01 — Calculators

Calculate cable cross-section

The cross-section of a cable is determined by two conditions: it has to carry the current thermally, and the voltage drop over the length has to stay within the permitted range. On longer runs the voltage drop is almost always the governing factor and that is what this tool calculates.

Result

Next standard cross-sectionSeries to IEC 60228: —mm²
Calculated requirement: —mm²
Permissible voltage drop: —V
Actual voltage drop at standard cross-section: —%

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A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Calculated on

Calculate cable cross-section

Inputs

System
Operating current: A
One-way cable length: m
Nominal voltage: V
Permissible voltage drop: %
Power factor cos φ
Conductor material

Result

Next standard cross-section: mm²
Calculated requirement: mm²
Permissible voltage drop: V
Actual voltage drop at standard cross-section: %

Formula

Three-phase: A = (√3 · L · I · cos φ) / (κ · Δu)
Single-phase: A = (2 · L · I · cos φ) / (κ · Δu)
DC: A = (2 · L · I) / (κ · Δu)
Δu = U · Δu% / 100

Assumptions and standards

κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
Calculated with resistive component only; inductive reactance is not included. For large cross-sections from about 95 mm² and long runs the result is therefore slightly optimistic.
Thermal current-carrying capacity by installation method (IEC 60364-5-52) has to be checked separately. This calculator does not check it.
Standard cross-sections to IEC 60228.

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

FISEKON GmbH – Fischer Elektrokonstruktion · https://fisekon.com

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

System: Three-phase (3~)
Operating current: 32 A
One-way cable length: 45 m
Nominal voltage: 400 V
Permissible voltage drop: 3 %
Power factor cos φ: 0.9
Conductor material: Copper (κ = 56 m/Ω·mm²)

Result

Next standard cross-section: 4 mm²
Calculated requirement: 3.34 mm²
Permissible voltage drop: 12 V
Actual voltage drop at standard cross-section: 2.51 %

Formula

Three-phase: A = (√3 · L · I · cos φ) / (κ · Δu)
Single-phase: A = (2 · L · I · cos φ) / (κ · Δu)
DC: A = (2 · L · I) / (κ · Δu)
Δu = U · Δu% / 100

Assumptions and standards

κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
Calculated with resistive component only; inductive reactance is not included. For large cross-sections from about 95 mm² and long runs the result is therefore slightly optimistic.
Thermal current-carrying capacity by installation method (IEC 60364-5-52) has to be checked separately. This calculator does not check it.
Standard cross-sections to IEC 60228.

Frequently asked

What voltage drop is permissible?

In Germany, 3 % for lighting and 5 % for other loads is customary for consumer installations, measured from the point of supply. Industrial plants often apply stricter internal limits, the operator’s specification always governs.

Why is the one-way length used?

The return path is already in the factor: 2 for single-phase and DC, √3 for three-phase. So enter the one-way distance from supply to load.

Does this replace design to standard?

No. The calculator gives the cross-section from voltage drop. A complete design also covers current-carrying capacity by installation method, grouping, ambient temperature, short-circuit withstand and protection against indirect contact.

Articles on this calculator

31 May 2026Why cable sizing rarely fails on currentCurrent-carrying capacity is usually satisfied quickly. What determines the cross-section in practice is the voltage drop over the length and that is regularly underestimated.

All articles

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

Electrical design

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