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.
Example
You enter
- Service or main-feeder rating (A) 200
- Conductor material copper
You get
- Required ampacity (83%) 166 A
- Service-entrance conductor 2/0
- Size ampacity (A) 175
Details, formula, and sources
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. A service-load calculation produces the amperage; this turns it into the wire. Dwellings only; adjustments and the neutral are separate. A design aid; the AHJ governs.
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.
NEC is free to read at nfpa.org/freeaccess. The single-phase 120/240 V dwelling scope, the ambient/fill adjustments, and the neutral sizing are stated, not modeled.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: service_A, material, A_req, size, size_ampacity_A
- 83% allowance ungrounded service-entrance or main-feeder conductors of a one-family dwelling (or an individual dwelling unit) at 83% of the service rating, single-phase 120/240 VNEC 310.12(A)/(B)
- Ampacity column Table 310.16 at 75 degC (the common service-equipment termination rating), single set to 400 kcmil; parallel sets are out of scopeNEC 310.16 / 110.14(C)