Enter values and press Calculate.
Every wire has resistance, so some voltage is lost as current flows along it — the voltage drop. Too much drop makes lights dim, motors run hot, and electronics misbehave. Most standards recommend keeping the total drop below 3–5% of the supply voltage.
| Quantity | Formula |
|---|---|
| Single-phase / DC drop | Vdrop = 2 × I × ρ × L / A |
| Three-phase drop | Vdrop = √3 × I × ρ × L / A |
| Percentage drop | %drop = Vdrop / Vsource × 100 |
| Resistivity ρ (Ω·mm²/m) | Copper ≈ 0.0175, Aluminium ≈ 0.0282 |
The single-phase formula uses a factor of 2 because current flows down and back through two conductors; three-phase uses √3. To reduce drop: use a thicker conductor (larger A), a shorter run, or copper instead of aluminium.
The voltage lost in a conductor because of its resistance as current flows through it. It equals the current times the wire resistance, and it reduces the voltage available at the load.
Most standards recommend keeping the total drop under 3% for lighting and 5% for general circuits. Exceeding this causes dim lights, weak motors, and wasted energy.
Because the current travels out along one conductor and back along another, so it passes through twice the one-way cable length. DC circuits use the same factor of 2.
In a balanced three-phase system the line-to-line voltage drop works out to √3 times the current, resistivity, length and inverse area — a standard result of three-phase analysis.
About 0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminium at normal temperatures. Resistivity rises with temperature, so hot cables drop a little more.
Use a larger cross-section (thicker wire), shorten the run, use copper instead of aluminium, or raise the system voltage so the same power needs less current.
The absolute drop depends on current, not voltage, but the same drop is a much bigger percentage of a 12 V supply than a 230 V supply — so 12 V and 24 V systems need thick cables.
Yes — the dropped voltage times the current is power dissipated as heat in the cable (I²R loss). Lower drop means lower losses and cooler wiring.
No. Ampacity is the maximum current a cable can carry without overheating; voltage drop is a separate limit about performance. A cable can pass the ampacity check but still drop too much on a long run.
Enter the one-way length — the formula's factor of 2 (single-phase) already accounts for the return conductor.
This calculator uses conductor resistance only, which dominates for small and medium cables. For large cables or long AC runs, cable reactance and power factor also matter.
Copper resistance rises about 0.4% per °C. For a hot conductor (say 70 °C), the drop is roughly 20% higher than the 20 °C value — use a higher resistivity for worst-case design.
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