Dwelling Service/Feeder Conductor at 83% (NEC 310.12)
Why a 200 A house runs on 2/0 copper instead of the 3/0 a straight table lookup demands: NEC 310.12 lets a single-phase 120/240 V dwelling service (and its main feeder) use conductors rated 83% of the service rating. 0.83 x the rating, then the smallest Table 310.16 75 C conductor at or above it - 200 A -> 166 A -> 2/0 Cu or 4/0 Al, 100 A -> #4 Cu, exactly what Table 310.12 tabulates. The service-load tiles produce the amperage; this turns it into the wire. Dwellings only; adjustments and the neutral are separate. A design aid; the AHJ governs.
Formula and source
A_req = 0.83 x service_A; size = smallest Table 310.16 75 degC conductor (ordered smallest first, copper or aluminum) with ampacity >= A_req.
The dwelling service/feeder 83% allowance of NEC 2023 310.12(A)/(B) with the conductor selected from the Table 310.16 75 degC column (reproducing Table 310.12's tabulated 100 A -> #4 Cu, 200 A -> 2/0 Cu / 4/0 Al results), by name. A computational aid; the AHJ-adopted NEC edition governs.
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