Voltage Drop Calculator
Estimate the voltage lost along a wire run from length, current and conductor size.
Results update as you type.
About this calculator
Every conductor has some resistance, so a little voltage is always lost pushing current along a cable — that loss is the voltage drop. For a single-phase run the drop is Vdrop = 2 × ρ × L × I / A, where the factor 2 accounts for the current travelling out to the load and back along the return conductor; a three-phase run uses √3 instead of 2. Here ρ is the resistivity of the metal (copper ≈ 0.0175 Ω·mm²/m, aluminium ≈ 0.0282 Ω·mm²/m), L is the one-way length in metres, I the load current in amps and A the conductor cross-section in mm². This calculator also reports the resulting percentage drop, the voltage actually delivered at the load and the power wasted as heat in the cable (Ploss = Vdrop × I).
As a worked example, take 20 A flowing 50 m one-way through a 10 mm² copper cable on a 230 V single-phase supply: Vdrop = 2 × 0.0175 × 50 × 20 / 10 = 3.5 V, about 1.5%, so the load sees roughly 226.5 V and the cable wastes 3.5 × 20 = 70 W. Switch the conductor to aluminium and the same run drops about 5.6 V, because aluminium’s resistivity is around 60% higher. You can either type a custom cross-section or pick a standard AWG gauge from the advanced list, which overrides the area field with the gauge’s real mm² value.
Voltage drop matters because motors, lights and electronics misbehave when they are starved of voltage — dim lamps, sluggish motors and nuisance tripping all trace back to oversized drops on long runs. Electricians size cables so total drop stays within about 3% for branch circuits and 5% end-to-end; the result here flags whether your run meets the limit you set, and the fix for a failing run is a larger conductor, a shorter route or, occasionally, a higher supply voltage.
Frequently asked questions
How is voltage drop calculated?
For single-phase, Vdrop = 2 × ρ × L × I / A. The factor 2 accounts for the current travelling out and back; three-phase uses √3 instead of 2.
What voltage drop is acceptable?
A common rule of thumb is to keep total drop under 3% for branch circuits and 5% overall. Larger conductors (bigger area) reduce the drop.
Copper or aluminium — does it matter?
Yes. Aluminium has about 60% higher resistivity than copper, so an aluminium run of the same size drops proportionally more voltage.
Why does the formula use twice the length?
Current has to flow out to the load and back, so a single-phase circuit sees the drop of both conductors — hence the factor of 2 × length. Three-phase circuits use √3 (about 1.732) instead, reflecting the geometry of balanced three-phase power.
How do I reduce an excessive voltage drop?
Use a larger conductor (doubling the cross-section roughly halves the drop), shorten the run, split the load across parallel cables, or raise the supply voltage. Bigger copper is the usual fix because drop is inversely proportional to area.
How much power is lost in the cable?
The wasted power is the drop times the current, Ploss = Vdrop × I. A 3.5 V drop at 20 A wastes 70 W as heat in the wire — energy you pay for but never use, which is why long high-current runs justify thicker, lower-loss cable.
Can I use AWG gauges instead of a mm² area?
Yes. Pick a gauge in the advanced AWG list and it overrides the area field with that gauge’s cross-section in mm² (e.g. 10 AWG ≈ 5.26 mm²), so you can work in the gauge system North American installers use.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/voltage-drop/curl
curl "https://calculator.free/api/v1/voltage-drop/?material=copper&length=50¤t=20&area=10&source=230"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/voltage-drop/?" + new URLSearchParams({
"material": "copper",
"length": "50",
"current": "20",
"area": "10",
"source": "230"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.