The NEC grounding table—officially designated as Table 250.122 in the National Electrical Code—dictates the minimum wire size for Equipment Grounding Conductors (EGCs) based on the rating of the overcurrent protective device (breaker or fuse). To use this chart, locate your breaker's amp rating in the left column, then read across to the Copper or Aluminum column depending on your wire material. The values below represent the absolute minimum cross-sectional area required to safely clear a fault current without melting or creating a fire hazard.
The NEC 250.122 Grounding Table (EGC Sizing)
Below is the complete data table for standard residential and commercial branch circuits and feeders. Bookmark this section for quick-jump lookups on the most common breaker sizes (20A, 60A, 100A, and 200A).
| Overcurrent Device Rating (Amps) | Copper Wire Size (AWG/kcmil) | Aluminum Wire Size (AWG/kcmil) |
|---|---|---|
| 15A | 14 AWG | 12 AWG |
| 20A (Most Common Branch) | 12 AWG | 10 AWG |
| 30A | 10 AWG | 8 AWG |
| 40A | 10 AWG | 8 AWG |
| 50A | 10 AWG | 8 AWG |
| 60A (Subpanels / EV Chargers) | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
| 110A | 8 AWG | 6 AWG |
| 150A | 6 AWG | 4 AWG |
| 200A (Standard Residential Service) | 6 AWG | 4 AWG |
| 300A | 4 AWG | 2 AWG |
| 400A (Large Commercial / Luxury Home) | 3 AWG | 1 AWG |
Source: NFPA 70 National Electrical Code (NEC), Table 250.122. Note: For overcurrent devices rated between standard table values, always round up to the next highest standard size row.
Proportional Upsizing: When Base Values Change
The most common mistake DIYers and junior electricians make with the grounding table is assuming the values are fixed. They are not. NEC 250.122(B) mandates that if you increase the size of your ungrounded (hot) conductors to compensate for voltage drop, you must proportionally increase the size of the EGC.
The table cannot account for long wire runs. If you are running a 100A feeder to a detached garage 150 feet away, standard 3 AWG copper will result in unacceptable voltage drop. You might upsize the hot conductors to 1/0 AWG copper. If you do this, the 8 AWG copper EGC listed in the table is now legally and physically undersized.
Upsized EGC Area = (Table EGC Area) × (Upsized Hot Area ÷ Table Hot Area)
Note: You must use circular mils (cmil) for this math, not AWG numbers.
Worked Numeric Example (100A Feeder):
- Base Table Values: 100A breaker requires 3 AWG Cu hot wire (52,620 cmil) and 8 AWG Cu EGC (16,510 cmil).
- Upsized Hot Wire: To mitigate voltage drop, you install 1/0 AWG Cu hot wire (105,600 cmil).
- The Ratio: 105,600 ÷ 52,620 = 2.006. Your hot wire is roughly twice the required cross-sectional area.
- Upsized EGC Calculation: 16,510 cmil × 2.006 = 33,128 cmil.
- Final Selection: Looking at standard wire properties, 4 AWG copper is 41,740 cmil. Therefore, your minimum EGC for this run is 4 AWG copper, not the 8 AWG listed in the base table.
For a deeper dive into the physics of fault current clearing and why this proportional rule prevents the EGC from acting as a fuse during a short circuit, review the technical breakdowns at EC&M's NEC 250.122 guide.
Installation Realities: Lugs, Torque, and Wire Prep
Knowing the wire size from the grounding table is only half the battle. Proper termination ensures the EGC actually performs during a fault event.
- Bare vs. Insulated: The NEC permits bare copper for EGCs in most residential branch circuits (like standard 12/2 NM-B Romex). However, for feeders, panel-to-panel runs, or outdoor conduit, use insulated green or bare copper THHN. If using insulated wire in a conduit with other circuits, the green insulation prevents accidental cross-connection with neutral conductors.
- Lug Sizing: A common jobsite headache occurs when the proportional upsizing rule forces an EGC that is too large for the grounding lug on the panel or disconnect. If your calculated EGC is 2 AWG, but the panel's ground bar is only rated up to 4 AWG, you must install an auxiliary ground bar rated for the larger wire, or use a listed mechanical lug adapter. Never jam a wire into an undersized lug and over-torque it; this deforms the copper and creates a high-resistance fault path.
- Torque Specifications: Grounding connections must be tight. While the NEC grounding table doesn't specify torque, NEC 110.14(D) requires you to follow the manufacturer's instructions. For standard 14-10 AWG ground screws on a receptacle, torque is typically 12-14 in-lbs. For panel ground bars with 6-2 AWG wires, torque values often range from 35 to 45 in-lbs. Always use a calibrated torque screwdriver.
What the Grounding Table Cannot Tell You
Table 250.122 is strictly for Equipment Grounding Conductors (EGCs)—the wires that run alongside your hot and neutral conductors to bond metal enclosures and provide a fault current path back to the panel. It does not cover other grounding system components.
1. Grounding Electrode Conductors (GEC)
If you are sizing the wire that connects your main service panel to the physical earth (ground rods, ufer ground, or metal water pipe), Table 250.122 is the wrong chart. You must use NEC Table 250.66. The GEC is sized based on the cross-sectional area of the largest service entrance conductor, not the breaker rating. For a standard 200A residential service with 4/0 AWG aluminum service entrance wires, Table 250.66 dictates a minimum 2 AWG bare copper GEC to the ground rods.
2. Parallel Conductor Runs
For massive feeders (typically 400A and above) where conductors are run in parallel across multiple conduit runs, the grounding table requires a specific application. Per NEC 250.122(F), you must run a separate EGC in each parallel conduit. Furthermore, the EGC in each conduit must be sized based on the amp rating of the main overcurrent device, not divided by the number of parallel runs. If you have a 800A breaker with two parallel sets of conductors, you must pull a full-sized 1/0 AWG copper EGC in both conduits, not a halved size.
3. Main Bonding Jumpers
The wire or strap that bonds the neutral bar to the ground bar inside the main service disconnect is a Main Bonding Jumper. Its sizing is governed by NEC 250.28, which bases the size on the area of the service entrance conductors, completely bypassing the overcurrent device ratings found in the grounding table.
