Quick answer: wire size for a 100 amp breaker
Minimum conductor for 100 A with not more than three current-carrying conductors at 30 °C (86 °F) ambient.
| Wire and terminal rating | Minimum size | Table ampacity |
|---|---|---|
| Copper, 60 °C column (NM cable, unmarked terminals) | 1 AWG | 110 A |
| Copper, 75 °C column (THHN/THWN-2, 75 °C terminals) | 3 AWG | 100 A |
| Aluminum, 60 °C column | 1/0 AWG | 100 A |
| Aluminum, 75 °C column (XHHW-2, SER, 75 °C terminals) | 1 AWG | 100 A |
Most 100 A breakers and panelboards are marked 60/75 °C, so the 75 °C column applies when the wire is also rated 75 °C or higher (NEC 110.14(C)(1)(a) lets circuits of 100 A or less use 75 °C ampacity only when the equipment is listed and marked for it). NM cable is held to the 60 °C column by NEC 334.80. The 90 °C column (115 A for 3 AWG copper) is only a starting point for derating and cannot be used at the terminals.
100 amp service vs 100 amp subpanel
| Installation | Copper | Aluminum | Rule |
|---|---|---|---|
| Dwelling service or main feeder, 120/240 V, carrying the whole house load | 4 AWG | 2 AWG | NEC 310.12: 83% of 100 A = 83 A |
| Subpanel feeder, detached garage, shop or any non-dwelling service | 3 AWG | 1 AWG | Table 310.16, 75 °C, full 100 A |
| NM or SE cable held to 60 °C | 1 AWG | 1/0 AWG | Table 310.16, 60 °C |
The 83% rule is the most common source of confusion. It only applies to a single-phase dwelling service or the feeder that carries the entire dwelling load, so a 100 A subpanel fed from a 200 A main panel needs the full-size 3 AWG copper or 1 AWG aluminum. A feeder to a subpanel also needs four wires (two hots, an insulated neutral and an equipment ground), because NEC 250.24(A)(5) and 250.142(B) keep the neutral and ground separate after the service disconnect.
Wire size for 100 amps by distance (240 V)
Smallest size that keeps a 120/240 V single-phase run at or under a 3% voltage drop. The 80 A columns are for a continuous or typical load on a 100 A breaker. Ampacity still sets the floor, so the table never goes below 3 AWG copper or 1 AWG aluminum.
| Run | Cu, 100 A | Cu, 80 A | Al, 100 A | Al, 80 A |
|---|---|---|---|---|
| 50 ft | 3 AWG | 3 AWG | 1 AWG | 1 AWG |
| 75 ft | 3 AWG | 3 AWG | 1 AWG | 1 AWG |
| 100 ft | 3 AWG | 3 AWG | 1 AWG | 1 AWG |
| 125 ft | 3 AWG | 3 AWG | 1 AWG | 1 AWG |
| 150 ft | 2 AWG | 3 AWG | 1/0 AWG | 1 AWG |
| 200 ft | 1 AWG | 2 AWG | 2/0 AWG | 1/0 AWG |
| 250 ft | 1/0 AWG | 1 AWG | 3/0 AWG | 2/0 AWG |
| 300 ft | 2/0 AWG | 1/0 AWG | 4/0 AWG | 3/0 AWG |
Example: a 100 A subpanel in a detached shop 250 ft from the house, with an expected load of 80 A at 240 V. 3 AWG copper only reaches 183 ft at 80 A and 2 AWG reaches 231 ft, so the run needs 1 AWG copper, which reaches 292 ft. In aluminum, 1/0 reaches 223 ft and 2/0 reaches 283 ft, so 2/0 aluminum. Because the hots were upsized, the 8 AWG copper ground must be upsized in proportion (see ground).
Maximum length at 100 amps by wire size
The longest one-way run, in feet, before each size reaches the drop shown. Three-phase columns use the √3 formula. These are DC resistance figures; for large conductors in steel conduit at a low power factor, reactance adds a little more drop.
