Voltage Drop Calculator
Estimate resistive voltage drop in copper or aluminum conductors from design current, one-way path length, and cross-section (mm²). Results show volts and percent of nominal voltage for single-phase or balanced three-phase runs—useful before detailed NEC/IEC checks.
Next step after sizing current: validate conductor thermal sizing with the Cable Size Calculator and protection with the Breaker Size Calculator.
Quick Answer
For many industrial LV circuits, keeping steady-state resistive drop near ~3% on feeders and ~5% end-to-end is a common planning conversation—exact limits depend on your code edition, circuit type, and special loads. This calculator gives a PF≈1 resistive screening estimate at 20 °C conductor temperature.
Quick presets
Load example values into the calculator below, then refine for your site.
Advanced inputs
Quick: current, length, and mm² presets. Advanced: phase model and Vnom denominator must align with how you quote % drop in your spec.
About this calculator
Screens steady-state resistive drop on copper or aluminum runs so you can compare feeders and branches before full reactance, temperature, and code table work. For breaker and cable workflows in the same protection suite, use the protection calculators hub.
Results
Engineering disclaimer
Reactance, power factor, parallel runs, conduit temperature, and harmonic currents are not modeled. This is not a substitute for NEC Chapter 9 / IEC 60364-5-52 tables or engineer-stamped calculations.
Results
Drop (% of nominal 230 V): 2.40%
Material / model: Copper at 20 °C resistivity, Single-phase (2 conductors, go and return).
Formula (resistive model)
Resistance one way: R = ρ · L / A, where ρ is conductor resistivity (Ω·mm²/m), L is one-way length (m), and A is cross-section (mm²). Copper reference ρ ≈ 0.017241 at 20 °C; aluminum ≈ 0.028265.
Single-phase: approximate line drop V ≈ I · 2R (out and back).
Three-phase (balanced, resistive): approximate V ≈ √3 · I · R using the same one-way R per conductor.
Percent drop = (V / Vnom) · 100% using the nominal voltage you enter for the denominator.
Worked example (screening)
Given: 16 A, 25 m one-way, 2.5 mm² copper, single-phase, 230 V nominal.
Find: R = 0.017241 × 25 / 2.5 ≈ 0.172 Ω one way; V ≈ 16 × 2 × 0.172 ≈ 5.5 V; percent ≈ 5.5 / 230 ≈ 2.4% (run the preset button above to match).
Guides
Frequently asked questions
Why is my percent drop different from NEC examples?
NEC examples include temperature correction, alternating-current resistance/reactance adjustments, and power factor. This page uses a simplified 20 °C DC resistance model for screening only.
Should I use line-to-line or line-to-neutral voltage for %?
Use the same reference your team uses for the circuit being studied. Be explicit in reports; mixing references changes the percentage even if the physical drop in volts is unchanged.
What if I need a smaller drop?
Increase conductor area, shorten the route, reduce current, or change topology (additional parallel feeders). Confirm ampacity and protection independently.
Does aluminum behave the same way in the tool?
The same formula applies with a higher resistivity constant. Terminations, oxidation control, and ampacity tables still require manufacturer and code guidance.
Related tools and hub
Use this result with: Cable Size Calculator, Breaker Size Calculator, and kVA to Amps Calculator.