Voltage Drop

Resistance method on the Chapter 9 Table 8 basis, or the impedance method on Table 9 with power factor — sizing floored at Table 310.16 ampacity. No number you enter leaves your browser.

Inputs

What do you need?

Drop for a run Size for a target Max length for a size

Method

Impedance accounts for reactance and power factor: use it for large conductors, long AC runs, steel conduit, or low power factor.

System

Voltage (V) — line-to-line

Three-phase uses the line-to-line voltage (208, 480, 600). For a 120 or 277 V branch, choose Single-phase.

Load current (A)

Conductor

Size

Parallel sets per phase

One-way length

Conductor temperature

Sets the resistance correction and the Table 310.16 ampacity column; 75 °C matches the table and most terminations (110.14(C)).

Result

Enter the load and the run, then Calculate.

Typical approach, not engineering. Resistance method: VD = 2 · I · L · R_kft ⁄ 1000 for DC and single-phase, √3 · I · L · R_kft ⁄ 1000 for three-phase, where L is one-way length in feet and R_kft is the conductor resistance in Ω per 1,000 ft (Chapter 9 Table 8, 75 °C, corrected for temperature per Table 8 Note 2). Written per foot instead, R = K ⁄ cmil with K = 12.9 Ω·cmil/ft copper, 21.2 aluminum, and the same drop is VD = 2 · K · I · L ⁄ cmil (√3 · K · I · L ⁄ cmil three-phase) with no ÷ 1000 — the two forms differ only by R_kft = 1000 · K ⁄ cmil. The circuit multiplier (2 or √3) appears once. The impedance method uses Chapter 9 Table 9 (three single conductors in conduit, 75 °C, 60 Hz): VD = √3 · I · L · Zₑ ⁄ 1000 for three-phase, and VD = 2 · I · L · Zₑ ⁄ 1000 for DC and single-phase — the circuit multiplier replaces √3, it is not applied on top of it — with Zₑ = R cos θ + X sin θ at your power factor (Table 9 Note 2) — R and X depend on conduit material; 700, 800, and 900 kcmil are not listed, so sizing steps past them. Sizing installs the larger of the voltage-drop minimum and the Table 310.16 ampacity minimum at the chosen temperature column (240.4(D) small-conductor caps; parallel sets from 1/0 AWG per 310.10(G)) — ambient correction, conductor-count adjustment, and continuous-load factors are on the Wire Sizing calculator. The 3 % and 5 % figures are NEC Informational-Note recommendations, not requirements. A licensed engineer confirms; nothing here is a seal or stamp.