Ground Wire Sizing Chart: NEC Table 250.122 Explained

The size of your ground wire—technically called the Equipment Grounding Conductor (EGC)—is dictated strictly by the rating of the overcurrent protective device (breaker or fuse) protecting the circuit, not by the actual load the device draws. For a standard 15-amp residential branch circuit, you need a minimum 14 AWG copper ground wire; for a 20-amp circuit, you need 12 AWG copper. If you are pulling a feeder to a subpanel protected by a 100-amp breaker, you must step up to 8 AWG copper or 6 AWG aluminum.

Below is the complete reference chart based on the National Electrical Code (NEC), followed by the specific rules for applying it to your installation.

The Master Ground Wire Sizing Chart (NEC Table 250.122)

This data is extracted directly from NEC Table 250.122 (NFPA 70). It provides the minimum allowable sizes for Equipment Grounding Conductors based on the ampere rating of the circuit breaker or fuse.

How to Read and Apply the Sizing Columns

When using this ground wire sizing chart, the most critical decision is selecting the correct material column. The NEC maintains separate columns for copper and aluminum (or copper-clad aluminum) because aluminum has a higher electrical resistance and lower thermal conductivity than copper.

Which column applies to your installation? If you are pulling standard NM-B (Romex) or THHN copper wire through conduit, use the Copper EGC column. If you are pulling SER cable or large aluminum feeder wires (common for 100A to 200A subpanels to save money on long runs), you must use the Aluminum EGC column. You cannot mix materials arbitrarily; the EGC material should generally match the ungrounded (hot) conductors to prevent galvanic corrosion at termination lugs, though copper pigtails to aluminum feeders are permitted with proper anti-oxidant compound and rated lugs.

Temperature Ratings: Unlike current-carrying conductors where you must navigate the 60°C, 75°C, and 90°C ampacity columns in NEC Table 310.16, Equipment Grounding Conductors do not use temperature columns for sizing. Table 250.122 is a flat lookup based purely on the breaker rating. The physical size of the wire inherently handles the thermal fault conditions required by the code. For physical conduit fill calculations, you will reference NEC Chapter 9, Table 8 for the bare or insulated cross-sectional area.

Proportional Increases and What the Chart Misses

A common point of confusion on the bench is how "derating" applies to ground wires. Let's clarify the physics and the code: EGCs are not derated for ambient heat or conduit bundling in the same way current-carrying conductors are (per NEC 310.15). Because an EGC only carries current during a fault condition (a short circuit), it does not generate continuous heat. Therefore, you do not apply the 80% bundling derating factors to your ground wire.

However, there is a different modification rule you must know: Proportional Upsizing (NEC 250.122(B)).

If you are forced to upsize your ungrounded (hot) wires to compensate for voltage drop on a long run, you must increase the size of your ground wire proportionally based on the circular mil area.

• Worked Example: You are installing a 100-amp subpanel 250 feet away. To keep voltage drop under 3%, you upsize your copper hot wires from the standard 3 AWG to 1/0 AWG.
• The Math: 3 AWG is 52,620 circular mils. 1/0 AWG is 105,600 circular mils. The ratio is roughly 2:1 (you doubled the wire size).
• The Rule: The standard ground for 100A is 8 AWG (16,510 circular mils). Because you doubled the hot wire size, you must double the ground wire size to roughly 33,020 circular mils. Looking at Chapter 9, Table 8, you must step up to a 4 AWG copper ground wire for this feeder.

What the Table Cannot Tell You

This chart is strictly for Equipment Grounding Conductors (EGCs)—the wires that run alongside your hot and neutral wires to provide a low-impedance fault path back to the panel. It does not apply to the Grounding Electrode Conductor (GEC). The GEC is the wire that connects your main service panel's neutral bar to the earth (ground rods, ufer grounds, or metal water pipes). Sizing a GEC requires an entirely different chart: NEC Table 250.66, which is based on the size of the largest service entrance conductor, not the breaker rating. Furthermore, this chart will not tell you if the wire will physically fit in your conduit; always run a conduit fill calculation using Chapter 9, Table 1 to ensure you aren't exceeding the 40% fill limit for three or more wires.

Ground Wire Sizing Chart FAQ

What size ground wire do I need for a 60 amp breaker?

According to NEC Table 250.122, a 60-amp breaker requires a minimum 10 AWG copper or 8 AWG aluminum equipment grounding conductor. This holds true whether the 60A breaker is protecting an EV charger, a subpanel feeder, or a large workshop tool. Note that while 10 AWG is the minimum for the ground, your hot wires will likely need to be 6 AWG copper (or 4 AWG aluminum) to safely carry 60 amps continuously without overheating.

Can I use a smaller ground wire if my actual load is low?

No. The NEC sizes the equipment grounding conductor based on the rating of the overcurrent protective device (the breaker), not the actual calculated load of the appliance. If you have a 5-amp compressor plugged into a 20-amp breaker, the breaker can still allow up to 20 amps to flow during a short-circuit fault before tripping. The ground wire must be sized (12 AWG copper) to safely clear that maximum 20-amp fault current without melting or creating a fire hazard.

Does a ground wire need to be the same size as the hot wire?

Not necessarily, and this is where the chart saves you money. For smaller circuits (15A to 30A), the ground wire often ends up being the same size as the hot wire simply because 14 AWG and 12 AWG are the smallest practical wires manufactured for building wiring. However, as amperage increases, the ground wire becomes significantly smaller than the hot wire. For example, a 200-amp feeder requires 2/0 AWG copper hot wires, but the chart shows you only need a 6 AWG copper ground wire. The physics of a short circuit dictates that the massive magnetic trip mechanism in the breaker will clear the fault in milliseconds, meaning the ground wire doesn't need the continuous thermal mass of the current-carrying conductors.

How do I size a grounding wire for a 200 amp subpanel?

First, clarify which wire you mean. If you mean the Equipment Grounding Conductor (the wire running in the conduit with the hots and neutral to clear faults), the chart above dictates a minimum 6 AWG copper or 4 AWG aluminum. However, if you are asking about the Grounding Electrode Conductor (the wire connecting the subpanel to a local ground rod), NEC 250.66 typically requires a minimum 8 AWG copper or 6 AWG aluminum for a standard ground rod connection, regardless of the panel size. Always install both: an EGC back to the main panel, and a local GEC to earth at the subpanel.