A free voltage drop calculator covering NEC 210.19 limits for single-phase and three-phase circuits. Handles copper and aluminum at 60°C, 75°C, and 90°C insulation; includes AC reactance and power factor; works for DC sizing too.
Where the calculator lives. The voltage drop tab is one mode of the full Wire & Cable calculator — you'll need ampacity sizing alongside voltage drop to make the correct conductor selection (NEC requires the larger of the two), so we kept them in one tool. Open the calculator, then click the “Voltage Drop” tab.
Open the voltage drop calculatorOn short runs (under ~50 ft) at any current, ampacity will set the conductor size. On long runs at moderate current — feeders to outbuildings, branch circuits to remote machinery, DC battery interconnects — voltage drop usually sets the size, often forcing you up one or two trade sizes above the ampacity-required minimum.
The Wire & Cable calculator's Cable Sizing tab handles this automatically: it runs both passes and reports the larger size. The Voltage Drop tab lets you check a specific size you have on hand, with full reactance math. Use the Cable Sizing tab when you don't have a size in mind yet; use the Voltage Drop tab when you do.
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped feeder voltage-drop study from a working engineer — typically within 48 hours of receiving your one-line and load data.
NEC 210.19 (informational, not strict code) recommends ≤ 3% on branch circuits and ≤ 5% on the combined feeder + branch run. Most engineers treat the 3% number as a hard ceiling for sizing decisions, especially on motor and lighting circuits where voltage drop affects equipment performance directly.
Single-phase: VD = 2 × I × R × L / 1000. Three-phase: VD = √3 × I × R × L / 1000 (real component) or √3 × I × (R cos θ + X sin θ) × L / 1000 when reactance matters. The calculator uses the second form with reactance and power factor inputs.
Yes. Set system type to single-phase, set voltage to your DC bus voltage (12, 24, 48, 380 V), and use a power factor of 1.0. The math collapses to the standard DC formula automatically. Most DC and battery-bank installs target 2% voltage drop instead of 3%.