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Voltage Drop Calculator

Calculate voltage drop, percentage drop and estimated load voltage for DC, single-phase AC and balanced three-phase AC circuits using cable length, conductor material, cross-section and power factor.

Example: a 230 V single-phase circuit carries 10 A through 30 m of one-way 2.5 mm² copper cable. What is the voltage drop?

For AC circuits: if cable-specific data is unavailable, 0.08 Ω/km is used as an approximate default reactance from the Schneider guide. At power factor 1, reactance does not affect the result.

Voltage drop 5.688 V 2 × 10 A × 9.48 Ω/km × 0.03 km = 5.688 V
Voltage-drop percentage 2.473043478260869565% Percentage = voltage drop ÷ source voltage × 100.
Estimated voltage at load 224.312 V R ≈ 9.48 Ω/km

✓ Uses steady-load voltage-drop formulas and the conductor-resistance approximation published in the Schneider Electric Installation Guide

How is it calculated? For DC, ΔV = 2 × I × R × L. For single-phase AC, ΔV = 2 × I × (R cosφ + X sinφ) × L. For balanced three-phase AC, ΔV = √3 × I × (R cosφ + X sinφ) × L. L is one-way cable length in kilometres. Conductor resistance is estimated as R = 23.7 ÷ S for copper or R = 37.6 ÷ S for aluminium, with S in mm². This is an engineering estimate, not a substitute for cable-manufacturer data, electrical codes or qualified design.

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Voltage-drop formulas and quick checks

  • DC: ΔV = 2 × I × R × L
  • Single-phase AC: ΔV = 2 × I × (R cosφ + X sinφ) × L
  • Balanced three-phase: ΔV = √3 × I × (R cosφ + X sinφ) × L
  • Approximate copper resistance: R = 23.7 ÷ S Ω/km
  • Approximate aluminium resistance: R = 37.6 ÷ S Ω/km
  • Voltage-drop percentage = ΔV ÷ source voltage × 100
  • Estimated load voltage = source voltage − ΔV

How it works

  1. Choose DC, single-phase AC, or balanced three-phase AC.
  2. Enter source voltage, load current, one-way cable length, conductor material and conductor cross-section in square millimetres.
  3. For AC circuits you can enter power factor and cable inductive reactance. If cable-specific X is unavailable, the interface starts with 0.08 Ω/km as the Schneider guide’s approximate default.
  4. Conductor resistance is estimated as R = 23.7 ÷ S for copper or R = 37.6 ÷ S for aluminium, where R is in Ω/km and S is in mm².
  5. Single-phase drop uses ΔV = 2 × I × (R cosφ + X sinφ) × L. Balanced three-phase uses √3 instead of 2. DC uses the resistive relationship ΔV = 2 × I × R × L.
  6. The calculator reports voltage drop in volts, drop as a percentage of source voltage, and the arithmetic voltage remaining at the load.
  7. This is a steady-load estimate. Manufacturer cable resistance/reactance, temperature, installation method and local electrical-code requirements can change the real design result.
Calculation-method sources

FAQ

Should I enter one-way cable length or round-trip length?

Enter one-way cable length. The calculator applies the two-conductor factor automatically for DC and single-phase circuits, and √3 for balanced three-phase circuits.

Can it calculate both copper and aluminium cable voltage drop?

Yes. It uses the Schneider guide’s simplified conductor-resistance approximation for copper or aluminium based on cross-sectional area.

What does power factor change?

For AC, power factor participates in the impedance voltage-drop term with conductor resistance and inductive reactance. At power factor 1, sinφ is zero and X has no effect.

What should I use for cable reactance X if I do not know it?

The calculator starts at 0.08 Ω/km, an approximate value given by the Schneider guide when no other information is available. Manufacturer-specific cable data is preferable.

Does this calculator select a code-compliant wire size?

No. It estimates voltage drop only. It does not verify ampacity, overcurrent protection, conductor temperature, installation method or local electrical-code requirements.

Why can a cable-manufacturer calculator give a different answer?

Manufacturer software may use cable-specific resistance at operating temperature and reactance for the exact cable construction and installation, which is more specific than this general estimate.

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