100 amps has two answers, and which one you need depends on what the wire feeds. For a feeder to a subpanel, or any other 100 A circuit, use Table 310.16: 1 AWG copper at 60 °C or 3 AWG copper at 75 °C, and in aluminum 1/0 at 60 °C or 1 AWG at 75 °C. For the service or main feeder that carries a whole home's load, NEC 310.12 allows smaller wire: 4 AWG copper or 2 AWG aluminum rated 75 °C.
A subpanel feeder gets no reduction. The 83% rule covers only conductors that carry the dwelling's entire load, and a feeder from the main panel to a subpanel in a garage, basement or addition carries part of it. So it is sized at the full 100 A from Table 310.16. If the subpanel serves continuous loads, such as an EV charger, the feeder is sized at 125% of those loads plus the rest, under 215.4(A)(1) (215.2(A)(1) in 2023).
100 A subpanel feeders are commonly run as aluminum SER cable or as individual conductors such as THHN or XHHW in conduit. Which column you read depends on the conductors' temperature rating and on the terminals at both ends: the 75 °C column applies only when the breaker in the main panel and the lugs in the subpanel are rated 75 °C too. That choice is the difference between 1 AWG and 3 AWG copper, or between 1/0 and 1 AWG aluminum, so read the markings, or have your electrician confirm them.
The service case is NEC 310.12. It covers a one-family home or an individual dwelling unit on a 120/240 V single-phase system, for a service or feeder rated 100 to 400 A that carries the unit's entire load. That includes the service conductors to the main panel and a feeder that carries everything the home uses. Those conductors may be sized at 83% of the rating. One 2026 change matters here: the 310.12 table may now be used only for conductors rated 75 °C or more, so 60 °C cable such as NM-B is sized from its own column. The service section on this page gives both results.
Length seldom changes a 100 A size until the run is long. At 240 V and the full 100 A, the length table keeps the Table 310.16 sizes out to 100 feet. At 150 feet, aluminum at 75 °C moves from 1 AWG to 1/0, and 3 AWG copper sits just under the 3% line. At 200 feet, copper needs 1 AWG in both columns and aluminum 2/0. The table assumes the whole 100 A is drawn at the far end; for your actual load, the wire size calculator works it out, and 3% is a recommendation rather than a rule.
What this page doesn't answer is whether a home or a subpanel needs 100 A in the first place. That comes from a load calculation (NEC Article 120 in 2026, Article 220 in 2023) that adds up lighting, appliances, heating and cooling with the code's demand factors. It's work for an electrician. Once the rating is settled, the sizes here follow from it.