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Power & Utilities · Interactive Engineering Model

Voltage Drop Calculator

Volt drop · Percentage · Longest run

Change the load, the run and the conductor and see where the permitted drop actually falls — volt drop in volts and per cent with the reactive term included, the loss it costs, and the longest run that stays inside your limit.

Interactive model Screening estimate for cable runs and preliminary selection No registration Inputs stay in your browser

A

Balanced line current per phase at the design condition.

Displacement power factor of the load, assumed lagging as almost all industrial load is. A leading power factor reverses the sign of the reactive term and this tool does not model it.

One-way route length of the circuit. The return path is already in the formula.

Cables run in parallel per phase. Doubling them halves both resistance and reactance.

Start from an example: 200 A over 150 m of 95 mm² copper at 0.9 power factor →

Advanced parameters4
Cable
Ω/km

AC resistance of one conductor at operating temperature. A 95 mm² copper core is about 0.24 Ω/km at 20 °C and roughly 20% higher at 90 °C.

Ω/km

Inductive reactance of one conductor, typically 0.07–0.11 Ω/km for LV cables and largely independent of size.

System
V

Nominal line-to-line voltage, used for the percentage and the maximum length.

The limit you are designing to. Many rules use 3% for lighting and 5% overall, but check the one that applies.

Source Load — — Run length — R per km — X per km — Volt drop over the run — Of nominal

Voltage drop against power factor at constant current

  • —Voltage drop
  • —Drop in volts
  • —Voltage at the load
  • —Longest run within the limit
  • —Conductor loss
  • —Resistive share of the drop
  • —Drop per metre

Move a parameter and the run reacts: the cable, the drop along it, and the longest run the limit allows.

The link carries the figures you entered, in the part of the address that is never sent to a server.

Change one assumption at a time. The model updates from the same calculation used for the reported result.

Try this

Keep the load fixed and increase the cable run. Then add conductor capacity and see how the permissible distance changes.

How the model works

  1. The runEvery metre of conductor adds resistance and reactance. Both act on the same current, so the drop grows in proportion to the length of the run.
  2. The currentDrop follows the current directly. Conductor loss does not — it follows the square of it, which is why a heavily loaded run heats long before it drops out of limits.
  3. The splitThe inset shows how the drop divides. Resistance acts in phase with the current, reactance on the quadrature component, so a falling power factor moves weight onto the part a bigger conductor helps least.
  4. The limitThe permitted drop sets the longest run the circuit may have. The marker on the axis is that distance; when the load sits beyond it, the drop is already outside the limit.

Need the whole cable schedule, not one run?

We can build the calculation across every circuit, with real conductor data at operating temperature, and keep it in step with the design as it changes.

Discuss your use case →

Calculation basis 5 steps · view the method →
  1. Effective R and X = per-km values ÷ conductors per phase—
  2. Drop = √3 × current × length × (R·cos φ + X·sin φ)—
  3. Percentage drop = drop ÷ nominal line voltage—
  4. Longest run = permitted drop × voltage ÷ drop per metre—
  5. Conductor loss = 3 × current² × effective resistance × length—
Assumptions & limitations Screening estimate for cable runs and preliminary selection · 8 assumptions →

This is a screening calculation for a balanced three-phase run at steady load. It excludes harmonics, motor starting, transient and fault conditions, conductor temperature correction, grouping and installation derating, and any local wiring rule about permitted drop. Use the applicable standard, the manufacturer's cable data at operating temperature, and a proper cable schedule before issuing a design.

  • Balanced three-phase load at steady state; the neutral carries nothing.
  • The power factor is assumed lagging; with a leading power factor the reactive term subtracts instead of adding, and the drop is smaller than shown.
  • Resistance is taken as entered — at operating temperature it is typically 15–25% above the 20 °C value.
  • Harmonic currents, motor starting and fault conditions are excluded.
  • Grouping, ambient and installation-method derating are not applied.
  • The load is treated as a single point at the end of the run, not distributed along it.
  • Skin and proximity effects are assumed to be already inside the quoted AC resistance.
  • The percentage is taken against nominal voltage rather than the actual sending-end voltage.

Worked examples

What this model reveals

Volt drop is a distance problem wearing the clothes of a cable problem. Current and length multiply, and conductor impedance with power factor decides how much of that becomes voltage. The permitted drop then sets a distance rather than a cable size — which is why adding conductor buys length. The model shows the drop along the whole run and where the limit falls relative to the load.

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