02 — Tools
Calculate voltage drop
The reverse of the cross-section calculation: the cross-section is fixed and the question is what still arrives at the end of the cable. This is the calculation you need for an existing installation or a later extension.
Result
- Voltage drop
- —%
- Voltage drop, absolute
- —V
- Voltage at the load
- —V
Please fill all fields with valid numbers.
A guide, not a design to standard. 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
- One-way cable length
- 45 m
- Conductor cross-section
- 6 mm²
- Nominal voltage
- 400 V
- Power factor cos φ
- 0.9
- Conductor material
- Copper (κ = 56 m/Ω·mm²)
Result
- Voltage drop
- 1.67 %
- Voltage drop, absolute
- 6.68 V
- Voltage at the load
- 393 V
Formula
- Three-phase: Δu = (√3 · L · I · cos φ) / (κ · A)
- Single-phase: Δu = (2 · L · I · cos φ) / (κ · A)
- DC: Δu = (2 · L · I) / (κ · A)
- Δu% = Δu / U · 100
Assumptions and standards
- κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
- Resistive component only, without inductive reactance.
- With heated conductors the resistance rises: at 70 °C the actual voltage drop is around 20 % above the value calculated here.
Frequently asked
Why does the measured value differ from the calculated one?
The formula assumes a conductor temperature of 20 °C. In operation the conductor is warmer and its resistance rises – by about a fifth at 70 °C. On top of that comes inductive reactance, which becomes noticeable at large cross-sections.
Does this apply to long motor cables on a drive?
Only to a limited extent. Inverter-fed cables add switching edges, leakage currents and screening effects. The calculation is usable for a rough sizing but not sufficient for the final decision.
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.
Contact
Tell us what it is about.
A phone call or three lines is enough. From the very start you talk to the people who will later work on your project – not to a distribution list.
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