The minimum size of an equipment grounding conductor (EGC) is determined strictly by the rating of the overcurrent protective device (breaker or fuse) protecting the circuit, not the actual continuous load of the appliance. According to NEC Article 250.122, a 40-amp breaker requires a specific minimum ground wire size regardless of whether the connected load draws 12 amps or 38 amps. This ensures the wire can safely carry the massive, instantaneous fault current required to trip the breaker during a short circuit.
How to Read the Grounding Wire Size Chart
Before pulling wire, you need to understand how to read the standard grounding wire size chart. The table below is based on the National Electrical Code (NEC) Table 250.122. Here is how to apply the columns to your specific installation:
- Overcurrent Device Rating: Look at the breaker or fuse size protecting the circuit. A 50A breaker means you look at the 50A row (or the next highest standard size if your exact breaker isn't listed, though standard sizes are fully covered below).
- Copper vs. Aluminum Column: You must match the grounding wire material to the column that corresponds to the ungrounded (hot) conductors you are using. If you are pulling THHN copper through conduit, use the Copper column. If you are running aluminum URD underground, use the Aluminum column. Never mix materials without proper anti-oxidant compound and rated lugs.
The chart below is for the Equipment Grounding Conductor (EGC) — the wire that runs alongside your hot and neutral wires to ground your tools and panels. It is not for the Grounding Electrode Conductor (GEC), which is the heavy wire connecting your main service panel to your ground rods or water pipe. Sizing a GEC requires a completely different chart (NEC Table 250.66) based on your service entrance conductor size, not your breaker size.
Complete Equipment Grounding Conductor (EGC) Sizing Table
Quick-Jump Bookmark Rows: For standard residential branch circuits and feeders, the most frequently queried values are 20A (12 AWG Cu), 60A (10 AWG Cu), 100A (8 AWG Cu), and 200A (6 AWG Cu).
| Overcurrent Device Rating (Amps) | Copper Wire Size (AWG) | Aluminum Wire Size (AWG) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 110 | 6 | 4 |
| 200 | 6 | 4 |
| 250 | 4 | 2 |
| 300 | 3 | 1 |
| 400 | 1 | 1/0 |
When the Chart Doesn't Apply: Upsizing, Derating, and Edge Cases
While the table above provides the baseline minimums, real-world jobsite conditions frequently force you to deviate from these base values. Understanding how derating rows and voltage drop calculations modify the base value is where most DIYers fail inspections.
How Derating and Upsizing Modify the Base Value
First, a crucial clarification: unlike current-carrying ungrounded (hot) conductors, equipment grounding conductors do not require ambient temperature derating (NEC 310.15). Because an EGC only carries current during a fault condition—which lasts milliseconds before the breaker trips—heat buildup from continuous load is not a factor.
However, the base value is modified by proportional upsizing for voltage drop under NEC 250.122(B). If your circuit run is so long that you must upsize the hot and neutral wires to prevent excessive voltage drop, you must upsize the ground wire by the exact same proportional cross-sectional area.
Worked Numeric Example:
You are wiring a 60A EV charger 150 feet from the panel. The base chart calls for 6 AWG copper hots and a 10 AWG copper ground. However, a voltage drop calculation shows 6 AWG will yield a 4.5% drop (exceeding the recommended 3%). You upsize the hot wires to 4 AWG copper.
Because you increased the hot wire cross-sectional area by a factor of roughly 1.6 (from 26,240 circular mils to 41,740 circular mils), you must multiply the ground wire area by 1.6 as well. A 10 AWG ground (10,380 circular mils) multiplied by 1.6 equals 16,608 circular mils. The next standard wire size that covers this is 8 AWG. If you pull a 10 AWG ground with 4 AWG hots, you will fail inspection.
What the Table Cannot Tell You
This chart is blind to two major installation variables:
- Grounding Electrode Conductors (GEC): As mentioned, if you are bonding a ground rod, metal water pipe, or ufer ground to your main service disconnect, Table 250.122 is the wrong tool. You must use Table 250.66, which scales based on the size of your service entrance conductors (e.g., 2/0 AWG service entrance requires a 4 AWG copper GEC).
- Parallel Conductor Runs: If you are running parallel sets of wire for a 400A+ service, you cannot just run one massive ground wire. NEC 250.122(F) requires an equipment grounding conductor in each parallel raceway or cable, and each one must be sized based on the overcurrent device rating, not divided among the runs.
For comprehensive safety standards regarding grounding and bonding in residential and commercial environments, always cross-reference your local Authority Having Jurisdiction (AHJ) with OSHA electrical safety guidelines and the latest NEC handbook.
Frequently Asked Questions About Grounding Wire Sizes
Does a grounding wire need to be the same size as the hot wire?
No, and in most residential circuits, it is significantly smaller. The hot wire is sized to carry the continuous operational load of the appliance without melting. The grounding wire only needs to carry a massive short-circuit current for the fraction of a second it takes for the breaker's magnetic trip mechanism to activate. For example, a 100A subpanel feeder uses 3 AWG copper for the hots, but only requires an 8 AWG copper wire for the equipment ground. The only time they are the same size is on very small circuits (like a 15A circuit using 14 AWG for both) or massive industrial feeders where the EGC table maxes out.
What size ground wire do I need for a 100-amp subpanel?
According to the chart, a 100-amp breaker requires an 8 AWG copper or 6 AWG aluminum equipment grounding conductor. However, if this subpanel is in a detached structure (like a garage or shed), NEC 250.32 requires you to also install a local grounding electrode system (like two ground rods). The wire connecting the subpanel's ground busbar to those local rods is a Grounding Electrode Conductor (GEC), which is typically sized at 8 AWG bare copper for standard residential ground rod setups, completely independent of the 8 AWG EGC you pulled with the feeder.
Can I use a bare copper wire for an equipment grounding conductor?
Yes, bare copper is not only allowed but is the industry standard and preferred method for equipment grounding conductors. Inside standard NM-B (Romex) cable, the included ground wire is always bare copper. When pulling individual THHN wires through PVC or EMT conduit, using a bare copper wire for the EGC is perfectly legal and saves money on insulation. The only exception is if you are pulling aluminum wire; aluminum EGCs must be insulated to prevent galvanic corrosion when interacting with dissimilar metals or masonry, and bare aluminum is strictly prohibited for underground grounding electrodes due to rapid soil degradation.
