Table 250.122
15 A: 14 AWG copper. 20 A: 12. 30–60 A: 10. 100 A: 8 AWG. 200 A: 6 AWG. 400 A: 3 AWG. Aluminum runs two sizes larger. 250.122(A): never required larger than the circuit conductors.
Ground wire size chart (NEC 250.122 & 250.66)
Ground wire size by NEC Table 250.122: a 60 A breaker needs 10 AWG copper, 100 An 8 AWG, 200 A 6 AWG. Upsize the circuit for distance and the ground grows too (250.122(B)): 6 → 4 AWG takes it to 8 AWG. Services use Table 250.66.
Drawing at true size…
Quick Project Finder · 100 A subpanel
Subpanel, 100 A · 200 ft one way, THHN copper in EMT
USE:
2 AWG COPPER WIRE · 100 A 2-POLE BREAKER · 1 1/4" EMT CONDUIT
Wire
2 AWG
copper THHN
Breaker
100 A
2-pole
Conduit
1 1/4" EMT
ground 6 AWG
Safe for up to 115 A (75 °C THHN terminals) on a 100 A breaker, and carries a 80 A load up to 231 feet one way with under 3% voltage drop.
- Meets NEC 2023 Table 310.16 at 75°C — verify with your local AHJ/inspector110.14(C)
- 2.59 % drop at 200 ft210.19 IN
- Upsized from 3 AWG for distance; ground 8 AWG → 6 AWG250.122(B)
The answer
- Wire
- 3 × 2 AWG THHN copper + 6 AWG groundTHHN copper in EMT
- Breaker
- 100 A 2-pole2-pole, 240 V
- Ground
- 6 AWG copperTable 250.122 for 100 A, grown with the wire
- Conduit
- 1 1/4" EMT26.6 % full of 40 %
- Max distance at this size
- 231 ft one wayunder 3 % drop at 80 A
How it was worked out
- Allowable ampacity
- 115 A2 AWG, 75 °C column
- Load used for the drop
- 80 AA 100 A subpanel feeder, four wires. Aluminum is the usual, cheaper choice at th
- Smallest size before distance
- 3 AWG
- With Aluminum SER / XHHW-2
- 3 AWG75 °C terminals allow the THHN column
2 AWG
- Copper 60 / 75 / 90 °C
- 95 / 115 / 130 ANM-B uses the 60 °C column (334.80)
- Aluminum 60 / 75 / 90 °C
- 75 / 90 / 100 A
- Largest breaker (Cu, 75 °C)
- 110 Aampacity ≥ breaker
- Solid diameter
- 0.258"6.54 mm · ASTM B258 formula
- Stranded (7 strands)
- 0.292"7.42 mm · Table 8
- THHN outside
- 0.384"9.75 mm · 0.1158 in² · Table 5
- Area
- 66,360 cmil33.63 mm² · nearest IEC 35 mm²
Grounding conductor tables 30 rows
Sort any column, print a shop sheet, download CSV. Click a row to load it in the calculator.
| Actions | |||
|---|---|---|---|
| 14 AWG | 12 AWG | ||
| 12 AWG | 10 AWG | ||
| 10 AWG | 8 AWG | ||
| 10 AWG | 8 AWG | ||
| 10 AWG | 8 AWG | ||
| 8 AWG | 6 AWG | ||
| 6 AWG | 4 AWG | ||
| 4 AWG | 2 AWG | ||
| 3 AWG | 1 AWG | ||
| 2 AWG | 1/0 AWG | ||
| 1 AWG | 2/0 AWG | ||
| 1/0 AWG | 3/0 AWG | ||
| 2/0 AWG | 4/0 AWG | ||
| 3/0 AWG | 250 kcmil |
Never required larger than the circuit conductors (250.122(A)); grows in proportion when they are upsized (250.122(B)).