Copper
| Size | 240 V, 3% | 240 V at 80 A, 3% | 240 V, 5% | 208 V 3-ph, 3% | 480 V 3-ph, 3% |
|---|---|---|---|---|---|
| 3 AWG | 146 | 183 | 244 | 147 | 339 |
| 2 AWG | 185 | 231 | 309 | 185 | 428 |
| 1 AWG | 233 | 292 | 389 | 233 | 539 |
| 1/0 AWG | 295 | 368 | 491 | 295 | 681 |
| 2/0 AWG | 372 | 465 | 620 | 372 | 859 |
| 3/0 AWG | 469 | 587 | 783 | 470 | 1085 |
| 4/0 AWG | 592 | 740 | 986 | 592 | 1367 |
| 250 kcmil | 699 | 873 | 1165 | 699 | 1614 |
Aluminum
| Size | 240 V, 3% | 240 V at 80 A, 3% | 240 V, 5% | 208 V 3-ph, 3% | 480 V 3-ph, 3% |
|---|---|---|---|---|---|
| 1 AWG | 142 | 177 | 237 | 142 | 328 |
| 1/0 AWG | 179 | 223 | 298 | 179 | 413 |
| 2/0 AWG | 226 | 283 | 377 | 226 | 522 |
| 3/0 AWG | 285 | 357 | 476 | 285 | 659 |
| 4/0 AWG | 359 | 449 | 600 | 360 | 831 |
| 250 kcmil | 425 | 531 | 708 | 425 | 981 |
| 300 kcmil | 509 | 636 | 848 | 509 | 1175 |
| 350 kcmil | 595 | 743 | 991 | 595 | 1374 |
NEC 215.2(A)(1) Informational Note 2 recommends 3% on a feeder and 5% total to the farthest outlet. A 3% drop at 240 V is 7.2 V. Use the voltage drop calculator for your exact load, or the three-phase voltage drop calculator for 208 to 600 V.
Ground wire for 100 amps
- Equipment grounding conductor: 8 AWG copper or 6 AWG aluminum, from NEC Table 250.122 for a 100 A breaker.
- Upsized hots: 250.122(B) requires the ground to grow by the same ratio of circular mils. Going from 3 AWG (52,620 cmil) to 1 AWG (83,690 cmil) is a ratio of 1.59, so the 8 AWG ground (16,510 cmil) must be at least 26,259 cmil. 6 AWG copper is 26,240 cmil, just under, so the strict answer is 4 AWG copper.
- Service grounding electrode conductor: for 4 AWG copper or 2 AWG aluminum service conductors, NEC Table 250.66 calls for 8 AWG copper or 6 AWG aluminum. A conductor to a ground rod never has to be larger than 6 AWG copper (250.66(A)).
Conduit size for 100 amp wire
Smallest conduit at 40% fill (NEC Chapter 9 Table 1) using THHN/THWN-2 areas from Chapter 9 Table 5. "Three wires" is two hots and a neutral; "four wires" adds a second neutral or a third phase.
| Run | Conductors | Area, in² | EMT | PVC 40 | EMT, 4 wires |
|---|---|---|---|---|---|
| Feeder, copper 75 °C | 3 × 3 AWG + 8 AWG | 0.329 | 1 | 1 | 1-1/4 |
| Feeder, aluminum 75 °C | 3 × 1 AWG + 6 AWG | 0.519 | 1-1/4 | 1-1/4 | 1-1/2 |
| Dwelling service (310.12), copper | 3 × 4 AWG + 8 AWG | 0.284 | 1 | 1 | 1-1/4 |
| Dwelling service (310.12), aluminum | 3 × 2 AWG + 6 AWG | 0.398 | 1-1/4 | 1-1/4 | 1-1/4 |
| Upsized for distance (250 ft example), copper | 3 × 1 AWG + 4 AWG | 0.551 | 1-1/4 | 1-1/4 | 1-1/2 |
XHHW-2 aluminum has slightly different areas than THHN, so check the conduit fill calculator for your insulation. Many electricians go one trade size up for an easier pull on long runs.
How much load can a 100 amp circuit carry?
| System | At 100 A | Continuous (80 A) |
|---|---|---|
| 120 V single phase | 12.0 kW | 9.6 kW |
| 240 V single phase | 24.0 kW | 19.2 kW |
| 208 V three phase | 36.0 kW | 28.8 kW |
| 480 V three phase | 83.1 kW | 66.5 kW |
| 600 V three phase | 103.9 kW | 83.1 kW |
Loads that run three hours or more must be sized at 125% under NEC 210.19(A) and 215.2(A)(1), which limits a 100 A breaker to 80 A of continuous load. The 240 V figures assume a balanced load and unity power factor. The amps to watts calculator handles power factor.
When more than three wires share a conduit
With four to six current-carrying conductors in one raceway, NEC 310.15(C)(1) cuts ampacity to 80%. Starting from the 90 °C column, 3 AWG copper drops to 92 A and 2 AWG copper to 104 A, so a 100 A circuit in a shared conduit needs 2 AWG copper. Hot attics and rooftops add an ambient correction too; see wire derating.
In Canada
CEC Table 2 (copper) and Table 4 (aluminum) give the same ampacities for these sizes, so 3 AWG copper or 1 AWG aluminum carries 100 A at 75 °C. Canada does not use the NEC 310.12 83% rule; single-dwelling service conductors have their own minimum sizes in CEC Table 39, so check the CEC before sizing a Canadian service. Rule 8-102 makes the 3% and 5% voltage drop limits mandatory. Use the Canadian wire size calculator for distances in metres.
This page is a quick reference. Confirm against the code edition your jurisdiction has adopted, the equipment markings and your inspector.