When electricians and DIYers search for a "grounding ampacity chart," they are almost always looking for NEC Table 250.122. However, the term "ampacity" is technically a misnomer in this context. Ampacity (defined in NEC Article 100) refers to the maximum continuous current a conductor can carry without exceeding its temperature rating. Equipment Grounding Conductors (EGCs) do not carry continuous load current; they only carry fault current long enough to trip the breaker. Therefore, the NEC sizes the EGC based on the rating of the overcurrent protective device (OCPD), not the load ampacity.
Below is the complete reference chart, the rules for selecting your material column, and the specific edge cases where the base table values must be modified.
The NEC Table 250.122 Grounding Conductor Sizing Chart
How to read this table: Locate the rating of your breaker or fuse in the left column. Read across to the Copper (Cu) column for standard branch circuits and most feeders, or the Aluminum (Al) column if you are pulling large-gauge aluminum feeder cable (like SER or XHHW) where cost savings justify the larger physical wire size. These values represent the minimum allowable size; you can always use a larger ground wire, but never smaller.
| Overcurrent Device Rating (Amps) | Copper (Cu) EGC Size (AWG/kcmil) | Aluminum (Al) EGC Size (AWG/kcmil) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 50 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
Note: For OCPD ratings not listed in this abbreviated chart (e.g., 110A, 225A), the NEC mandates using the next higher standard OCPD rating row to find your minimum ground size. Always consult the full NFPA 70 National Electrical Code for ratings above 400A or specific non-standard fuse ratings.
Which Column Applies and How Modifications Work
Choosing Copper vs. Aluminum
For 95% of residential and light commercial work, the Copper column applies. Standard NM-B (Romex) and THHN in conduit utilize copper grounding wires or bare copper grounds. The Aluminum column applies strictly when your ungrounded (hot) conductors are aluminum. You cannot mix metals in the same cable assembly, and while you can technically run a copper ground alongside aluminum hots in a conduit, it is highly impractical and rarely done outside of specific transition lugs. If your hots are aluminum, your ground must be aluminum (or copper-clad aluminum rated for the application).
How Derating Modifies the Base Value
This is where many apprentices and DIYers get confused. Standard ambient temperature and conduit fill derating factors (NEC 310.15) do not apply to the base values in Table 250.122. Because the EGC only carries current during a fault (which lasts milliseconds), it does not generate sustained heat. You do not need to upsize your ground wire just because you have 4 current-carrying conductors in a conduit or a 110°F attic.
However, there is one massive exception that functions as a modification rule: Voltage Drop Proportional Upsizing (NEC 250.122(B)).
Worked Numeric Example:
You are running a 60A circuit. Standard wire is 6 AWG Cu (26,240 circular mils). Table 250.122 dictates a 10 AWG Cu ground (10,380 circular mils). Because the run is 250 feet, you upsize the hot wires to 4 AWG Cu (41,740 circular mils) to limit voltage drop.
1. Calculate the upsize ratio: 41,740 / 26,240 = 1.59.
2. Apply ratio to the ground: 10,380 cmil × 1.59 = 16,504 cmil.
3. Find the new wire size: 8 AWG Cu is 16,510 cmil.
Result: Your ground wire must be upsized from 10 AWG to 8 AWG to remain code-compliant.
What the Table Cannot Tell You (Edge Cases)
Table 250.122 is the baseline, but real-world jobsite conditions frequently trigger edge cases that the chart alone cannot resolve. Understanding these prevents failed inspections and dangerous fault-clearing failures.
1. Parallel Conductor Runs
If you are pulling parallel feeders (e.g., two sets of 500 kcmil for an 800A service), you might assume you only need one massive ground wire. NEC 250.122(F) strictly forbids this. When conductors are run in parallel across multiple raceways, you must install a full-sized Table 250.122 EGC in every single raceway. If the chart calls for a 2/0 AWG ground for an 800A breaker, you must pull a 2/0 AWG ground in Pipe A, and another 2/0 AWG ground in Pipe B. This ensures that if one raceway is physically destroyed, the remaining raceway still has a fully capable fault-current path.
2. Motor Circuits and High Inrush Loads
Motors draw massive inrush current, meaning the breaker protecting the circuit is often sized much larger than the ampacity of the wire. For example, a 20A full-load motor might be wired with 12 AWG THHN (rated 25A) but protected by a 40A inverse-time breaker to allow for startup surges. In this scenario, you do not use the 12 AWG wire size to find your ground; you use the 40A breaker rating. Looking at the chart, a 40A OCPD requires a 10 AWG ground. Always size the EGC to the breaker, not the motor wire.
3. Adjustable Trip Breakers
If you are using a molded-case circuit breaker with an adjustable trip setting (common in industrial panels), Table 250.122 requires you to size the EGC based on the maximum possible trip setting of the breaker, regardless of where the dial is currently set. If a 400A frame breaker is dialed down to 250A, you still must use the 400A row (3 AWG Cu), because someone could turn the dial up in the future. The only exception is if the breaker has a restricted access, sealed adjustment mechanism approved by the AHJ (Authority Having Jurisdiction).
For deeper dives into complex grounding topologies and separately derived systems, reference the safety grounding guidelines outlined by All About Circuits and always verify your final design with your local electrical inspector, as local amendments to the NEC can occasionally mandate larger minimum EGC sizes for specific municipal feeders.
