductors.
- 15A Lighting/Receptacle Circuit: 14 AWG (Usually pre-included in 14/2 NM-B).
- 20A Appliance/Receptacle Circuit: 12 AWG (Pre-included in 12/2 NM-B).
- 30A Dryer/HVAC Circuit: 10 AWG (Pre-included in 10/2 or 10/3 NM-B).
- 50A Range or Hot Tub: 10 AWG (Wait, the chart says 10 AWG for 60A, but 50A breakers also require 10 AWG minimum. If pulling individual THHN, use 10 AWG bare).
- 60A Subpanel Feeder: 10 AWG (Minimum, though many electricians pull 8 AWG or 6 AWG for future-proofing and physical durability).
- 100A Subpanel Feeder: 8 AWG.
- 200A Main Service or Subpanel: 6 AWG.
Decision Path: EGC vs. GEC (Which Table Do You Actually Need?)
The most common mistake on the jobsite is confusing the Equipment Grounding Conductor (EGC) with the Grounding Electrode Conductor (GEC). Use the decision tree below to terminate your search at the correct code table.
| If your installation is... | Then you are sizing a... | Use this NEC Table | Concrete Pick for 200A Service |
|---|---|---|---|
| Running inside a wall/conduit alongside hot wires to an outlet, appliance, or subpanel to clear a short circuit. | Equipment Grounding Conductor (EGC) | Table 250.122 | 6 AWG Copper |
| Connecting the main service panel's neutral/ground bus bar to a ground rod, ufer ground, or metal water pipe. | Grounding Electrode Conductor (GEC) | Table 250.66 | 4 AWG Bare Copper |
| Bonding metal conduit, water pipes, or structural steel back to the service ground bus to ensure equipotential. | Bonding Jumper | Table 250.102(C)(1) | Sized identically to the GEC (4 AWG for 200A) |
Upsizing, Derating, and What the Chart Cannot Tell You
Table 250.122 provides the minimum baseline. It does not account for voltage drop, physical damage, or conduit fill. Here is how modifications apply and what the chart leaves out.
1. Proportional Upsizing for Voltage Drop (NEC 250.122(B))
If you upsized your hot wires to mitigate voltage drop on a long feeder run, you must upsize the EGC proportionally. The chart's base value no longer applies.
Example: You are running a 100A subpanel 200 feet away. To keep voltage drop under 3%, you upsize your hot wires from 3 AWG to 1/0 AWG copper. Because you increased the circular mil area of the ungrounded conductors, you must increase the EGC by the exact same ratio. Your 8 AWG EGC from the chart must now be bumped up to a 4 AWG EGC. For detailed math on this, refer to Mike Holt's NEC grounding tutorials.
2. What the Table Cannot Tell You: Physical Protection
The chart only dictates electrical ampacity for fault clearing. It does not dictate physical protection. If you are running a 6 AWG or smaller GEC or EGC on the exterior of a wall or in an exposed location where it could be subject to physical damage (like a garage wall or near a driveway), NEC 250.64(B) requires you to protect it with rigid metal conduit, intermediate metal conduit, or schedule 80 PVC. A bare 6 AWG wire stapled to an exposed block wall will fail inspection.
3. Paralleled Conductors
If you are running parallel feeders (e.g., two sets of 350 kcmil for a 800A service), you do not use one massive ground wire. You must run a separate EGC in each parallel raceway, sized based on the overcurrent device protecting that specific parallel set, or sized per 250.122(F) based on the total equivalent area.
Concrete Picks and Final Verification
Do not leave your grounding installation to guesswork. Follow this final verification sequence before energizing the panel:
- Verify the Conductor Type: Ensure your EGC is bare copper, green-insulated copper, or green with a yellow stripe. Never use white or gray for an EGC.
- Check the Termination Torque: Grounding lugs and bus bars require specific torque. A standard 10-32 grounding screw on a residential panel typically requires 20 to 25 inch-pounds of torque. Use a calibrated torque screwdriver (like the Klein Tools 651 or CDI torque driver). Loose ground connections cause high-impedance fault paths, meaning the breaker won't trip during a short.
- Confirm the Bus Bar Bond: In a main service panel, the neutral and ground bars must be bonded (connected). In a subpanel, they must remain isolated. The EGC lands on the ground bar; the neutral lands on the neutral bar.
- Measure Continuity: With the power off and locked out, use your multimeter to measure resistance between the EGC at the furthest outlet and the main panel ground bus. You should read less than 1.0 ohm.
By strictly applying Table 250.122 for branch circuits and feeders, and Table 250.66 for service electrodes, you eliminate the most common grounding errors. When in doubt, upsize the ground wire; there is no electrical penalty for a grounding conductor that is larger than the code minimum, only a safer, lower-impedance fault path.
