Wire Size for 100 Amp Circuit
Find the copper or aluminum wire for a 100 A subpanel, garage feeder or dwelling service, and check the voltage drop for your run.
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Assumes NEC 2023 Table 310.16: copper or aluminum, up to 3 current-carrying conductors in a raceway or cable, 30 °C ambient, 60 °C terminals for a 100 A circuit (110.14(C)), 3 % drop target; the dwelling service size follows NEC 310.12 (83 %, 75 °C); ground wire not included. Runs in your browser. Verify with your local code and a licensed electrician before you install.
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How to use the 100 Amp Wire Size Calculator
- Keep 100 in “Breaker rating (A)”, or type the rating of your breaker or main.
- Enter the nameplate or calculated current in “Load current (A)”. Leave it empty to size for the full breaker rating, which is the safe choice for a subpanel feeder.
- Tick “Continuous load (3 hours or more)” for a charger or heater. The design current becomes 125 % of the load and must stay at or under 100 A.
- Pick the “Supply type” and enter “Voltage (V)”: 240 for a residential subpanel, or the line-to-line voltage for three phase, for example 208.
- Type the “One-way length” in ft or m, measured along the cable or conduit route from the panel to the subpanel.
- Choose Copper or Aluminum under “Conductor” and the “Insulation temperature rating” printed on the cable, for example 90 °C for THWN-2 or XHHW-2 in conduit (the terminal setting still caps the answer at the 60 or 75 °C column).
- Leave “Allowed voltage drop (%)” at 3 unless your project sets another limit.
- Open the advanced settings only when they apply: “Terminal temperature rating” (set 75 °C only when the breaker and the subpanel lugs are listed for it), “Ambient temperature” and the number of current-carrying conductors.
- Read “Recommended wire size”, “Deciding limit” and the “Dwelling service or feeder” rows, then the steps and warnings.
The example buttons load a copper subpanel, an aluminum garage feed, an EV charger, a dwelling service and a three phase feeder. “Copy result”, “Copy link” and “Print / save as PDF” keep the answer.
What size wire do I need for a 100 amp subpanel?
A subpanel fed from a 100 A breaker needs a conductor whose allowed ampacity reaches 100 A. With Auto terminals (60 °C for circuits up to 100 A, NEC 110.14(C)(1)) that is 1 AWG copper (110 A in the 60 °C column) or 1/0 aluminum (100 A). The sizes people quote, 3 AWG copper and 1 AWG aluminum, are the 75 °C column values and apply only when both ends are listed for 75 °C. Select “75 °C (listed equipment)” in the advanced terminal setting to see them.
| Settings (100 A breaker, 80 A load, 240 V, 75 ft) | Copper | Aluminum |
|---|---|---|
| Terminals Auto (60 °C) | 1 AWG | 1/0 AWG |
| Terminals listed for 75 °C | 3 AWG | 1 AWG |
| Dwelling service or feeder, NEC 310.12 | 4 AWG | 2 AWG |
What size wire for a 100 amp service?
For the service or feeder that carries the whole load of a one-family dwelling, NEC 310.12 lets the conductors be 83 % of the rating: 83 A for a 100 A service. In the 75 °C column that is 4 AWG copper (85 A) or 2 AWG aluminum (90 A). The tool shows these in the “Dwelling service or feeder” rows. They do not apply to a garage or shop subpanel, which uses the breaker-rated size above. The service rows appear only when NEC 310.12 applies to your inputs: nominal 120/240 V single phase (not 208 V), a 100 to 400 A rating, a calculated load entered, conductors rated 75 °C or more, up to 3 current-carrying conductors and a 30 °C ambient. For three phase, 60 °C cable such as NM-B, more conductors, a hotter ambient or a rating outside 100-400 A, the tool says so and gives no smaller size. If your calculated load (125 % for continuous loads) is above the 83 % figure, the rows show the larger size that carries the load; with the load left empty they show no 83 % size and ask for it.
How far can you run 100 amp wire?
At an 80 A load on 240 V, 1 AWG copper reaches about 293 ft one way before the drop passes 3 %; at a full 100 A load it reaches about 234 ft. Beyond that the calculator moves up a size.
| Copper unless noted, 240 V | One-way length | Result |
|---|---|---|
| 80 A load | 100 ft | 1 AWG, 1.03 % drop |
| 80 A load | 200 ft | 1 AWG, 2.05 % drop |
| 100 A load | 300 ft | 2/0 AWG, 2.42 % drop |
| 80 A load, aluminum | 150 ft | 1/0 AWG, 2.01 % drop |
The ground wire is separate: for a 100 A breaker the equipment grounding conductor is 8 AWG copper or 6 AWG aluminum, found with the equipment grounding conductor calculator.
Worked examples
| Input | Result |
|---|---|
| 100 A breaker, 80 A load, 240 V, 75 ft, copper 75 °C | 1 AWG (0.769 %) |
| 100 A breaker, 100 A load, 240 V, 30 ft, copper 75 °C | 1 AWG (0.384 %) |
| 100 A breaker, 80 A load, 240 V, 40 ft, copper 75 °C | 1 AWG (0.41 %) |
| 100 A breaker, 80 A load, 240 V, 150 ft, aluminum 75 °C | 1/0 AWG (2.01 %) |
| 100 A breaker, 80 A load, 240 V, 40 ft, copper 60 °C | 1 AWG (0.41 %) |
Calculated by the tool above; open it with the same values to see each step.
Questions, briefly answered
What size wire for a 100 amp subpanel?
1 AWG copper or 1/0 aluminum with Auto terminals, or 3 AWG copper and 1 AWG aluminum when the breaker and the subpanel lugs are listed for 75 °C.
What size wire for a 100 amp service?
For a one-family dwelling service, NEC 310.12 allows 4 AWG copper or 2 AWG aluminum because the conductors may be rated at 83 % of 100 A. It applies only to the dwelling service or a feeder that carries the whole dwelling load; check that your utility and inspector accept it.
Is 4 gauge wire good for 100 amps?
Only as a dwelling service or feeder under NEC 310.12 (4 AWG copper has 85 A in the 75 °C column, above the 83 A needed). As a subpanel feeder it is too small, because the conductor must carry the 100 A breaker.
Is 3 AWG enough for a subpanel on a 100 A breaker?
Yes, if the breaker and the subpanel are both listed for 75 °C and the cable is rated 75 °C or more; 3 AWG copper has 100 A in the 75 °C column. With 60 °C terminals it falls short and the answer is 1 AWG.
What size aluminum wire for 100 amps?
1/0 aluminum with Auto terminals, 1 AWG with 75 °C listed equipment, and 2 AWG as a dwelling service. Aluminum needs lugs marked AL/CU.
How far can I run 1 gauge wire at 100 amps?
About 234 ft one way at 240 V with a full 100 A load and a 3 % target; the “Longest run of the recommended size” row updates with your own load.
What size ground wire for a 100 amp breaker?
8 AWG copper or 6 AWG aluminum under NEC Table 250.122. It is not included in the wire size above.
What size wire for a detached garage on a 100 A feeder?
The same breaker-rated size: 1 AWG copper, or 1/0 aluminum for a run of about 150 ft (2.0 % drop at 80 A), unless the listed 75 °C equipment allows the smaller sizes. A feeder to another building also needs its own grounding electrode.