Ground Wire Size Calculator

Size an equipment grounding conductor from the breaker rating, or a grounding electrode conductor from the service conductors, per NEC Tables 250.122 and 250.66.

What this does not check

These are minimum sizes from the tables. They do not account for an EGC upsized under 250.122(B), for parallel raceways, or for any engineered fault-current study your job may require.
It sizes only. It does not tell you whether an EGC is required, whether it may be bare or insulated, or how it must be terminated — those are separate parts of Article 250.
The GEC table returns the size before the 250.66(A) allowance. A connection to a rod, pipe or plate electrode never has to exceed 6 AWG copper, so the table answer is often larger than what you actually run to a ground rod.
Aluminium is not permitted everywhere. 250.64(A) restricts aluminium grounding conductors near earth and in contact with masonry; check the termination and the environment before you buy it.

How to use it

Pick which conductor you are sizing first — that choice decides which table applies and what the calculator asks you for.

For an equipment grounding conductor, enter the rating of the overcurrent device protecting the circuit. Not the load, not the conductor ampacity: the breaker or fuse rating. For a grounding electrode conductor, pick the size of the largest ungrounded service-entrance conductor instead.

Then choose copper or aluminium. The two columns are not interchangeable — an aluminium grounding conductor is always at least one size larger than the copper equivalent.

Two conductors people mix up

The single most common mistake here is treating these as one wire with one size. They are not.

The equipment grounding conductor is the fault-clearing path. Its job is to carry enough fault current, fast enough, to open the overcurrent device. That is why it is sized from the device rating — a bigger breaker needs a bigger fault path to trip it reliably.

The grounding electrode conductor connects the electrical system to earth. It is not a fault-clearing path in the same sense, and earth is not a low-impedance return. It is sized from the service conductors because it scales with the system, not with any one circuit.

Ask which job the wire is doing, and the table picks itself.

Worked example — equipment ground

A 100 A feeder to a subpanel, run in copper.

Table 250.122 brackets by overcurrent device rating. 100 A falls in the row whose ceiling is 100 A, which calls for 8 AWG copper — or 6 AWG if you are running aluminium.

Now the part that catches people. Say that feeder runs 180 feet and you upsize the ungrounded conductors from 3 AWG to 1 AWG to hold voltage drop. Under 250.122(B) the equipment grounding conductor has to increase in proportion to that upsize — leaving it at 8 AWG because the breaker is still 100 A is a failed inspection.

Worked example — electrode conductor

A 200 A residential service with 2/0 copper service-entrance conductors.

Table 250.66 puts 2/0 in the “2/0 – 3/0” row, which calls for 4 AWG copper.

But if that conductor is going to a driven ground rod, 250.66(A) caps the required size at 6 AWG copper regardless. Both numbers are correct — the 4 AWG is what the table says, the 6 AWG is what the allowance lets you run to that particular electrode. To a concrete-encased electrode the cap is different again, so read the subsection that matches the electrode you actually have.

Where it goes wrong in the field

The upsize rule in 250.122(B) is the one that fails inspections, because it is invisible from the breaker: the device rating did not change, so nothing prompts you to revisit the ground. Long runs, solar circuits, and anything corrected for voltage drop are the usual candidates.

The other one is running the table size to a ground rod and paying for conductor you never needed, which is harmless to safety and expensive over a job.

Code references

NEC Table 250.122Minimum equipment grounding conductor by overcurrent device rating2023 edition
NEC 250.122(B)Proportional increase when ungrounded conductors are upsized2023 edition
NEC Table 250.66Grounding electrode conductor by largest service-entrance conductor2023 edition
NEC 250.66(A)Rod, pipe and plate electrode connections need not exceed 6 AWG copper2023 edition

Section numbers and table values move between editions, and jurisdictions amend them. Confirm against the edition your authority having jurisdiction enforces.

Common questions

What size ground wire for a 100 amp breaker?: 8 AWG copper, or 6 AWG aluminium, from Table 250.122. That is the equipment grounding conductor for the circuit. A 100 A service is a different question — the grounding electrode conductor for it comes from Table 250.66 and depends on the service-entrance conductor size, not the breaker.
What is the difference between an EGC and a GEC?: The equipment grounding conductor bonds metal enclosures back to the source so a fault trips the breaker, and it is sized from the overcurrent device. The grounding electrode conductor connects the system to earth — a rod, a water pipe, a Ufer — and is sized from the service conductors. They are different conductors, sized from different tables, doing different jobs.
Do I have to upsize the ground wire if I upsize the circuit conductors?: Yes, if the ungrounded conductors were increased in size, 250.122(B) requires the equipment grounding conductor to be increased proportionally. Upsizing a run for voltage drop and leaving the ground at table minimum is one of the most common inspection failures on long circuits.
Does the ground wire to a ground rod have to be as big as the table says?: No. Under 250.66(A), the portion of the grounding electrode conductor run to a rod, pipe or plate electrode never has to be larger than 6 AWG copper or 4 AWG aluminium, even when the table would call for more. That allowance applies to that electrode connection, not to the whole grounding system.

For the method behind this — worked examples, the tables, and where it goes wrong — read the full guide.