100 amp wire size
For a 100 amp circuit sized at its full rating, the minimum is 3 AWG copper or 1 AWG aluminum in the 75 °C column, which applies to terminations on circuits above 100 A and to equipment marked for 75 °C. The NM-B cable used for branch circuits stops at 2 AWG, rated 95 A, so the 60 °C column offers no full-rating answer at this size.
Minimum wire size for 100 A
| Conductor | 60 °C column | 75 °C column |
|---|---|---|
| Copper | Not listed | 3 AWG |
| Aluminum | Not listed | 1 AWG |
| Dwelling service, 83% (83 A) | — | 4 AWG Cu · 2 AWG Al |
Usually chosen: 3 AWG copper or 1 AWG aluminum for a 100 A subpanel feeder; 4 AWG copper or 2 AWG aluminum for a 100 A dwelling service or main feeder under the 83% rule.
Breaker rating taken as the required ampacity; 240.4(D) limits applied. The charts list no 60 °C aluminum, and 60 °C copper only to 2 AWG (95 A), the largest NM-B. Manufacturer chart values (2017 NEC basis). Verify against the NEC edition adopted locally.
A dwelling service or main feeder of 100 A is a different case: NEC 310.12 lets those conductors be sized at 83% of the rating, which brings the minimum down to 4 AWG copper or 2 AWG aluminum. Which rule applies depends on whether the conductors carry the whole load of the dwelling.
Where 100 amp circuits are used
100 A is the smallest service the NEC allows for a one-family dwelling (230.79(C)) and remains common in smaller and older houses, condominiums and apartments. Many houses built with 100 A services have been upgraded as electric ranges, heat pumps and EV chargers were added.
A 100 A feeder is a popular size for a subpanel in a large garage, a pole barn or an addition, and for the second panel of a house split between two panels. A 100 A feeder from a 200 A main panel to a garage subpanel does not carry the entire dwelling load, so it is sized at the full 100 A.
Dwelling services and main feeders: the 83% rule
NEC 310.12(A) and (B) apply to services and feeders rated 100 A through 400 A that supply the entire load of a one-family dwelling, or of an individual unit in a two-family or multifamily dwelling. Those conductors may have an ampacity of not less than 83% of the service or feeder rating. The wording here follows the 2020 edition; the rule is long-standing and is carried into later editions, which should be checked for local adoption.
At 100 A, 83% is 83 A. In the 75 °C column, 4 AWG copper at 85 A and 2 AWG aluminum at 90 A are the smallest sizes that qualify. 310.12(C) adds that a feeder to an individual dwelling unit never has to be larger than the service conductors sized this way.
The 83% rule does not reach a feeder that serves only part of the load, such as a subpanel for a garage. It also does not cover the neutral, which is sized from the calculated unbalanced load, and correction for a hot location such as a rooftop or a sun-exposed wall still has to be applied.
Voltage drop at 100 A
3 AWG copper carrying the full 100 A reaches 3% at about 147 ft at 240 V. Service drops and laterals on large rural lots often run farther, which is one reason utilities and installers favor aluminum in sizes such as 1/0 for long 100 A runs.
The 120 V column shows the drop when the load is entirely on one leg, a worst case for a single-phase panel; a balanced panel sees figures closer to the 240 V column.
| One-way length (ft) | Drop at 120 V (%) | Drop at 240 V (%) | Copper for 3% at 120 V | Copper for 3% at 240 V |
|---|---|---|---|---|
| 25 | 1.02 | 0.51 | 3 AWG | 3 AWG |
| 50 | 2.04 | 1.02 | 3 AWG | 3 AWG |
| 75 | 3.06 | 1.53 | 2 AWG | 3 AWG |
| 100 | 4.07 | 2.04 | 1 AWG | 3 AWG |
| 150 | 6.11 | 3.06 | 2/0 AWG | 2 AWG |
| 200 | 8.15 | 4.07 | 3/0 AWG | 1 AWG |
| 250 | 10.19 | 5.09 | 4/0 AWG | 1/0 AWG |
| 300 | 12.22 | 6.11 | 250 kcmil | 2/0 AWG |
Full rating on a single-phase circuit, copper at 75 °C, stranded.
Breaker pairing
A 100 A feeder breaker or main breaker protects these conductors. With 2 AWG copper NM-B rated 95 A, the next-size-up rule of 240.4(B) permits a 100 A breaker where the calculated load is 95 A or less, since 95 A is not a standard rating.
2 AWG aluminum at 90 A cannot use the same allowance outside the 310.12 rule, because 90 A is itself a standard rating. It needs a 90 A breaker unless it is a dwelling service or main feeder sized at 83%.
NM-B cable or THHN in conduit
Feeders at this rating are usually SER cable, conductors in conduit or, for underground runs, URD or USE-2 aluminum. NM-B in 2 AWG exists but is uncommon; the 60 °C limit leaves it at 95 A.
With conduit, THHN/THWN-2 conductors land on 75 °C rated main lugs or breakers, which are standard in panelboards at this size.
Copper or aluminum
Aluminum dominates 100 A services and feeders. The minimums are 1 AWG aluminum at full rating and 2 AWG aluminum under the 83% rule, against 3 AWG and 4 AWG copper.
Panel lugs at this size are almost always listed for AL/CU. Aluminum terminations need the manufacturer's torque, applied with a calibrated tool as 110.14(D) requires, and an antioxidant compound where the lug instructions call for it.
Common mistakes
Applying the 83% rule to a subpanel feeder
A 100 A feeder from the main panel to a garage subpanel does not carry the entire dwelling load. It is sized at 100 A, which means 3 AWG copper or 1 AWG aluminum.
Assuming 4 AWG copper is a 100 A wire everywhere
4 AWG copper is rated 85 A at 75 °C. It qualifies for a 100 A dwelling service only under 310.12; on any other 100 A circuit it is undersized.
Using the wrong equipment grounding conductor
A 100 A feeder needs an 8 AWG copper or 6 AWG aluminum equipment grounding conductor under Table 250.122, and it must be increased in proportion if the phase conductors are upsized for voltage drop.
Forgetting temperature correction on exterior runs
Conductors in conduit on a sunny roof or exterior wall run hotter than 30 °C. The correction factors can push a 100 A feeder up one or two sizes.
Frequently asked questions
What size wire is needed for a 100 amp service?
For a dwelling service under NEC 310.12, the minimum is 4 AWG copper or 2 AWG aluminum, sized at 83% of 100 A. A 100 A circuit outside that rule needs 3 AWG copper or 1 AWG aluminum.
What size wire is needed for a 100 amp subpanel?
3 AWG copper or 1 AWG aluminum at the 75 °C column, because a subpanel feeder usually does not carry the whole dwelling load. Long runs may need a larger size for voltage drop.
Can 2 gauge aluminum be used for 100 amps?
For a dwelling service or main feeder under the 83% rule, yes. For a subpanel feeder, 2 AWG aluminum is rated 90 A and needs a 90 A breaker; 1 AWG aluminum is the 100 A size.
Is 4 gauge copper good for 100 amps?
Only as dwelling service or main feeder conductors sized under NEC 310.12. In every other case 4 AWG copper is an 85 A conductor at 75 °C.
How far can 100 amp wire run before it needs upsizing?
At the full 100 A and 240 V, 3 AWG copper reaches 3% voltage drop at about 147 ft. The voltage drop table gives the size needed for longer runs.