When electricians and DIYers search for an NEC ground wire size chart, they are looking for NEC Table 250.122. This table dictates the minimum size for an Equipment Grounding Conductor (EGC) based entirely on the rating of the overcurrent protective device (OCPD)—your breaker or fuse—not the actual load current. Sizing a ground wire based on load rather than the breaker is one of the most common and dangerous mistakes in residential and commercial wiring, as it can prevent a breaker from tripping during a short circuit.
- 15A Breaker: 14 AWG Copper / 12 AWG Aluminum
- 20A Breaker: 12 AWG Copper / 10 AWG Aluminum
- 30A Breaker: 10 AWG Copper / 8 AWG Aluminum
- 50A Breaker: 10 AWG Copper / 8 AWG Aluminum
- 100A Breaker: 8 AWG Copper / 6 AWG Aluminum
- 200A Breaker: 6 AWG Copper / 4 AWG Aluminum
How to Read the NEC Table 250.122 Ground Wire Size Chart
Before pulling wire, you need to understand how to read the columns and what assumptions the table makes. Table 250.122 is divided into three primary columns: the OCPD rating (in Amps), the minimum Copper wire size (AWG/kcmil), and the minimum Aluminum wire size (AWG/kcmil).
Which column applies to your installation? If you are pulling standard THHN/THWN-2 or NM-B (Romex) copper wire, use the Copper column. If you are pulling XHHW-2 aluminum feeder wire (common for subpanels and service entrances), use the Aluminum column. The table assumes standard installation temperatures, but unlike ungrounded (hot) conductors, the ampacity temperature columns (60°C, 75°C, 90°C) do not change the baseline EGC size. The EGC only carries current during a fault, so its size is dictated by the magnetic trip threshold of the breaker, not continuous thermal heating.
| OCPD Rating (Amps) | Copper Wire (AWG/kcmil) | Aluminum Wire (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 |
| 800 | 1/0 | 3/0 |
| 1000 | 2/0 | 4/0 |
| 1200 | 3/0 | 250 kcmil |
| Source: NFPA 70 (National Electrical Code), Table 250.122. Always consult your local AHJ, as regional amendments may apply. | ||
Critical Sizing Rules: Derating, Upsizing, and Parallel Runs
The chart above provides the baseline minimums. However, real-world jobsite conditions frequently require you to deviate from the base table. Understanding how derating and upsizing modify these values is where novice installers fail inspections.
How Derating Rows Modify the Base Value
Here is a crucial piece of bench and jobsite knowledge: Ampacity derating (NEC 310.15) does not apply to Equipment Grounding Conductors. When you bundle multiple hot wires in a conduit, you must derate their ampacity due to heat buildup. Because the EGC only carries current for a fraction of a second during a fault, it does not generate sustained heat and is exempt from thermal derating.
However, if derating your hot wires forces you to upsize them to maintain safe ampacity, you trigger the Proportionality Rule (NEC 250.122(B)). If you increase the cross-sectional area of your ungrounded conductors to compensate for derating or voltage drop, you must proportionally increase the size of your EGC.
You are running a 100A feeder. Table 250.122 calls for an 8 AWG copper ground (16,510 circular mils). Due to a 150-foot run, voltage drop requires you to upsize your hot wires from 3 AWG (52,620 cmil) to 1/0 AWG (105,600 cmil).
Ratio: 105,600 / 52,620 = 2.00.
New Ground Size: 16,510 cmil × 2.00 = 33,020 cmil.
You must now install a 4 AWG copper ground (41,740 cmil) instead of the baseline 8 AWG.
What the Table Cannot Tell You
Table 250.122 is strictly for Equipment Grounding Conductors (EGCs) that run with your circuit wires to clear faults. It cannot tell you the size of your Grounding Electrode Conductor (GEC)—the wire that connects your panel to your ground rods or ufer ground. For that, you must use NEC Table 250.66. It also does not size Main Bonding Jumpers or system bonding jumpers, which are covered in NEC 250.28.
Common Grounding Conductor Sizing Mistakes
- Sizing by Load Instead of Breaker: If you have a 45A continuous load on a 60A breaker, your hot wires might be sized for 60A, but your ground must be sized for the 60A breaker (10 AWG copper), not the 45A load. The breaker dictates the maximum fault current the ground must safely carry before tripping.
- Shared Grounds in Multi-Wire Branch Circuits (MWBC): If you run two 20A circuits in the same conduit and share a single ground wire, that shared ground must be sized for the largest OCPD protecting the circuits. If one is 20A and one is 30A, the shared ground must be 10 AWG, not 12 AWG.
- Parallel Feeder Errors: When running parallel conduits for a 400A service, you cannot run one massive ground wire in a single pipe. NEC 300.3(B) and 250.122(F) require you to run a full-sized 3 AWG copper ground in every single parallel raceway.
NEC Ground Wire Size Chart FAQ
Do I need to size the ground wire based on the breaker or the actual load?
You must always size the Equipment Grounding Conductor based on the rating of the Overcurrent Protective Device (the breaker or fuse), not the actual load. The ground wire's sole purpose is to provide a low-impedance path back to the source to allow enough fault current to flow to instantly trip the breaker. If you size it for the load, a short circuit might not generate enough current to trip a larger breaker, leaving the circuit energized and creating a severe shock or fire hazard.
What size ground wire do I need for a 200 amp service?
This is a trick question that requires distinguishing between an EGC and a GEC. If you are pulling a 200A feeder to a subpanel, you use Table 250.122, which requires a 6 AWG copper or 4 AWG aluminum EGC. However, if you are wiring the main service entrance and connecting the panel to your ground rods (the Grounding Electrode Conductor), you must use Table 250.66, which requires a 4 AWG copper or 2 AWG aluminum GEC for 2/0 service conductors.
Does the equipment grounding conductor need to be insulated?
For most standard residential and commercial installations, a bare copper wire is perfectly legal and widely used as an EGC. However, there are exceptions. If you are pulling wire in a healthcare facility (NEC Article 517), or if the ground wire is being pulled through flexible metal conduit (FMC) where the conduit itself is not relied upon as the ground path, insulated green (or green with a yellow stripe) wire is often required by local inspectors or specific project specs to prevent accidental contact with hot terminals inside crowded junction boxes.
If I upsize my hot wires for voltage drop, do I need to upsize the ground?
Yes. Under NEC 250.122(B), if you increase the size of your ungrounded (hot) conductors to mitigate voltage drop on long runs, you must increase the equipment grounding conductor proportionally. You calculate the ratio of the circular mil area of the new hot wire versus the original hot wire, and multiply the baseline ground wire's circular mil area by that exact ratio, stepping up to the next standard AWG size. For a deeper dive into voltage drop calculations and proportional sizing, refer to resources like EC&M's code guides.