| 20 A | 12 AWG | 10 AWG | × 1.59 | 12 AWG | 10 AWG |
|---|---|---|---|---|---|
| 20 A | 12 AWG | 8 AWG | × 2.53 | 12 AWG | 8 AWG |
| 30 A | 10 AWG | 8 AWG | × 1.59 | 10 AWG | 8 AWG |
| 30 A | 10 AWG | 6 AWG | × 2.53 | 10 AWG | 6 AWG |
| 50 A | 8 AWG | 6 AWG | × 1.59 | 10 AWG | 8 AWG |
| 60 A | 6 AWG | 4 AWG | × 1.59 | 10 AWG | 8 AWG |
| 60 A | 6 AWG | 2 AWG | × 2.53 | 10 AWG | 6 AWG |
| 100 A | 3 AWG | 1 AWG | × 1.59 | 8 AWG | 6 AWG |
| 100 A | 3 AWG | 1/0 AWG | × 2.01 | 8 AWG | 4 AWG |
| 200 A | 3/0 AWG | 250 kcmil | × 1.49 | 6 AWG | 4 AWG |
| 2 or smaller | 1/0 or smaller | 8 AWG | 6 AWG |
|---|---|---|---|
| 1 or 1/0 | 2/0 or 3/0 | 6 AWG | 4 AWG |
| 2/0 or 3/0 | 4/0 or 250 | 4 AWG | 2 AWG |
| Over 3/0 through 350 | Over 250 through 500 | 2 AWG | 1/0 AWG |
| Over 350 through 600 | Over 500 through 900 | 1/0 AWG | 3/0 AWG |
| Over 600 through 1100 | Over 900 through 1750 | 2/0 AWG | 4/0 AWG |
A conductor to a rod, pipe or plate electrode alone is never required larger than 6 AWG copper (250.66(A)); to a concrete-encased electrode, 4 AWG (250.66(B)).
05 / Field guide
Sized by the breaker,
grown with the wire.
The equipment grounding conductor carries fault current long enough for the breaker to trip, so NEC Table 250.122 sizes it by the breaker or fuse protecting the circuit, not by the circuit wire. The grounding electrode conductor that ties a service to its ground rod or footing steel is a different wire, sized by Table 250.66 from the service conductors.
Upsizing for distance: 250.122(B)
When the circuit conductors are made larger than the minimum to cut voltage drop, the wire ground grows by the same circular-mil ratio. A 60 A circuit moved from 6 to 4 AWG grows by 1.59×: the 10 AWG ground (10,380 cmil) must reach 16,511 cmil, which is 8 AWG.
ground cmil ≥ Table 250.122 ground cmil × (installed wire cmil ÷ minimum wire cmil)
Table 250.66
A 200 A service on 2/0 AWG copper takes a 4 AWG copper grounding electrode conductor; the conductor to a ground rod alone never needs to exceed 6 AWG copper (250.66(A)), to a concrete-encased electrode 4 AWG (250.66(B)).
Detached buildings and subpanels
A feeder to a detached garage or shop runs four wires: two hots, a neutral and an equipment grounding conductor sized by Table 250.122. At the remote panel the neutral and ground stay separate, and the building gets its own grounding electrode, usually ground rods bonded to the panel (250.32). The same separation applies to any subpanel in the house.
Parallel runs, colours and old circuits
Where a feeder runs in parallel through several raceways, each raceway gets its own full-size grounding conductor from Table 250.122, based on the breaker for the whole feeder (250.122(F)). A grounding conductor is bare, green or green with yellow stripes (250.119). For a 100 A feeder upsized from 3 AWG to 1/0 copper, the 8 AWG ground grows to 4 AWG.
An old two-wire circuit without a ground can be protected by a GFCI and its receptacles labelled “No equipment ground” (406.4(D)(2)); the GFCI protects people but does not replace a ground for surge protectors or equipment that needs one.
Questions
Answered plainly.
01What size ground wire for a 100 amp subpanel?
+−
8 AWG copper or 6 AWG aluminum by Table 250.122, larger if the feeder was upsized for distance. The subpanel keeps neutral and ground separate.
02What size ground wire for a 200 amp service?
+−
The grounding electrode conductor is 4 AWG copper for 2/0 AWG copper service conductors (Table 250.66), or 6 AWG copper to rods only. Equipment grounds for 200 A feeders are 6 AWG copper.
03Does the ground wire have to be the same size as the hots?
+−
No. It is sized by the breaker (Table 250.122), so it is often smaller, but never required larger than the circuit conductors.
04Can the ground be bare?
+−
Yes, in cables like NM-B and in most raceways; some installations (spas, some feeders) require an insulated copper ground.
05Why does the ground get bigger on a long run?
+−
A longer, heavier circuit can deliver more fault current; 250.122(B) keeps the ground able to carry it so the breaker trips quickly.
06Does a detached garage need a ground rod?
+−
Yes. A building supplied by a feeder needs its own grounding electrode system (250.32), bonded to the equipment grounding conductor at its panel; the neutral is not bonded there.
07What color is the ground wire?
+−
Bare copper, green, or green with one or more yellow stripes (250.119). Larger conductors may be re-identified with green tape at each end.