Ground wire size calculator

Worked examples

100 A subpanel feeder, copper EGC

Table 250.122, row “not exceeding 100 A”: 8 AWG copper, or 6 AWG aluminum. The same 8 AWG serves 61 A to 100 A; at 101 A the row changes to 6 AWG. Open this run

20 A circuit, hots upsized from 12 to 10 AWG for a long run

Table value 12 AWG. 250.122(B): 10,380 ÷ 6,530 cmil = × 1.590, so the EGC needs 6,530 × 1.590 = 10,380 cmil: 10 AWG. In practice an upsized NM or MC cable already carries the larger ground; with single conductors in conduit it is easy to miss. Open this run

200 A feeder, 3/0 copper upsized to 250 kcmil

Table 250.122 at 200 A: 6 AWG copper (26,240 cmil). 250,000 ÷ 167,800 = × 1.490, so 39,094 cmil is needed and 6 AWG is short; the next size, 4 AWG (41,740 cmil), is the answer. Open this run

200 A service on 2/0 copper, GEC to the water pipe and rods

Table 250.66, row “2/0 or 3/0”: 4 AWG copper. If the conductor runs only to ground rods, 250.66(A) caps it at 6 AWG copper; only to a concrete-encased electrode, 250.66(B) caps it at 4 AWG copper. Open this run

Three parallel sets of 500 kcmil copper

Note 1 sums the areas: 3 × 500 = 1,500 kcmil, which is “over 1100”. GEC: 3/0 copper or 250 kcmil aluminum, unless it runs only to rods (6 AWG copper). Open this run

How it is worked out

The figures behind it

NEC Table 250.122, Minimum Size Equipment Grounding Conductors

15 A to 6000 A, copper and aluminum or copper-clad aluminum columns. Read from the NFPA-typeset 2017 text and cross-checked cell by cell against a reproduction headed for NEC 2020 through 2026; UpCodes’ 2020 and 2023 entries confirm the 15 A and 6000 A end rows. Checked 2026-10-11. Source

NEC Table 250.66, Grounding Electrode Conductor for AC Systems

Seven rows from “2 or smaller” to “over 1100” (copper service conductors) and “1/0 or smaller” to “over 1750” (aluminum). Same sources and cross-check as Table 250.122. Row bounds are converted to circular mils with Chapter 9 Table 8 so parallel sets can be summed. Source

250.122(A), (B), (C), (D), (F) and 250.66(A), (B), (C)

Paraphrased from the 2017 text. The 2020 and later wording of 250.122(B) could not be read from an open NFPA-typeset copy; the arithmetic here is the proportional circular-mil increase the rule has required since the 2017 text. Check the edition your jurisdiction enforces. Source

NEC Chapter 9 Table 8, Conductor Properties

Circular-mil areas and DC resistance at 75 °C (ohm/kFT) for 18 AWG to 1000 kcmil, copper uncoated and aluminum. Read from the NFPA-typeset 2017 text and cross-checked cell by cell with a 2014 reproduction; the table is unchanged since at least 2005, so the 2023 values are inferred from that history rather than read from a 2023 copy. Checked 2026-09-05. Source

Table 250.122: equipment grounding conductor by breaker size

Table 250.66: grounding electrode conductor by service conductor size

Questions people ask

What size ground wire for a 100 amp subpanel?

8 AWG copper or 6 AWG aluminum for the equipment grounding conductor in the feeder (Table 250.122, row 100 A). A separate building also needs its own grounding electrode system; that GEC is sized from the feeder conductors by Table 250.66.

What size ground wire for a 200 amp service?

At the service it is the grounding electrode conductor, sized by Table 250.66 from the service conductors: 4 AWG copper for 2/0 or 3/0 copper (or 4/0 or 250 aluminum) service conductors, and 6 AWG copper if it runs only to ground rods. A 200 A feeder to a subpanel carries a 6 AWG copper or 4 AWG aluminum EGC by Table 250.122.

What size ground for a 30, 40, 50 or 60 amp circuit?

10 AWG copper for all four: Table 250.122 has one row covering 21 A to 60 A. Aluminum: 8 AWG. 15 A takes 14 AWG copper and 20 A takes 12 AWG copper.

Is the ground wire the same size as the hot wires?

Up to 20 A it usually is, because the table matches the circuit conductors. Above that it is smaller: a 100 A feeder on 3 AWG copper carries an 8 AWG copper EGC. It is never required to be larger than the circuit conductors (250.122(A)).

If I upsize the wire for voltage drop, do I upsize the ground?

Yes, for wire-type EGCs. 250.122(B) requires the EGC to grow in proportion to the circular-mil increase of the ungrounded conductors. Going from 12 to 10 AWG on a 20 A circuit takes the EGC from 12 to 10 AWG too.

What is the difference between an EGC and a GEC?

The equipment grounding conductor runs with the circuit and gives fault current a path back to the source so the breaker trips; it is sized by the breaker (Table 250.122). The grounding electrode conductor connects the system to the earth through rods, pipe or footing steel; it is sized by the service conductors (Table 250.66). 250.121 says an EGC is not to be used as a GEC, with one exception for a wire that meets the rules for both.

Does the ground wire count in conduit fill and box fill?

Yes. Chapter 9 Note 3 counts equipment grounding conductors in conduit fill at their actual dimensions, and 314.16(B)(5) counts all equipment grounding conductors in a box as one volume allowance (the 2020 and later editions add a quarter allowance for each beyond four). They do not count toward the ampacity adjustment for current-carrying conductors.