The size of your equipment grounding conductor (EGC) is dictated by a single number: the rating of the overcurrent protective device (breaker or fuse) protecting the circuit. It is not based on the actual load current, and it does not use the temperature ampacity columns you rely on for hot wires. To find your exact wire size, you use the NEC grounding conductor chart, officially published as NEC Table 250.122.
Below is the complete reference table, followed by the exact mathematical procedure for upsizing your ground when voltage drop forces you to increase your ungrounded (hot) conductors.
The Quick-Answer Grounding Conductor Chart (NEC Table 250.122)
This table provides the minimum size equipment grounding conductors for raceways and cables. The values below are sourced directly from the National Fire Protection Association (NFPA) 70, National Electrical Code (NEC), Article 250.122. Bookmark this section for quick jobsite lookups.
| Overcurrent Device Rating (Amps) | Copper Wire (AWG/kcmil) | Aluminum Wire (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 50 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 150 | 6 | 4 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
| 800 | 1/0 | 3/0 |
| 1000 | 2/0 | 4/0 |
How to Read the Chart: Columns, Ratings, and the Copper Default
Unlike the ampacity tables in NEC 310.16, the grounding conductor chart does not require you to cross-reference 60°C, 75°C, or 90°C temperature columns. The EGC size is purely a function of the breaker rating ahead of it. Here is how to interpret the columns and apply them to your installation:
- Overcurrent Device Rating: This is the trip rating of your breaker or fuse. If you have a 50A breaker protecting a circuit, you look at the 50A row. If your breaker size falls between two rows (e.g., a 70A breaker), you must round up to the next highest row (100A).
- Copper vs. Aluminum: Choose the column that matches your conductor material. For 99% of residential branch circuits and subpanel feeders, you will use the Copper column. Aluminum grounding conductors are rare in standard residential wiring and cannot be used in direct contact with masonry or earth, nor can they be installed where subject to corrosive conditions.
- The '10 AWG Plateau': Notice that 30A, 40A, 50A, and 60A all share the exact same minimum copper grounding conductor: #10 AWG. This is a common point of confusion for apprentices who assume a 60A circuit needs a larger ground than a 30A circuit. It does not, unless the hot wires were upsized.
The Voltage Drop Trap: When You Must Upsize the Ground
The most frequent code violation involving this chart occurs when installers upsize ungrounded (hot) conductors to mitigate voltage drop on long runs, but fail to upsize the grounding conductor proportionally. This is governed by NEC 250.122(B).
If you increase the size of your hot wires for any reason (voltage drop, ambient temperature derating, or conduit fill adjustments), the equipment grounding conductor must be increased in size proportionally based on circular mils.
1. Find the circular mil (cmil) ratio:
Upsized Hot (#3 AWG) = 52,620 cmil
Base Hot (#6 AWG) = 26,240 cmil
Ratio = 52,620 / 26,240 = 2.005
2. Apply ratio to the base ground:
Base Ground for 60A (from chart) = #10 AWG = 10,380 cmil
Required Ground cmil = 10,380 x 2.005 = 20,811 cmil
3. Select the new wire size:
Looking at NEC Chapter 9, Table 8, #8 AWG is 16,510 cmil (too small). #6 AWG is 26,240 cmil.
Final Pick: You must pull a #6 AWG copper grounding conductor, not the #10 AWG the base chart suggests.
For a deeper dive into the mathematical proofs behind proportional EGC sizing, EC&M's analysis on sizing equipment grounding conductors provides excellent field scenarios.
Step-by-Step Decision Path: Pick Your Exact Ground Wire
Use this decision tree to terminate your wire selection process with a single, code-compliant part number. Do not guess; follow the logic path.
| Condition / Scenario | Action Required | Final Concrete Pick (Copper) |
|---|---|---|
| Standard 20A branch circuit, run under 50 feet. | Lookup 20A row. No upsizing required. | #12 AWG (Bare or Green THHN) |
| 60A EV charger or subpanel, standard distance. | Lookup 60A row. No upsizing required. | #10 AWG (Bare or Green THHN) |
| 100A subpanel, long run requiring #2 AWG hots (upsized from #3). | Calculate cmil ratio. Multiply base #8 ground cmil by ratio. | #4 AWG (Proportionally upsized) |
| 200A main service entrance, standard distance. | Lookup 200A row. (Note: Service entrances use GEC rules, but if sizing an EGC for a downstream 200A panel, use chart). | #6 AWG (Bare copper) |
| Breaker is 45A (non-standard size). | Round up to the next standard table row (60A). | #10 AWG (Bare or Green THHN) |
What This Chart Cannot Tell You (And Where to Look Next)
While NEC Table 250.122 is the definitive grounding conductor chart for equipment and branch circuits, it has strict boundaries. It will not help you if you are dealing with the following scenarios:
- Grounding Electrode Conductors (GEC): If you are connecting your main service panel to a ground rod, ufer ground, or metal water pipe, you are not installing an EGC; you are installing a GEC. You must use NEC Table 250.66, which sizes the wire based on the physical size of your largest service entrance conductor, not the breaker rating.
- Main Bonding Jumpers: The wire or busbar that bonds the neutral bar to the ground bar inside the main service disconnect is sized per NEC 250.28, which also relies on the area of the largest ungrounded service conductor.
- Parallel Conductor Runs: If you are running parallel sets of wire (e.g., two sets of 500 kcmil for an 800A service), NEC 250.122(F) requires that a full-size equipment grounding conductor be run in each parallel raceway. You cannot run one massive ground in a single conduit and daisy-chain the equipment. Each conduit gets its own EGC sized to the breaker protecting the entire parallel set.
Default Recommendation: For all standard branch circuits and feeders where voltage drop is within acceptable limits (under 3%), default to the Copper column of Table 250.122, pull a bare copper wire to save on insulation costs and conduit fill space, and terminate it to a 75°C rated ground bar torqued to the manufacturer's specification. If your run exceeds 100 feet, run the voltage drop math first, and apply the 250.122(B) proportional multiplier before cutting your ground wire.
