Reference · NEC 2023
Voltage drop formula
Basic — K-factor (Ch. 9 Table 8 notes)
Three-phase: VD = √3 × K × I × L ÷ CM
%VD = VD ÷ V × 100
K = 12.9 copper, 21.2 aluminum (Ω·cmil/ft at 75°C). I = amps, L = one-way feet, CM = circular mils (e.g. #12 = 6530, #10 = 10380). The 2 covers out + back; √3 covers 120° phase spread. Fast, field-proven for branch circuits in PVC at PF ≈ 1.
Advanced — Table 9 effective impedance (Note 2)
Single / DC: VD = 2 × I × Zeff × L ÷ 1000
Three-phase: VD = √3 × I × Zeff × L ÷ 1000
R/X in Ω/1000ft at 75°C per conduit (PVC / aluminum / steel). Steel (magnetic) conduit raises X ~70% — K alone under-reads large steel-raceway feeders. Motors: PF 0.8–0.9. At PF = 1, Zeff = R and both methods agree within a few percent.
mV/A/m — same math, IEC writing
VD = mV/A/m × I × L ÷ 1000 (L in metres). Our Advanced panel shows the equivalent mV/A/m as a display-only cross-check for BS 7671 / IEC datasheets. US field work stays in K or Ω/kft.
Numeric check
#12 Cu, 20A, 100ft, 120V 1φ: Basic 2×12.9×20×100÷6530 = 7.90V (6.58%). Advanced PVC PF 1.0: 2×20×1.98×100÷1000 = 7.92V — agreement confirms both tables.