Wire Size & Voltage Drop Calculator

Voltage drop uses the K-factor (circular-mils) method, the form every field reference presents. K is the conductor resistivity constant — 12.9 for copper and 21.2 for aluminum (ohm-cmil/ft at ~75 °C). I is the load current, L is the one-way run length in feet, and CM is the conductor's cross-sectional area in circular mils. The leading 2 (single-phase and DC) and √3 ≈ 1.732 (three-phase) bake in the return path / line-to-line factor, so you enter one-way distance and never double it yourself.

single-phase / DC:  VD = (2 × K × I × L) / CM
three-phase:        VD = (√3 × K × I × L) / CM
percent drop:       VD% = VD / V_source
voltage at load:    V_end = V_source − VD

find-size recommended = larger (greater CM) of:
  ampacity leg — first size whose 75 °C column (capped by
    NEC 240.4(D): 14→15 A, 12→20 A, 10→30 A) ≥ sizing amps
    (sizing amps = load × 1.25 when continuous)
  drop leg     — first size whose VD% ≤ your target

The ampacity leg walks the NEC 310.16 copper column (75 °C by default; 60/90 °C reachable in the advanced block) and applies the 240.4(D) small-conductor caps. Aluminum reuses the copper ampacity column in this version — the K-factor already captures aluminum's higher resistance in the drop leg. The recommended size is the larger of the ampacity and drop results, and the breakdown names which leg governed.

Method & edition drift. This is the simplified reactance-free K-factor method; large three-phase feeders may need the AC effective-impedance method (NEC Ch. 9 Table 9). The NEC ampacity-table reference moved from 310.15(B)(16) (2017 NEC) to 310.16 (2020+ NEC), and the adopted edition varies by jurisdiction — this tool is pinned to the 310.16 basis and disclaims that drift. Always confirm against the edition your AHJ has adopted.

Sizing settles what goes in the wall; what the customer pays is a separate build-up of labor hours, parts and overhead, which the electrical work pricing calculator assembles fixture by fixture. Once the run is agreed, the conductor and length you landed on are worth writing onto the electrical work order template so the crew installs what was priced, and they carry through to the electrical invoice template at the end. Sister calculators for the other expansion trades are collected on the cross-trade tools hub.

Formula: VD = (2 × K × I × L) / CM · 3φ: VD = (√3 × K × I × L) / CM · VD% = VD / V

See Sources below ↓