Cable cross-section calculator

Input

System

Operating currentA

One-way cable lengthm

Nominal voltageV

Permissible voltage drop%

Typically 3 % for lighting and 5 % for other loads.

Power factor cos φ

Conductor material

Installation method

For PVC-insulated copper cables, 70 °C. With XLPE insulation the result is on the safe side. Rubber cables such as H07RN-F are not covered.

Ambient temperature

The air at the cable, not in the room. Inside an enclosure or at the machine it is often well above 30 °C.

Circuits installed together

Loaded circuits laid together in trunking, in a bundle or on a wall. This cable counts.

Result

Required cross-sectionLarger of voltage drop and current-carrying capacity: —mm²
Governing condition: —
Cross-section by current-carrying capacityIEC 60364-5-52, Annex B: —mm²
Cross-section by voltage dropSeries to IEC 60228: —mm²
Current-carrying capacity I_zAt least the protective device rating: —A
Calculated by voltage drop: —mm²
Permissible voltage drop: —V
Actual voltage drop at the selected cross-section: —%

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
Installation method
Ambient temperature
Circuits installed together

Result

Required cross-section: mm²
Governing condition
Cross-section by current-carrying capacity: mm²
Cross-section by voltage drop: mm²
Current-carrying capacity I_z: A
Calculated by voltage drop: mm²
Permissible voltage drop: V
Actual voltage drop at the selected 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
A_req = max(A_voltage drop, A_current capacity)

Assumptions and standards

κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
Calculated with the 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.
Current-carrying capacity to IEC 60364-5-52, Annex B, for PVC-insulated copper cables, 70 °C conductor temperature, reference 30 °C air. Corrected with the ambient temperature factor and the grouping factor for bunched circuits.
Three conductors are loaded in three-phase systems, two in single-phase and DC. Harmonic currents in the neutral are not considered.
The capacity of the result must at least cover the rated current of the protective device (I_B ≤ I_n ≤ I_z). No capacity values are stored for aluminium.
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

The grey values are an example. Simply enter your own.

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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.

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Method and questions

Cable cross-section

Required cross-section from voltage drop and current-carrying capacity.

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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)
Installation method: C · Multi-core cable directly on a wall
Ambient temperature: 30 °C
Circuits installed together: this cable only

Result

Required cross-section: 4 mm²
Governing condition: Both conditions lead to the same cross-section.
Cross-section by current-carrying capacity: 4 mm²
Cross-section by voltage drop: 4 mm²
Current-carrying capacity I_z: 32 A
Calculated by voltage drop: 3.34 mm²
Permissible voltage drop: 12 V
Actual voltage drop at the selected cross-section: 2.51 %

Formula and standards

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
A_req = max(A_voltage drop, A_current capacity)

Assumptions and standards

κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
Calculated with the 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.
Current-carrying capacity to IEC 60364-5-52, Annex B, for PVC-insulated copper cables, 70 °C conductor temperature, reference 30 °C air. Corrected with the ambient temperature factor and the grouping factor for bunched circuits.
Three conductors are loaded in three-phase systems, two in single-phase and DC. Harmonic currents in the neutral are not considered.
The capacity of the result must at least cover the rated current of the protective device (I_B ≤ I_n ≤ I_z). No capacity values are stored for aluminium.
Standard cross-sections to IEC 60228.

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, which is 2 for single-phase and DC and √3 for three-phase. So enter the one-way distance from supply to load.

Does this replace design to standard?

No. The calculator combines voltage drop and current-carrying capacity. A complete design also covers coordination with the protective device, short-circuit withstand and the disconnection condition.