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.

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

See Sources below ↓