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05 — Calculators

Calculate maximum cable length

This question usually comes from site, not from the desk: the cross-section is fixed, the route turns out longer than planned — will it still do? This calculation inverts the voltage-drop formula and answers in metres.

Input

Typically 3 % for lighting and 5 % for other loads, measured from the point of supply.

Result

Maximum cable length
—m
Voltage drop at that length
—V
Length at the next size upWhat one size more copper buys in distance.
—m

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.

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
Conductor cross-section
6 mm²
Nominal voltage
400 V
Permissible voltage drop
3 %
Power factor cos φ
0.9
Conductor material
Copper (κ = 56 m/Ω·mm²)

Result

Maximum cable length
80.8 m
Voltage drop at that length
12 V
Length at the next size up
135 m

Formula

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

Assumptions and standards

  • κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
  • Voltage drop is the only criterion here. Current-carrying capacity, short-circuit protection and the disconnection condition limit the length independently.
  • At operating temperature the resistance rises; the length actually permissible is then roughly one sixth below the calculated value.

Frequently asked

Does this settle the length?

No. As length increases, the short-circuit current at the far end falls. At some point it is no longer enough to trip the protective device within the required time — then the disconnection condition, not the voltage drop, is the tighter limit.

Which permissible voltage drop should I use?

The common values are 3 percent for lighting and 5 percent for other loads. What matters is the point the calculation starts from and how much has already been used up. Downstream of a sub-distribution board, less is left for the final circuits.

Why show the next size up?

Because in practice that is the real question: not how far you get, but what it costs to get further. Length grows in proportion to cross-section.

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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