Code reference · NEC Table 310.16; Ch.9 Table 8; 210.19(A) IN; 110.14(C)
Wire size for 100 amps at 100 feet (240 V)
100 A at 100 ft one-way on a 240 V circuit: the engine-computed answer is 3 AWG copper (or 1 AWG aluminum). That size carries the load under Table 310.16 with standard derates and lands at 2.04% voltage drop — inside the NEC's recommended 3% for a branch circuit (210.19(A) Informational Note).
Here ampacity is the binding constraint — 3 AWG is already required just to carry 100 A at the 75°C termination column, and at 100 ft its 2.04% drop stays inside the guideline. Only past roughly 100 ft would drop force an upsize.
Notes that survive inspection: the 3%/5% figures are Informational Notes — recommended, not enforceable, in most jurisdictions (some local amendments make them binding); a continuous load (3+ hours) needs conductors and OCPD at 125% of the load (210.19(A)/210.20(A)); and these numbers assume ≤3 current-carrying conductors at 30°C — attics and bundled raceways derate further (310.15).
Run the numbers yourself
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More references: Wire size for 20 amps at 100 feet (240 V) · Wire size for 30 amps at 100 feet (240 V) · Wire size for 30 amps at 150 feet (240 V) · Wire size for 40 amps at 100 feet (240 V) · Wire size for 50 amps at 50 feet (240 V) · Wire size for 50 amps at 100 feet (240 V) · all 97 references