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
Checked against published worked examples from: EC&M.
Details, formula, and sources
ScopeNEC 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.
Formula or tableA_req = 0.83 x service_A; size = smallest Table 310.16 75 degC conductor (ordered smallest first, copper or aluminum) with ampacity >= A_req.
Source and editionThe 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.
Free accessNEC 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.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesservice_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)