Cable Voltage Drop Calculator
Voltage drop is usually quoted as a single figure, which hides the fact that on a long run at poor power factor the reactive term can rival the resistive one. This separates the two, so you can see whether a bigger conductor will actually help.
Choosing something to solve for makes Voltage drop an editable target and computes the chosen input from it.
Formula
| R, X | resistance and reactance per unit length of one conductor |
| L | one-way run length (m) |
| phi | load displacement angle, cos(phi) is the power factor |
| k | supply arrangement factor |
What this model assumes, and where it stops
Assumptions
- Balanced load and balanced three-phase currents.
- Sinusoidal current at the fundamental only.
- Constant resistance and reactance along the run, at the operating temperature.
- Displacement power factor - no harmonic distortion.
- A single load at the far end, not distributed along the run.
Limitations
- Conductor resistance rises about 0.4% per kelvin. Cable tables are usually quoted at the maximum conductor temperature, so using a 20 degC value understates the drop on a fully loaded circuit.
- Reactance depends strongly on conductor spacing and arrangement. Spaced single-core cables have several times the reactance of a multicore, and a single tabulated value cannot cover that.
- Harmonics raise both the loss and the effective impedance, and neither is represented here.
- For a distributed load, the drop is roughly half what a lumped end load gives - this page assumes the end load.
- The exact phasor solution differs slightly from this linear approximation at large drops; above about 10% the approximation starts to matter.
- Nothing here checks ampacity, derating or protection coordination, which are usually the binding constraints.
When you need a 3D field solution instead
Closed-form models like the one above hold on idealised geometry. These are the cases where they stop being good enough and a full 3D electromagnetic and thermal solution is the only way to get a trustworthy answer:
- Working out the actual reactance of a specific conductor arrangement rather than taking a table value.
- Busbar systems where geometry sets the inductance and the phases couple strongly.
- Harmonic-rich loads, where skin and proximity effects raise the effective resistance frequency by frequency.
- Parallel cable runs, where circulating currents and unequal sharing depend on the physical layout.
- Cases needing the magnetic field around the run as well as the drop along it.
Common questions
What voltage drop is acceptable?
Installation codes typically allow 3 to 5% total from origin to load, split between feeder and final circuit - IEC 60364-5-52 suggests 4% as a default and separate limits apply for motor starting. The right number is the one your equipment tolerates: drives and contactors care about the voltage at their terminals during inrush, not the steady-state figure.
Why does doubling the conductor size not halve my voltage drop?
Because only the resistive part scales with copper. In the default 100 A, 50 m case at 95 mm2 the drop is 1.71 V of which 79% is resistive; going to 300 mm2 leaves 0.81 V, but the resistive share falls to 55% and the reactive part - set by conductor spacing, not size - stays at 0.36 V. Past that point you fix voltage drop with power factor correction or shorter routes, not more copper.
Where do the factor 2 and the factor sqrt(3) come from?
Single-phase current flows out and back, so the drop acts over twice the one-way length. In a balanced three-phase system the return currents cancel in the neutral and the line-to-line drop works out to sqrt(3) times the one-way per-conductor drop. The calculator applies the right factor from the supply selection; the length you enter is always one-way.
References
- IEC 60364-5-52 - Electrical installations, selection and erection of wiring systems
- IEC 60287 - Electric cables, calculation of the current rating