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Cable • Wire • Voltage Drop

Calculate Voltage Drop

Calculate the voltage drop of a cable in volts and percent for single-phase or three-phase systems.

Calculation Values

single (one-way) cable length

use 1 for purely resistive loads

copper 58, aluminum 37

enter 1 or 3

Note: This calculator provides a technical estimate for guidance only. For electrical installations, standards, protective measures, installation method, ambient temperature, grouping, protective devices and manufacturer specifications must also be checked.

Formula Used

Single-phase: ΔU = (2 × L × I × cos φ) / (κ × A)

Three-phase: ΔU = (√3 × L × I × cos φ) / (κ × A)

ΔU% = ΔU / U × 100

ΔU is the voltage drop, L the one-way cable length, I the current, κ the conductivity and A the conductor cross-section.

Frequently Asked Questions

What is a permissible voltage drop?

The permissible voltage drop is the maximum accepted voltage loss between the feed point and the load.

Why do single-phase and three-phase differ?

Single-phase circuits account for the outgoing and return conductor with a factor of 2, while balanced three-phase circuits use the line factor √3.

Which conductivity should I use?

Copper commonly uses κ = 58 and aluminum κ = 37. Depending on temperature and source, different values may apply.

Practical guidance

Assess voltage loss in an existing cable

Current, one-way length, conductor size and material determine the calculated voltage loss.

Input values explained

Current
Enter the expected operating current, not a fuse rating. Motors and electronic power supplies may draw much higher starting or peak currents.
Cable length
Enter the one-way distance from supply point to load. The required return path is already represented by the selected formula.
Conductor cross-section
Enter the cross-section of one loaded conductor in mm², not the sum of all cores in a cable.
Voltage
Match the voltage to the selected electrical system. Single-phase calculations use line-to-neutral voltage, while three-phase calculations use voltage between two line conductors.
Power factor cos φ
cos φ is the ratio of active to apparent power. It is close to 1 for resistive heating loads; use the data-sheet value for motors and transformers.
Conductivity κ
κ is the calculation value for the conductor material. Common approximations are 58 for copper and 37 for aluminium; heating increases actual resistance.
Electrical system
Choose single-phase AC for a 230 V circuit and three-phase for a balanced three-phase system. This selection changes the formula factor.

Example: 16 A over 25 m copper

A 230 V single-phase circuit uses 2.5 mm² copper over 25 m at 16 A and cos φ 0.90.

ΔU = (2 × 25 × 16 × 0.90) ÷ (58 × 2.5) ≈ 4.97 V = 2.16%

About 225 V remains at the load. Whether 2.16% is acceptable depends on the complete installation and load.

Consider the full distribution path

Voltage drops from successive cable sections add together, so one section should not automatically consume the full design allowance.

Calculation limits

  • Warm-conductor resistance is represented only approximately.
  • Terminal resistance and upstream voltage drop are excluded.