Voltage Drop Calculator
Estimate how much voltage is lost along a cable run — important for long runs, where too much drop dims lights, weakens motors and wastes energy. Enter the run length, conductor size, current and system voltage.
How to use this calculator
- Enter the one-way run length — the calculator doubles it for the return path.
- Enter the conductor size, current and system voltage.
- Check the drop against the 3% guideline; over that, use thicker cable.
Formula used
Current flows out and back, so the resistive length is twice the run. Resistivity ρ is ~0.0175 Ω·mm²/m for copper, ~0.0282 for aluminium. A common design guideline keeps total drop under 3% (some codes allow up to 5%).
Example calculation
16 A over a 30 m run of 2.5 mm² copper at 230 V: resistance = 0.0175 × 60 ÷ 2.5 = 0.42 Ω, drop = 16 × 0.42 = 6.7 V (2.9%) — just within the 3% guide.
Push it to 40 m and the drop exceeds 3%, so you'd step up to 4 mm² cable.
Why long runs need thicker cable
Every conductor has resistance, so current flowing through it loses some voltage as heat — a loss that grows with run length and current, and shrinks with conductor thickness. On short household runs it's negligible, but on long runs (outbuildings, garden supplies, industrial layouts) excessive drop starves the load: lights dim, motors run hot and struggle to start, heaters underperform.
The standard fix is counter-intuitive to newcomers: to reduce drop you make the wire thicker (more area = less resistance), not change the voltage. Regulations cap acceptable drop — commonly 3% for lighting and 5% for other loads — precisely to force adequate cable sizing on long runs. The return path matters too: current makes a round trip, so the effective resistive length is twice the physical distance to the load.
Why use this calculator?
- Check whether a planned cable run stays within the drop limit.
- Size up cable correctly for long runs before wiring.
- See the power wasted as heat in undersized cable.
Frequently asked questions
What is an acceptable voltage drop?
A common design guideline is under 3% for lighting circuits and under 5% total for other loads, though exact limits are set by local wiring regulations. Lower is always better for efficiency and equipment performance.
How do I reduce voltage drop?
Use a thicker conductor (larger mm²) — the most common fix — or shorten the run, reduce the current, or increase the system voltage if the design allows. Doubling the cross-section roughly halves the drop.
Why does the calculator double the cable length?
Current flows out to the load and back through the return conductor, so it travels twice the one-way distance. The resistance that causes voltage drop acts over that full round-trip length.