The NEC grounding size chart (officially Table 250.122 in the National Electrical Code) dictates the minimum Equipment Grounding Conductor (EGC) size based on the rating of the overcurrent protective device (breaker or fuse). For the most common residential branch circuits, the minimums are straightforward: a 15A breaker requires a 14 AWG copper ground, and a 20A breaker requires a 12 AWG copper ground. However, matching the ground wire to the current-carrying conductors (using 12 AWG for 20A and 10 AWG for 30A) is the universal standard practice on the jobsite to avoid confusion and ensure mechanical durability.
How to Read the NEC Grounding Size Chart (Table 250.122)
Before pulling wire, you need to understand how to read the spec sheet. Table 250.122 is organized by the Overcurrent Device Rating (the breaker size), not the actual calculated load of the circuit. If you have a 40A calculated load but install a 50A breaker, you must size the ground for the 50A breaker.
Which column applies to your installation? The table provides two material columns: Copper and Aluminum/Copper-Clad. For 99% of residential branch circuits, indoor feeders, and standard subpanels, you will use the Copper column. The Aluminum column is reserved for heavy service entrance feeders or specific industrial runs where aluminum wire is explicitly permitted and terminated with rated lugs.
Temperature rating irrelevance: Unlike current-carrying conductors (where you must differentiate between the 60°C column for NM-B Romex and the 75°C/90°C columns for THHN in conduit), the EGC size in Table 250.122 is independent of insulation temperature rating. A 10 AWG ground is a 10 AWG ground, whether it is bare, green THHN, or bundled inside a 60°C NM-B jacket. The fault-current capacity of the metal itself is what matters here, not the insulation's thermal limit.
| Overcurrent Device Rating (Amps) | Copper Wire Size (AWG/kcmil) | Aluminum / Copper-Clad Wire 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 | 6 | 4 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
Source: NFPA 70 National Electrical Code (NEC), Table 250.122. Always verify against your local AHJ's adopted code cycle.
When Derating and Upsizing Modify the Base Value
A common point of confusion for apprentices and DIYers is how derating factors apply to grounding conductors. Here is the definitive rule: Equipment Grounding Conductors are not subject to ambient temperature derating or conduit bundling adjustment factors (NEC 310.15). Because the EGC only carries current during a fault condition (and only for a fraction of a second before the breaker trips), it does not generate sustained heat. You do not need to upsize the ground wire just because you have nine current-carrying conductors in the same conduit.
While ambient derating doesn't apply, voltage drop upsizing absolutely does. If you increase the size of your ungrounded (hot) conductors to compensate for voltage drop on a long run, you must proportionally increase the size of the EGC.
Worked Example: You are running a 50A circuit to a detached garage. Per the chart, the standard hot wires are 6 AWG Cu and the ground is 10 AWG Cu. However, the run is 150 feet, so you upsize the hot wires by two steps to 4 AWG Cu to keep voltage drop under 3%. Because you upsized the hots by two steps, you must upsize the ground by two steps as well: from 10 AWG to 8 AWG, and then to 6 AWG Cu. Your ground must now be 6 AWG Cu.
What the table cannot tell you: Table 250.122 only sizes the EGC (the wire that runs with your circuit back to the panel). It does not size the Grounding Electrode Conductor (GEC)—the wire that connects your main service panel to the ground rods or water pipe. For GEC sizing, you must use NEC Table 250.66. Furthermore, the chart assumes standard fault currents. In industrial settings with massive available fault currents (e.g., 65kA+), the magnetic let-through energy of the breaker might melt a minimum-sized EGC before the mechanical trip mechanism clears the fault, requiring an engineered upsized ground.
Bookmark-Friendly Quick-Jump Reference
For fast jobsite lookups, here are the most queried residential and light-commercial values extracted from the main chart:
- 15A / 20A Receptacle Circuits: 14 AWG Cu minimum (12 AWG Cu standard practice).
- 30A Dryer / RV Receptacle: 10 AWG Cu.
- 40A / 50A Range or EV Charger: 10 AWG Cu.
- 60A Subpanel Feeder: 10 AWG Cu.
- 100A Subpanel Feeder: 8 AWG Cu.
- 200A Main Service / Subpanel: 6 AWG Cu (or 4 AWG Al if using aluminum feeders).
Frequently Asked Questions
Can I use a smaller grounding wire if my circuit is very short?
No. The NEC grounding size chart sets an absolute minimum based on the overcurrent device rating, not the physical length of the wire. A 50A breaker requires a 10 AWG copper ground whether the outlet is 5 feet away or 50 feet away. The breaker only 'sees' the fault current; it does not know how long the wire is. Using a smaller wire risks the conductor melting during a short circuit before the breaker trips, leaving the equipment chassis energized.
Does the grounding size chart apply to grounding electrode conductors (GEC)?
No. This is a critical distinction in OSHA and NEC wiring design standards. The Equipment Grounding Conductor (EGC) runs with your circuit wires back to the panel busbar to clear faults. The Grounding Electrode Conductor (GEC) connects the panel busbar to the earth (ground rods, ufer ground, metal water pipe). The GEC is sized using a completely different table (NEC Table 250.66) based on the size of your largest service entrance conductor, not the breaker size.
What size ground wire do I need for a 100 amp subpanel?
For the feeder circuit supplying a 100A subpanel, Table 250.122 requires an 8 AWG Copper (or 6 AWG Aluminum) Equipment Grounding Conductor. However, if you are asking about the Grounding Electrode Conductor that bonds the subpanel to a local ground rod, that is governed by Table 250.66 and typically requires an 8 AWG Copper wire for a standard residential ground rod setup, regardless of the feeder size.
Do I need to count the ground wire when calculating conduit fill?
Yes. While the EGC does not count as a 'current-carrying conductor' for the purposes of thermal derating (ampacity adjustment), it physically occupies space inside the raceway. When calculating conduit fill capacity per NEC Chapter 9, Table 1, you must include the bare or insulated ground wire in your total cross-sectional area calculations. If you are pulling three 4 AWG THHN wires and one 8 AWG bare ground through a 1-inch PVC conduit, the ground wire counts toward your physical fill limit.
