Why is 4% the recommendation?

Excess voltage drop means motors, heaters and electronics get lower voltage than rated, giving reduced output, more heat and shorter life. IEC 60364 and national codes typically recommend max 3% for lighting and 5% for power loads. 4% is a safe universal limit many electricians use. Note: computers tolerate more, but motors with high starting current struggle at low voltage.

Copper has lower resistivity (~0.0175 Ω·mm²/m) than aluminium (~0.0282 Ω·mm²/m), so the same cross-section gives 61% more drop for aluminium. For equal drop, aluminium needs ~1.6× the cross-section. Aluminium is cheaper per metre but has heavier terminations, creeps (needs retightening) and corrodes more. For service entrances and runs over 100 m aluminium is common; for house circuits, copper almost always wins.

Why multiply by 2 for 1-phase but √3 for 3-phase?

In 1-phase, current goes through the line conductor and returns through the neutral, so twice the cable length, hence the factor 2. In balanced 3-phase, return currents cancel at the neutral point, so current effectively goes "one way", and trigonometry gives a factor √3 = 1.732 (because line voltage is √3 times phase voltage). Net: 3-phase has ~15% less voltage drop than 1-phase for the same power and length.

Can I use this for real design work?

No, not directly. The calculator is a simplified purely resistive computation that ignores cable reactance (important at large sections and long runs), power factor, conductor temperature (hotter conductor = higher resistance), grouping effects and installation-method correction. For approved design you must follow national codes (NEC, IEC 60364) and use professional software like DIALux, ETAP or the cable maker’s tools. Use this for rough estimation and to understand the principle.

/spenningsfall

Voltage Drop Calculator - Cable Size and Length

Calculate voltage drop in a cable from length, current, cross-section and conductor (copper or aluminium), for single- and three-phase.

Conductor material

System

Cable length (m, one way)

Current (A)

Cross-section (mm²)

Voltage (V)

Voltage drop
Voltage drop (%)

How it works

Voltage drop is how many volts are "lost" in the cable because of the conductor's resistance. For a single-phase circuit, ΔU = 2 · L · I · ρ / A, and for three-phase ΔU = √3 · L · I · ρ / A. Here L is the one-way cable length in metres, I the current in amperes, A the cross-section in mm², and ρ the resistivity (copper ≈ 0.0175, aluminium ≈ 0.0282 Ω·mm²/m).

This is a simplified, purely resistive calculation that ignores reactance, power factor and conductor temperature. As a rule of thumb, a maximum of around 4% voltage drop is often recommended. For designing real installations, follow the applicable regulations and manufacturer data. Everything is calculated locally in your browser.

About this tool

The voltage-drop calculator tells you how many volts are "lost" in a cable because of conductor resistance. Choose copper or aluminium, 1-phase or 3-phase, cable length, current, cross-section and system voltage. Useful for planning new circuits, sizing cable to a garage, cabin or EV charger, or checking that an existing installation stays within the 4% recommendation. All computed locally, no server.

How to use it

Pick the conductor material (copper or aluminium).
Choose 1-phase or 3-phase.
Enter one-way cable length, current, cross-section and voltage.
See the voltage drop in volts and percent in real time.

Examples

16 A circuit, 30 m

Input2.5 mm² copper, 1-phase 230 V, 16 A, 30 m

OutputΔU = 6.72 V (2.92%) - within 4%

EV charging 32 A

Input6 mm² copper, 3-phase 400 V, 32 A, 40 m

OutputΔU = 6.5 V (1.62%) - good margin

For EV charging the cable is typically loaded for many hours, so avoid running it at the limit. Going up one size (10 mm² instead of 6 mm²) reduces heat and gives more headroom.

Common use cases

Sizing cable to a shed, garage, cabin or annex.
Planning a circuit for an EV charger (11 kW, 22 kW) with long runs.
Checking if an existing cable is big enough when adding new equipment.
Deciding whether aluminium cable is acceptable vs copper for long runs.
Estimating energy loss in long cables over time.

Frequently asked questions

Why is 4% the recommendation?: Excess voltage drop means motors, heaters and electronics get lower voltage than rated, giving reduced output, more heat and shorter life. IEC 60364 and national codes typically recommend max 3% for lighting and 5% for power loads. 4% is a safe universal limit many electricians use. Note: computers tolerate more, but motors with high starting current struggle at low voltage.
Copper or aluminium?: Copper has lower resistivity (~0.0175 Ω·mm²/m) than aluminium (~0.0282 Ω·mm²/m), so the same cross-section gives 61% more drop for aluminium. For equal drop, aluminium needs ~1.6× the cross-section. Aluminium is cheaper per metre but has heavier terminations, creeps (needs retightening) and corrodes more. For service entrances and runs over 100 m aluminium is common; for house circuits, copper almost always wins.
Why multiply by 2 for 1-phase but √3 for 3-phase?: In 1-phase, current goes through the line conductor and returns through the neutral, so twice the cable length, hence the factor 2. In balanced 3-phase, return currents cancel at the neutral point, so current effectively goes "one way", and trigonometry gives a factor √3 = 1.732 (because line voltage is √3 times phase voltage). Net: 3-phase has ~15% less voltage drop than 1-phase for the same power and length.
Can I use this for real design work?: No, not directly. The calculator is a simplified purely resistive computation that ignores cable reactance (important at large sections and long runs), power factor, conductor temperature (hotter conductor = higher resistance), grouping effects and installation-method correction. For approved design you must follow national codes (NEC, IEC 60364) and use professional software like DIALux, ETAP or the cable maker’s tools. Use this for rough estimation and to understand the principle.

Technical background

Formula: ΔU = k · L · I · ρ / A, where k = 2 for 1-phase (there + back) and √3 for 3-phase (line voltage). L is one-way cable length (m), I current (A), A cross-section (mm²), ρ resistivity (Ω·mm²/m). Standard 20°C values: copper ρ = 0.0175, aluminium ρ = 0.0282. At higher temperature resistance rises (temperature coefficient α = 0.00393 for copper, 0.00403 for aluminium); at 70°C copper resistance is ~20% higher than at 20°C. The calculator uses room-temperature values as a conservative estimate for short loads. Power factor (cos φ) is assumed = 1 (purely resistive); for motors and transformers cos φ is typically 0.8–0.95, making real drop somewhat smaller. Reactance (X_L) of typical installation cables is 0.08–0.15 Ω/km, negligible for cross-sections below 50 mm² but noticeable for larger. All computations in IEEE 754 double, ample precision for field use. Do not use for design of HV installations or critical infrastructure without professional verification.