100 amp service wire size depends on the run: #3 copper or #1 aluminum for standard feeders, #4 copper or #2 aluminum for dwelling mains.
For the full breakdown, see our best 100 Amp Service Wire guide.
Two questions decide what size wire for 100 amp service you need: whether the run is a dwelling’s main service or a subpanel feeder, and the temperature rating of the terminals at both ends. For most 100 amp feeders, the answer is #3 AWG copper or #1 AWG aluminum. A dwelling main service can legally run #4 copper or #2 aluminum under the NEC’s 83% allowance.
Pick wrong and you either overpay for cable you didn’t need or create a conductor that runs hot behind the wall. The table below shows the sizes that actually apply, and the sections after it explain which rule governs your project.
100 Amp Wire Size: The Two Code Paths
For a standard 100 amp feeder or subpanel, NEC Table 310.16 lists #3 AWG copper and #1 AWG aluminum as 100 amp conductors at the 75°C column. If the run is a dwelling main service or main feeder,
It covers services and main feeders from 100 to 400 amps that supply the entire load of one-family, two-family, or multifamily dwellings. That’s why residential service charts show smaller wire than general feeder charts for the same breaker size — the two tables answer different questions.
Terminal ratings change the math. NEC 110.14(C)(1) requires conductor ampacity to match the terminal temperature rating, so equipment with 60°C-only lugs forces a larger conductor than the 75°C baseline — NEC 240.6 confirms a 100 amp breaker is the standard overcurrent device for the circuit.