The correct ground wire size for a branch circuit or feeder is determined by the rating of the overcurrent protective device (the breaker or fuse), not the actual load current or the size of the hot wires. For a standard 20-amp breaker, you need a 12 AWG copper ground wire. For a 100-amp breaker, you need an 8 AWG copper ground wire. This sizing ensures the wire can safely carry fault current long enough for the breaker to trip without melting.
How to Read the NEC Ground Wire Size Chart
The table below is sourced directly from NEC Table 250.122 (National Electrical Code, NFPA 70). It dictates the minimum size for an Equipment Grounding Conductor (EGC). Before using this chart, you must understand how to read the columns:
- Breaker Rating: This is the ampere rating of the circuit breaker or fuse protecting the circuit. Look up your specific breaker size here.
- Copper Column: Use this column if your ground wire is copper (the standard for almost all residential branch circuits and indoor feeders).
- Aluminum Column: Use this column only if you are pulling an insulated aluminum equipment ground, which is rare in residential work but common in large commercial feeders. Never use bare aluminum for grounding.
| Breaker / Fuse Rating (Amps) | Copper Wire Size (AWG / kcmil) | Aluminum Wire Size (AWG / kcmil) |
|---|---|---|
| 15A | 14 | 12 |
| 20A | 12 | 10 |
| 30A | 10 | 8 |
| 40A | 10 | 8 |
| 50A | 10 | 8 |
| 60A | 10 | 8 |
| 100A | 8 | 6 |
| 110A | 8 | 6 |
| 150A | 6 | 4 |
| 200A | 6 | 4 |
| 250A | 4 | 2 |
| 300A | 4 | 2 |
| 400A | 3 | 1 |
| 500A | 2 | 1/0 |
| 600A | 1 | 2/0 |
When Derating and Upsizing Change Your Ground Wire Size
A common point of confusion is how ambient temperature derating or voltage drop calculations affect the ground wire. The EGC does not carry continuous load current, so you do not apply NEC 310.15 ambient temperature derating factors directly to the ground wire's ampacity. However, derating and voltage drop do affect your ground wire indirectly through NEC 250.122(B).
If you must upsize your ungrounded (hot) conductors to compensate for voltage drop on a long run, or because ambient temperature derating forced you to use a larger hot wire, you must proportionally increase the size of the equipment grounding conductor.
You are wiring a 60-amp subpanel 150 feet away. Table 250.122 says a 60A breaker requires a 10 AWG copper ground. Your hot wires are normally 6 AWG copper. However, to keep voltage drop under 3% at that distance, you upsize the hot wires to 4 AWG copper (an increase of two cross-sectional area steps).
Per NEC 250.122(B), you must upsize the ground wire by the same proportion. Moving up two steps from 10 AWG means your new minimum ground wire size is 6 AWG copper. If you pull 4 AWG hots and a 10 AWG ground, you will fail inspection.
For exact cross-sectional area proportions when upsizing, refer to NEC Chapter 9, Table 8. If you are using parallel conductor runs, the proportional increase applies to each individual EGC pulled in the parallel raceways.
What This Ground Wire Size Chart Cannot Tell You
While NEC Table 250.122 is the definitive NFPA 70 standard for branch circuits and feeders, it has strict limitations. Do not use this chart for the following scenarios:
- Grounding Electrode Conductors (GEC): This chart is for Equipment Grounding Conductors (EGC), which run from the breaker panel to the outlets or subpanels to clear faults. It does not apply to the wire connecting your main service panel to the ground rods or ufer ground (the GEC). For GEC sizing, you must use NEC Table 250.66, which sizes the wire based on the largest ungrounded service entrance conductor, not the breaker.
- Parallel Conductor Runs: If you are running parallel sets of conductors (e.g., two sets of 500 kcmil for a 800A service), Table 250.122 gives you the total minimum size. However, NEC 250.122(C) requires that if you pull your parallel hots in separate raceways, you must pull a full-size EGC in each raceway. You cannot split the ground wire size in half.
- Metal Raceways as a Ground: If you are using rigid metal conduit (RMC) or electrical metallic tubing (EMT) as your equipment grounding conductor per NEC 250.118, you do not need to pull a wire ground at all, provided the fittings are properly tightened and listed for grounding. The chart becomes irrelevant in this specific installation method.
Frequently Asked Questions About Ground Wire Sizing
Can I use a larger ground wire than the chart requires?
Yes. NEC Table 250.122 specifies the minimum size. Using a larger ground wire (e.g., pulling 10 AWG ground on a 20A breaker that only requires 12 AWG) is perfectly legal and often done by electricians who buy wire in bulk or want to reduce overall circuit impedance. The only limitation is physical conduit fill; a larger ground wire takes up more cross-sectional area in your raceway.
Does the equipment ground need to be the same size as the hot wire?
No, except on very small circuits. On a 15A circuit (14 AWG hot) or a 20A circuit (12 AWG hot), the ground wire happens to be the same size as the hot wire. However, as breaker sizes increase, the ground wire becomes proportionally smaller than the hot wire. For example, a 100A breaker requires 3 AWG copper hot wires, but only an 8 AWG copper ground wire. The ground only needs to carry current for the fraction of a second it takes for the breaker to trip during a dead short.
What size ground wire do I need for a 200-amp service panel?
This is a trick question that depends on which "ground" you mean. If you are running a 200-amp feeder from a main panel to a subpanel, the Equipment Grounding Conductor (EGC) requires 6 AWG copper per the chart above. However, if you are wiring the main 200-amp service entrance from the meter to the main panel, the Grounding Electrode Conductor (GEC) connecting to your ground rods requires 4 AWG copper (or 2 AWG aluminum) per NEC Table 250.66. Always verify if you are sizing an EGC or a GEC.
Do I count the ground wire for conduit fill calculations?
No. When calculating conduit fill to ensure you aren't overcrowding your PVC or EMT, NEC Chapter 9, Table 1, Note 2 explicitly states that equipment grounding conductors are not counted when applying the percentage fill rules. However, you must still account for the physical space they take up when pulling the wires, and if you are using a bare copper ground, its bare diameter must be used for any physical clearance checks.
