The standard grounding wire chart used by electricians and inspectors in the United States is NEC Table 250.122. This table dictates the minimum size of the Equipment Grounding Conductor (EGC) based strictly on the rating of the overcurrent protective device (breaker or fuse) protecting the circuit, not the load itself. For a standard 100-amp breaker, you need an 8 AWG copper ground; for a 200-amp breaker, you need a 6 AWG copper ground.
While the table provides the baseline, real-world jobsite conditions—like long wire runs requiring voltage drop compensation—will force you to modify these base values. Below is the complete reference chart, the math for modifying it, and answers to the most common long-tail questions we see on the bench and in the panel.
How to Read the Grounding Wire Chart
Before jumping to the rows, you need to understand how the columns apply to your specific installation. Table 250.122 is structured around the overcurrent device rating, which is the breaker or fuse size.
- Column 1 (Overcurrent Device Rating): This is the ampere rating of the breaker protecting the circuit. If you are installing a 50A breaker for a welder outlet, you look at the 50A row.
- Column 2 (Copper): The minimum AWG or kcmil size for copper conductors. This is the column 95% of residential and commercial DIYers will use, as bare copper is the standard for EGCs pulled in conduit.
- Column 3 (Aluminum / Copper-Clad): The minimum size if you are using aluminum wire. Aluminum requires a larger cross-section to safely carry the same fault current without melting.
Temperature Ratings: Unlike ungrounded (hot) conductors where you must choose between the 60°C, 75°C, or 90°C ampacity columns, temperature ratings do not dictate the base size of an EGC. The values in Table 250.122 are absolute minimums for fault-current survival. However, the physical insulation of the wire you pull (e.g., THHN vs. XHHW) must still be rated for the environment and termination temperatures of your lugs.
NEC Table 250.122: Minimum Equipment Grounding Conductor Sizes
The following table is sourced directly from NFPA 70 (National Electrical Code) Table 250.122. Bookmark this section for quick lookups on the jobsite.
| Overcurrent Device Rating (Amps) | Copper Wire Size (AWG/kcmil) | Aluminum / Copper-Clad Size (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 50 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 110 - 200 | 6 | 4 |
| 250 | 4 | 2 |
| 300 | 3 | 1 |
| 400 | 2 | 1/0 |
| 500 | 1 | 2/0 |
| 600 | 1/0 | 3/0 |
When the Chart Changes: Upsizing and Derating Rules
A common misconception is that because EGCs only carry current during a short-circuit fault, they are exempt from all conduit fill and derating rules. Here is the exact jobsite reality:
1. Conduit Fill Derating (NEC 310.15): Equipment grounding conductors do count toward the physical conduit fill percentage (e.g., the 40% rule for 3 or more wires). However, they do not count when applying the ampacity derating factors for heat buildup in crowded conduits. The EGC size from the chart above remains unchanged regardless of how many hot wires share the pipe.
2. Voltage Drop Upsizing (NEC 250.122(B)): This is where the base chart values change. If your circuit run is long and you must upsize the ungrounded (hot) conductors to prevent voltage drop, you must increase the EGC proportionally based on circular mils (cmil). You cannot just use the base table value.
You are running a 60A circuit. The base chart requires 6 AWG copper hots (26,240 cmil) and a 10 AWG copper ground (10,380 cmil). Due to a 150-foot run, you upsize the hots to 4 AWG (41,740 cmil) to mitigate voltage drop.
Step 1: Find the ratio of the new hot wire to the base hot wire: 41,740 / 26,240 = 1.59.
Step 2: Multiply the base ground wire cmil by this ratio: 10,380 cmil × 1.59 = 16,504 cmil.
Step 3: Look up 16,504 cmil in Chapter 9, Table 8. The next standard size up is 8 AWG (16,510 cmil).
Result: Your equipment ground must be upgraded from 10 AWG to 8 AWG.
What This Table Cannot Tell You (Edge Cases)
While Table 250.122 is the undisputed authority for branch circuits and feeders, it has strict boundaries. According to EC&M's National Electrical Code basics, you must look elsewhere for these scenarios:
- Grounding Electrode Conductors (GEC): If you are sizing the wire that connects your main panel to a ground rod, ufer ground, or metal water pipe, Table 250.122 is the wrong chart. You must use NEC Table 250.66, which sizes the GEC based on the size of the largest ungrounded service entrance conductor, not the breaker.
- Parallel Feeder Runs: If you are running multiple conduits in parallel for a 400A+ service, you cannot just pull one giant ground wire in one pipe. NEC 250.122(F) requires a full-sized EGC in every parallel raceway, sized based on the breaker protecting the entire parallel set.
- Motor Circuits: If your overcurrent device is sized significantly higher than the conductor ampacity (common with motor short-circuit protection), you size the EGC based on the motor full-load current and conductor size, not the massive breaker rating.
Grounding Wire Chart FAQ
What size grounding wire do I need for a 100-amp subpanel?
According to Table 250.122, a 100-amp overcurrent device requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. However, if you are feeding a subpanel, ensure you are pulling a dedicated EGC back to the main panel's ground bar. Do not rely on the neutral bus for grounding at the subpanel, as neutral and ground must be isolated at all subpanels per NEC 250.32.
Does the equipment grounding conductor need to be in the same conduit?
Yes. NEC 250.134 requires the EGC to be routed in the same raceway, cable, or trench as the circuit conductors. This is not just a bureaucratic rule; it is based on physics. Keeping the ground wire tight against the hot wires minimizes the magnetic loop area, which drastically reduces circuit impedance. High impedance during a fault means the breaker won't trip fast enough to clear the short, creating a severe fire and shock hazard.
Can I use a smaller ground wire if the run is very short?
No. There is no "short run" exception in Table 250.122. Even if the wire is only 3 feet long, a 50-amp breaker requires a 10 AWG copper ground. The table values are calculated based on the maximum available fault current the breaker will let through before tripping (let-through current). A smaller wire could vaporize under high-magnetic fault forces before the breaker's mechanical latch has time to physically open the contacts.
What is the difference between a grounding electrode conductor and an equipment grounding conductor?
An Equipment Grounding Conductor (EGC) (sized via Table 250.122) connects the metal frames of tools, appliances, and subpanels back to the main panel to provide a low-impedance path for fault current to trip the breaker. A Grounding Electrode Conductor (GEC) (sized via Table 250.66) connects the main panel's ground bus to the physical earth (ground rods, metal water pipes) to stabilize voltage against lightning strikes and utility line surges. The EGC saves you from a short circuit; the GEC saves your house from a lightning strike.
