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Voltage Drop Calculator

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No conductor is perfect, so some of the voltage you send down a cable is spent getting there rather than arriving. Over a short run it is negligible; over a long one it dims lamps, overheats motors and makes heaters underperform. This calculates the loss from the conductor size, the run length, the current and the temperature — and reports the longest run that would stay inside the 3% figure most wiring guidance treats as comfortable.

Aluminium is about 54% more resistive for the same size

m

The distance, not the wire used

A
V

DC and single-phase use twice the run; three-phase uses √3

°C

Copper gains about 0.4% resistance per degree

How to use the voltage drop calculator

  1. 1Pick the conductor size and whether it is copper or aluminium.
  2. 2Enter the one-way distance to the load, not the total length of wire — the calculation doubles it for you where the circuit type requires.
  3. 3Enter the current the load draws and the supply voltage.
  4. 4Choose the circuit type: DC and single-phase use twice the run, three-phase uses √3.
  5. 5Set the conductor temperature. A cable that has been working for an hour is warmer than the room, and warmer copper is more resistive.

Examples

A workshop socket circuit

Input
12 AWG copper, 30 m, 15 A, 120 V, single-phase
Result
5.24 V lost — 4.36%, over the comfortable limit

A long run at a fair current. Stepping up to 10 AWG brings it back under 3%.

The same load, shorter run

Input
12 AWG copper, 15 m, 15 A, 120 V
Result
2.62 V — 2.18%, comfortably inside

Loss is directly proportional to length, so halving the distance halves the drop exactly.

Low voltage, where drop bites hardest

Input
14 AWG copper, 20 m, 5 A, 12 V
Result
3.31 V — 27.6% of the supply

A 12 V system has no headroom to lose. This is why LED strip runs go dim at the far end.

About the voltage drop calculator

What is actually being lost

The cable is a resistor you did not intend to install. Current flowing through it develops a voltage across it, exactly as Ohm's law describes, and that voltage is subtracted from what reaches the load. The energy involved does not vanish — it becomes heat in the cable, which is why an undersized run for a heavy load feels warm along its length.

Two things set the size of the effect. The first is the conductor's resistance, which depends on the material, the cross-sectional area and the length: thicker is better, shorter is better, and copper beats aluminium by about 54% for the same size. The second is the current, and here the relationship is worth stating carefully. The voltage lost rises in proportion to the current, but the power wasted rises with the square of it, because power is I²R. Doubling the load doubles the volts lost and quadruples the heat.

That squared term is why voltage drop is an efficiency problem as well as a performance one. A run losing 5% of the voltage is also turning 5% of the energy into warm cable, continuously, for the life of the installation.

The four ways to fix a run that fails

Use a thicker conductor. Resistance is inversely proportional to cross-sectional area, and on the AWG scale three gauges down roughly doubles the area, so 10 AWG has about twice the copper of 13 AWG and half the drop. This is the usual answer and it costs money in proportion to the metal.

Shorten the run. Loss is directly proportional to length, so relocating a distribution point halfway to the load halves the problem. This is often cheaper than heavier cable over a long distance, which is precisely why electricity is distributed at high voltage and stepped down close to where it is used.

Raise the voltage. Since the drop is a percentage of what you started with, the same absolute loss matters half as much at twice the voltage — and if the load is a fixed power rather than a fixed resistance, doubling the voltage also halves the current, which halves the drop again. This is why long LED runs are increasingly 24 V or 48 V rather than 12 V.

Reduce the current. Splitting one heavy circuit into two lighter ones on separate cables halves the current in each and therefore halves the drop in each, at the cost of a second cable. Whether that beats one thicker cable depends on prices rather than on physics.

Frequently asked questions

Why is the run length doubled?
Because current has to get there and come back. A cable run of 30 metres contains 60 metres of conductor in the circuit — the live and the neutral, or the positive and the return — and both develop a voltage drop. Three-phase is the exception: the return current is shared between the phases rather than travelling its own conductor, so the multiplier is the square root of three rather than two.
Why does low-voltage wiring suffer so much more?
Because the drop is an absolute number of volts, and what matters is what fraction of the supply it represents. Losing 3 volts in a cable is a rounding error on a 230 V circuit and a quarter of the supply on a 12 V one. This is why long LED strip runs fade towards the far end, why solar and battery installations use conductors that look absurdly heavy for the current, and why raising the distribution voltage is the standard fix for a long run.
Where does the 3% figure come from?
It is a widely used design target rather than a universal legal limit. Several codes recommend keeping a branch circuit within 3% and the whole path from supply to load within 5%, with the exact numbers and their legal status depending on where you are. Below 3% almost nothing notices; above 5% incandescent lamps visibly dim, motors draw more current to make up the shortfall and run hotter, and resistive heaters lose output as the square of the voltage.
Does the temperature setting really matter?
Enough to change a marginal answer. Copper gains roughly 0.4% resistance per degree Celsius, so a conductor at 70 °C — a normal insulation rating, and a normal temperature for a cable working near its capacity — is about 20% more resistive than the same cable at 20 °C. A run calculated at room temperature that lands just inside 3% may sit outside it once loaded, which is why the field is here rather than assumed.
Can I use this to choose a cable size?
Only for the voltage drop half of the question, and that is genuinely half. A conductor also has to be able to carry the current without its insulation overheating, and that limit — ampacity — depends on the insulation rating, the ambient temperature, how many cables are bundled together and whether the run is in free air, buried or in a wall. Those figures live in the wiring regulations that apply where you are. A cable has to satisfy both constraints, and this page addresses one.

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