How to Size a Ground Wire: Ground Wire Size Chart and Rules | NEC Table

Ground wire size comes from the breaker rating, not the load: a 20-amp circuit needs a 12 AWG copper equipment grounding conductor.

The wire that lands on a 60-amp breaker is often the same 10 AWG copper you would use on a 30-amp one, and that fact alone shows why load current never enters the math. Learning how to size a ground wire starts with the overcurrent device — the breaker or fuse — because that device is what has to clear a fault.

What follows is the working version of NEC Table 250.122, the rule that sets minimum equipment grounding conductor (EGC) sizes for US branch circuits and feeders. Four steps, one chart, and the mistakes that fail inspections.

How Do You Size A Ground Wire, Step By Step?

Ground wire sizing takes four steps: read the rating of the overcurrent device protecting the circuit, choose the column that matches your conductor material, select the minimum equipment grounding conductor for that rating, and increase the ground if the circuit conductors were increased. The device rating drives all four.

1. Find the overcurrent device rating. That is the breaker or fuse number, not the amps the equipment draws. A 15-amp motor on a 60-amp breaker still reads from the 60-amp row.
2. Pick the material column. Copper and aluminum are not interchangeable at the same gauge — aluminum and copper-clad aluminum run about two AWG sizes larger for the same rating.
3. Read across to the minimum size. The chart in the next section holds the whole lookup.
4. Check for upsized circuit conductors. If the current-carrying conductors were increased for voltage drop on a long run, the ground may need to increase along with them.

A lookup like this is right when the breaker number and the ground size land on the same row every time you recheck them, and the ground never shrinks as the breaker grows.

The table applies to branch circuits and feeders, which covers most residential and light commercial work. Service-entrance grounding and a few special installations follow different rules, so confirm the adopted edition in your area before ordering wire for those.

Ground Wire Size Chart: What Table 250.122 Lists For Each Amp Rating

Table 250.122 sets 14 AWG copper as the minimum for a 15-amp device, 12 AWG for 20 amps, and 10 AWG across the entire 30-to-60-amp band. Above that, the minimum size climbs roughly two steps each time the rating doubles. The values below match NEC Table 250.122 as compiled by UpCodes.

Larger gear keeps the pattern going.

A 200-amp feeder is where most homeowners start buying wire, and the 6 AWG copper on that row is sold in pre-cut lengths that beat cutting from a bulk spool. Our comparison of 6 AWG 25-foot ground wire options covers what holds up outdoors.

Where Ground Wire Sizing Goes Wrong

The three failures that turn up most in DIY work are sizing the ground by load amps, swapping copper for aluminum at the same gauge, and treating a published chart as if it were the adopted code.

• Sizing by load current. The load tells you nothing here. A 15-amp pump on a 60-amp breaker still reads from the 60-amp row: 10 AWG copper.
• Assuming the metals match. A 100-amp device takes 8 AWG copper but 6 AWG aluminum. Same rating, larger wire.
• Ignoring the application. One all-purpose chart cannot cover branch circuits, feeders, and service grounding at once, because they do not all follow Table 250.122.
• Trusting a chart over the code. Published charts are summaries. Your adopted local edition, plus any amendments, is what an inspector checks.

None of that is cosmetic. An undersized equipment grounding conductor can slow fault clearing enough to leave metal parts energized, which turns a wiring error into a shock and fire hazard.