02 — Calculators
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.
- Opens the print dialog. Choose “Save as PDF” as the destination.
- Opens your email program with the values from this calculation. Nothing is sent to us until you send it yourself.
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
- 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.
- The formula is a linear approximation for voltage drops well below the nominal voltage. Once the calculated drop reaches the nominal voltage the design is unusable anyway, the voltage at the load is then left blank.
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.
More tools
- 01Cable cross-section
- 03Cable resistance
- 04Cable losses
- 05Maximum cable length
- 06Current-carrying capacity
- 07Parallel cables
- 08Conductor temperature
- 09Cable capacitance
- 10Short-circuit current
- 11Cable impedance
- 12Network impedance
- 13Short-circuit current at the far end
- 14Thermal short-circuit withstand
- 15Length and disconnection
- 16Motor current
- 17Torque
- 18Starting current
- 19Star-delta starting
- 20Soft starting
- 21Motor efficiency
- 22Speed control instead of throttling
- 23Speed and slip
- 24Setting the motor protection
- 25Single-phase motor
- 26Motor feeder
- 27Required motor rating
- 28PFD and SIL
- 29PFH and SIL
- 30Protective conductor size
- 31Earth rod
- 32Touch voltage
- 33Residual current protection
- 34Check discrimination
- 35Connecting a surge arrester
- 36Enclosure cooling
- 37Reference designation
- 38Reactive power compensation
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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