How do I size a residential circuit?
Three checks in one tool: voltage drop, the breaker and wire a load needs, and conduit fill. Copper + THHN assumed.
Result
3.3% drop
4.0 V lost on 12 AWG · target ≤ 3%
Over 3% — upsize
Smallest wire that stays under 3% here: 10 AWG
Loading 3D viewer…
How is this calculated?
Using the tool
Three checks in one tool: voltage drop on a wire run, the breaker and wire a load needs, and whether your conductors fit the conduit. Switch modes above. Copper + THHN assumed.
The formulas
Voltage drop = 2 × K × I × L ÷ CM (single-phase): K = 12.9 for copper, I the current, L the one-way length in feet, CM the conductor's circular mils. The NEC informational target for a branch circuit is 3% or less; beyond that, lights dim and motors run hot, so you step up a wire size.