🌍What size ground wire do you need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Ground wire size calculator

Breaker or fuse ahead of the circuit

Ground wire metal

Circuit conductor the table expects

Conductor you are actually pulling

8 AWG copper ground

behind a 100 A device · that is 8.4 mm²

Device rating	Copper ground	Aluminium ground
15 A and below	14 AWG	12 AWG
20 A and below	12 AWG	10 AWG
60 A and below	10 AWG	8 AWG
100 A and below	8 AWG	6 AWG
200 A and below	6 AWG	4 AWG
300 A and below	4 AWG	2 AWG
400 A and below	3 AWG	1 AWG
500 A and below	2 AWG	1/0 AWG
600 A and below	1 AWG	2/0 AWG
800 A and below	1/0 AWG	3/0 AWG
1000 A and below	2/0 AWG	4/0 AWG
1200 A and below	3/0 AWG	250 kcmil

Copper option: 8 AWG
Aluminium option: 6 AWG

Upsized the circuit for voltage drop? Set both size boxes and the ground grows with it.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

Try also

What size ground wire does a circuit need?

The AmpSizer ground wire size for a 100 amp circuit is 8 AWG copper, or 6 AWG aluminium. It comes from the rating of the device ahead of the circuit, not from the size of the conductors next to it — Table 250.122 is indexed on amps, not on wire.

That distinction matters because the two often disagree. A 100 A feeder pulled in 1 AWG aluminium and one pulled in 3 AWG copper take the same 8 AWG copper ground.

The exception is 250.122(B). If you made the circuit conductors bigger than they needed to be — almost always for voltage drop — the grounding conductor increases in the same proportion by area. Going from 8 AWG to 6 AWG is 1.59 times the area, so a 10 AWG ground becomes 8 AWG.

The whole table is below. It is short, it has not changed materially in decades, and it is worth knowing by heart for the sizes you use every week: 15 A takes 14, 20 A takes 12, everything from 25 to 60 A takes 10, and 100 A takes 8.

Minimum equipment grounding conductor by the rating of the device ahead of the circuit — NEC Table 250.122. Upsize it in proportion when the circuit conductors are upsized for voltage drop, per 250.122(B).
Device rating	Copper	Aluminium	Copper in mm²
Up to 15 A	14 AWG	12 AWG	2.08 mm²
Up to 20 A	12 AWG	10 AWG	3.31 mm²
Up to 60 A	10 AWG	8 AWG	5.261 mm²
Up to 100 A	8 AWG	6 AWG	8.367 mm²
Up to 200 A	6 AWG	4 AWG	13.3 mm²
Up to 300 A	4 AWG	2 AWG	21.15 mm²
Up to 400 A	3 AWG	1 AWG	26.67 mm²
Up to 500 A	2 AWG	1/0 AWG	33.62 mm²
Up to 600 A	1 AWG	2/0 AWG	42.41 mm²
Up to 800 A	1/0 AWG	3/0 AWG	53.49 mm²
Up to 1000 A	2/0 AWG	4/0 AWG	67.43 mm²
Up to 1200 A	3/0 AWG	250 kcmil	85.01 mm²

Why it is sized on amps and not on the wire

The grounding conductor has one job: carry enough fault current, for long enough, to make the overcurrent device operate. What decides that is the device — how much current it needs to see and for how long — not how thick the circuit conductors happen to be.

This is why the table steps in device ratings and why the same ground serves several conductor sizes. It is also why upsizing for voltage drop is treated as an exception rather than the rule: a longer run has more impedance in the fault path too, and 250.122(B) keeps the ground in proportion so the path does not get relatively weaker as the run gets longer.

Frequently asked questions

What size ground wire for a 100 amp sub panel?

8 AWG copper or 6 AWG aluminium for the equipment grounding conductor, from Table 250.122. Note that this is the ground that runs with the feeder — it is a different question from the grounding electrode conductor at a service, which is sized from Table 250.66 and is usually 4 AWG copper for a 100 to 200 A service.

Can the ground be smaller than the neutral?

Yes, and normally it is. The neutral is a current-carrying conductor sized like a hot; the equipment grounding conductor only has to carry fault current long enough to open the breaker. On a 100 A feeder the hots and neutral might be 3 AWG copper while the ground is 8 AWG.

Does the ground wire count toward conduit fill or derating?

Toward fill yes, toward derating no. Fill is about physical space in the pipe, so every conductor counts. Derating is about heat from current, and the grounding conductor only carries current during a fault, so 310.15(C)(1) leaves it out of the count.

Do I upsize the ground when I upsize for a long run?

Yes — 250.122(B), and it is the step most people miss. The ratio is by circular-mil area, not by size step: 8 AWG to 6 AWG is 26,240 over 16,510, or 1.59, so a 10 AWG ground goes to 8 AWG. Going up two sizes rather than one usually pushes the ground up two as well.

Is a bare ground allowed in conduit?

In most cases yes, and in metal raceway the raceway itself can serve as the equipment grounding conductor under 250.118. Many electricians pull a separate green or bare conductor anyway, because couplings and connectors are the weakest part of that path and a wire is easier to prove than a pipe.

How we calculate this

The equipment grounding conductor comes from NEC Table 250.122, indexed on the rating or setting of the overcurrent device ahead of the circuit. Where the ungrounded conductors are increased in size for any reason other than a code requirement to do so, 250.122(B) increases the grounding conductor proportionally by circular-mil area, which is the ratio this calculator applies using the areas in Chapter 9, Table 8. The table is a minimum: 250.122(A) never requires a grounding conductor larger than the circuit conductors themselves, and metal raceway can serve as the grounding path under 250.118 in place of a separate conductor.

Worked example

With Breaker or fuse ahead of the circuit 100 A, Ground wire metal Copper, Circuit conductor the table expects Not upsized, Conductor you are actually pulling Same, this page works out 8 AWG copper ground. behind a 100 A device · that is 8.4 mm²

Copper option8 AWG
Aluminium option6 AWG

Sources:

The three grounds that get confused

Equipment grounding conductor. The one this page sizes. It runs with the circuit, bonds the metal parts, and exists to carry fault current back so the breaker trips. Table 250.122.

Grounded conductor. The neutral. It carries normal current and is sized like a hot conductor. The code calls it “grounded”, which is exactly one letter away from the one above and the source of half the confusion in this subject.

Grounding electrode conductor. The one going to the rod, the plate or the rebar at the service. Sized from Table 250.66 and generally 4 AWG copper for a 100 to 200 A dwelling service. It has nothing to do with branch circuits.