The size of your ground wire (Equipment Grounding Conductor or EGC) is determined by the rating of the overcurrent protective device (breaker or fuse), not the actual load current. For a standard 15A or 20A residential breaker, you need a 14 AWG or 12 AWG copper ground wire, respectively. Use the ground wire sizing table below, based on NEC Table 250.122, to find the exact requirement for your circuit.
The NEC Ground Wire Sizing Table (NEC 250.122)
How to read this table: The data below is derived directly from Table 250.122 of the National Electrical Code (NFPA 70). Unlike current-carrying conductors where you must choose between 60°C, 75°C, or 90°C ampacity columns, EGC sizing is strictly tied to the OCPD rating. Choose the row matching your breaker size, then select the Copper or Aluminum column based on your wire material. The most frequently queried residential and light commercial sizes (15A, 20A, 50A, and 200A) are highlighted in bold for quick bookmark-friendly reference.
| Breaker / Fuse Rating (Amps) | Copper Wire (AWG/kcmil) | Aluminum Wire (AWG/kcmil) |
|---|---|---|
| 15A | 14 AWG | 12 AWG |
| 20A | 12 AWG | 10 AWG |
| 30A | 10 AWG | 8 AWG |
| 40A | 10 AWG | 8 AWG |
| 50A | 10 AWG | 8 AWG |
| 60A | 10 AWG | 8 AWG |
| 100A | 8 AWG | 6 AWG |
| 110A | 8 AWG | 6 AWG |
| 200A | 6 AWG | 4 AWG |
| 300A | 4 AWG | 2 AWG |
| 400A | 3 AWG | 1 AWG |
| 500A | 2 AWG | 1/0 AWG |
| 600A | 1 AWG | 2/0 AWG |
Source: NEC Table 250.122 (NFPA 70). Always verify with your local Authority Having Jurisdiction (AHJ).
Critical Adjustments: When the Base Table Isn't Enough
What the table cannot tell you: This chart assumes standard installation lengths and standard terminal temperatures. It does not account for voltage drop mitigations, parallel conductor runs, or installations where the metal raceway itself is being used as the EGC (per NEC 250.118). If your installation falls into these categories, the base table values are legally insufficient.
A common point of confusion is how bundling derating (NEC 310.15(C)(1)) affects ground wires. When you pull more than three current-carrying conductors in a raceway, you must derate the ampacity of the hot and neutral wires. However, the EGC does not carry continuous load current, so you do not apply bundling derating factors to the ground wire.
Proportional Upsizing for Voltage Drop (NEC 250.122(B))
If you increase the size of your ungrounded (hot) conductors to compensate for voltage drop over a long run, you must increase the EGC proportionally. You cannot simply use the base table value. The upsizing is calculated using the circular mil (cmil) area of the wires.
Worked Example: You are running a 50A feeder to a detached garage 250 feet away.
- Base Rule: A 50A breaker requires 6 AWG copper hots (26,240 cmil) and a 10 AWG copper ground (10,380 cmil).
- Voltage Drop Adjustment: To keep voltage drop under 3%, you upsize the hot wires to 2 AWG copper (66,360 cmil).
- The Math: Divide the new hot wire cmil by the base hot wire cmil (66,360 / 26,240 = 2.528). Multiply the base ground wire cmil by this ratio (10,380 × 2.528 = 26,240 cmil).
- The Result: Looking at NEC Chapter 9, Table 8, a 4 AWG copper wire (41,740 cmil) is the minimum size that exceeds 26,240 cmil. Your ground wire must be upsized from 10 AWG to 4 AWG.
Parallel Conductor Runs (NEC 250.122(C))
When running parallel feeders (e.g., multiple conduits carrying phases A, B, C, and Neutral), you do not divide the ground wire size. Each individual raceway or cable must contain its own full-sized EGC based on the rating of the upstream overcurrent device. If you have a 600A service split into two parallel conduits, each conduit gets its own 1 AWG copper ground wire, not a split 3 AWG.
Frequently Asked Questions (FAQ)
Can I use a smaller ground wire if my actual load is much lower than the breaker size?
No. The EGC is sized to the overcurrent protective device (the breaker), not the actual connected load. The purpose of the ground wire is to provide a low-impedance fault path that allows enough current to flow to instantly trip the breaker during a short circuit. If you use a 14 AWG ground on a 50A breaker because your load only draws 10A, a ground fault might not draw enough current to trip the 50A breaker before the 14 AWG ground wire melts, creating a severe fire and shock hazard.
Does the ground wire need to be insulated, or can I use bare copper?
For standard residential and commercial wiring using NM-B (Romex) or THHN in conduit, bare copper is perfectly legal and standard practice for the EGC. However, there are exceptions. If you are pulling wires in specific hazardous locations, or if local amendments require it for underground feeders, you may need an insulated ground. When insulation is required, it must be colored green or green with one or more yellow stripes (NEC 250.119).
How do I size the ground wire when running parallel feeders to a subpanel?
As outlined in NEC 250.122(C), when conductors are run in parallel in multiple raceways or cables, an equipment grounding conductor must be run with the parallel conductors in each raceway or cable. Each EGC must be sized based on the total rating of the overcurrent device protecting the parallel circuit. For example, if you have an 800A main breaker feeding four parallel conduits, you look up 800A on the table (which requires 1/0 AWG Copper). You must pull a 1/0 AWG Copper ground in all four conduits.
What happens if I use the aluminum column for a copper wire?
If you accidentally use the aluminum column to size a copper ground wire, you will end up with a wire that is physically larger than the code minimum. From a safety perspective, this is completely fine—a larger ground wire has lower impedance and handles fault currents better. However, from a practical standpoint, you are wasting money on expensive copper, and the thicker wire will be much stiffer and harder to terminate in standard outlet boxes or lug terminals. Always match the column to your actual wire material.
