The size of your equipment grounding conductor (EGC) is dictated strictly by the rating of the overcurrent protective device (breaker or fuse) protecting the circuit, not the ampacity of the circuit wires. For a standard 20-amp residential branch circuit, you need a 12 AWG copper ground. For a 60-amp subpanel feeder, you need a 10 AWG copper ground. Sizing the EGC based on the hot wire size is the most common mistake DIYers make, and it can result in a ground wire that melts before the breaker trips during a dead short.

Below is the master reference for sizing your ground wires, pulled directly from the National Electrical Code (NEC). Bookmark this page for your next rough-in.

The Master Equipment Grounding Conductor Size Chart

The following data is sourced directly from NEC Table 250.122 (Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment). To read this table correctly, locate the rating of the breaker or fuse installed in the panel. Trace that row to the right to find the minimum allowable gauge for copper or aluminum. The temperature rating of the conductor insulation (60°C, 75°C, or 90°C) does not alter the values in this specific table; the EGC size is purely a function of the overcurrent device rating.

NEC Table 250.122: Minimum Size Equipment Grounding Conductors
Rating of Overcurrent Device (Amps) Copper Wire (AWG/kcmil) Aluminum Wire (AWG/kcmil)
15A 14 12
20A 12 10
30A 10 8
40A 10 8
60A 10 8
100A 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
Pro-Tip for Panel Builders: Notice that 30A, 40A, and 60A breakers all share the same minimum EGC size (10 AWG Copper). Do not downsize the ground wire just because a 30A circuit uses 10 AWG hot wires; if you swap that breaker for a 40A or 60A later, the 10 AWG ground remains code-compliant. However, you can never use a 14 AWG ground on a 20A breaker, even if the circuit only powers a single 12-amp receptacle.

Which Column Applies and How Proportional Increases Work

For 95% of residential branch circuits and light commercial work, you will use the Copper column. Standard NM-B (Romex) and THHN in conduit rely on copper grounding conductors. The Aluminum column applies primarily to heavy feeders using SER cable or XHHW-2 aluminum wire in conduit, commonly seen in 200A residential service entrances or large subpanel feeds. Never mix copper and aluminum in the same grounding path without using properly rated bi-metallic lugs to prevent galvanic corrosion.

The Proportional Increase Rule (NEC 250.122(B))

The table above assumes your ungrounded (hot) conductors are sized exactly to the minimum required ampacity for the breaker. But what happens when you upsize your hot wires to mitigate voltage drop on a long run? According to NFPA 70 (NEC) Section 250.122(B), if you increase the size of the ungrounded conductors, the equipment grounding conductor must be increased proportionately based on the circular mil area.

Here is the exact bench math for a real-world scenario:

  • The Setup: You are running a 60A subpanel feeder 150 feet away. To keep voltage drop under 3%, you upsize your hot wires from the minimum #6 AWG to #4 AWG copper.
  • The Base Values: Table 250.122 dictates a #10 AWG copper ground for a 60A breaker. Chapter 9, Table 8 tells us #6 AWG has 26,240 circular mils (cmil), and #10 AWG has 10,380 cmil.
  • The Ratio: Your new #4 AWG wire has 41,740 cmil. Divide the new wire size by the base wire size: 41,740 / 26,240 = 1.59.
  • The New Ground Size: Multiply the base ground cmil by your ratio: 10,380 cmil × 1.59 = 16,504 cmil.
  • The Result: Looking back at Chapter 9, Table 8, an #8 AWG wire has 16,510 cmil. Therefore, you must upsize your equipment grounding conductor from #10 AWG to #8 AWG.

If you skip this math and pull a standard #10 ground alongside your upsized #4 hots, an inspector will fail your rough-in, and more importantly, the #10 ground may not clear a high-impedance fault at the far end of that 150-foot run.

Adjustment Factors vs. Proportional Increases

Do not confuse ampacity derating (NEC 310.15) with proportional increases. If you have to upsize your hot wires because you have nine current-carrying conductors in a single conduit (requiring a 70% derating factor), you do not need to upsize the EGC. The EGC only carries current during a fault condition, which lasts milliseconds. It does not generate continuous heat, so conduit fill derating does not apply to the ground wire.

What This Chart Cannot Tell You (Edge Cases & Parallel Runs)

While NEC Table 250.122 is the ultimate authority for standard circuits, it has blind spots. If your installation falls into one of the following categories, the table alone will not give you the correct answer.

Installation Scenario Why the Standard Chart Fails The Code-Compliant Solution
Parallel Feeder Runs You cannot simply divide the ground wire size across multiple conduits. A single #4 ground split across two conduits violates fault-current path integrity. Per NEC 250.122(F), you must run a full-sized EGC in every parallel raceway, sized based on the total overcurrent device rating (e.g., a 800A breaker requires a 1/0 Cu ground in each of the three parallel conduits).
Motor Circuits (NEC Article 430) Motor breakers (inverse time breakers) are often sized at 250% of the motor's full-load amps to handle inrush current. A 20A motor might sit on a 50A breaker. Size the EGC based on the 50A breaker rating (#10 Cu), not the 20A motor wire rating (#12 Cu). The breaker dictates the let-through energy during a fault.
High Fault Current Systems Table 250.122 assumes standard residential/light commercial available fault currents (typically 10kA to 22kA). Industrial panels can see 65kA to 100kA+. Engineers must calculate the specific let-through current of the protective device and verify the EGC can withstand the thermal and magnetic stresses of a massive fault without vaporizing.
Flexible Cords & Fixture Wire Table 250.122 applies to fixed wiring systems (NM-B, THHN, MC cable). It does not govern the internal ground wire of a manufactured lamp or appliance cord. Flexible cord EGC sizing is governed by NEC Table 409.5(B) and UL manufacturing standards, which generally require the ground to be the same size as the circuit conductors within the cord.

Finally, remember that local Authority Having Jurisdiction (AHJ) amendments can override baseline NEC tables. Some municipalities mandate a minimum 10 AWG copper ground for all branch circuits regardless of breaker size to provide a more robust physical wire that resists breaking during rough-in abuse. Always verify local amendments with your inspector before pulling wire for a large-scale project. For deeper reading on grounding electrode systems versus equipment grounding, refer to the code breakdowns published by Electrical Contractor Magazine.