Code reference · NEC Table 310.16; 310.12; 215.2
What wire size for a 100 A sub panel?
For a 100 A feeder to a subpanel, Table 310.16 at 75°C terminations gives: 3 AWG copper (100 A) or 1 AWG aluminum (100 A). That is the general answer for a subpanel feeder.
The 83% allowance in 310.12 (4 AWG copper / 2 AWG aluminum for 100 A) applies only to a dwelling's MAIN service or the feeder carrying the entire dwelling load — not to an ordinary subpanel feeding part of the load.
Don't forget: four conductors (two hots, neutral, equipment ground — grounds and neutrals separated at a subpanel), voltage drop on long runs (upsize past ~100–150 ft), and local amendments.
Run the numbers yourself
Need this as a permit-ready document?
One Document Credit ($29) buys a full clause-cited compliance PDF — wire sizing, voltage drop, overcurrent protection, every value traceable to the section it came from.
Get your document →Every numeric value on this page is computed by the CircuitProof calculation engine — the same golden-tested code that renders our permit documents — not typed by hand. Code text is paraphrased and cited; always verify against your jurisdiction's adopted edition and a licensed professional before installation. Design documentation, not a stamped drawing.
More references: NEC Table 310.16 ampacity, explained · The NEC 705.12 120% rule, actually explained · NEC 625: EV charger circuit sizing · NEC 706: battery energy storage disconnects · NEC 240.4(D): the small-conductor overcurrent caps · Voltage drop and the NEC: what's required vs recommended · all 97 references