Calculators
Calculate cable resistance
Cable resistance sits behind almost every other calculation, behind voltage drop, power loss and loop impedance. The tabulated figure applies at 20 °C. In service the conductor is warmer, and the difference is not a rounding error.
Result
- Resistance at operating temperature
- —Ω
- Resistance at 20 °C
- —Ω
- Loop resistance (out and back)
- —Ω
- Increase due to heating
- —%
Please fill all fields with valid numbers.
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.
The grey values are an example. Simply enter your own.
Matching services
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.
Method and questions
Cable resistance
Resistance of a cable from length, cross-section and conductor temperature.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- One-way cable length
- 45 m
- Conductor cross-section
- 6 mm²
- Conductor material
- Copper (κ = 56 m/Ω·mm²)
- Conductor temperature
- 70 °C
Result
- Resistance at operating temperature
- 0.16 Ω
- Resistance at 20 °C
- 0.134 Ω
- Loop resistance (out and back)
- 0.32 Ω
- Increase due to heating
- 19.6 %
Formula and standards
Formula
- R₂₀ = L / (κ · A)
- R(ϑ) = R₂₀ · [1 + α · (ϑ − 20 °C)]
- α = 0.00393 /K for copper, 0.00403 /K for aluminium
- Loop resistance = 2 · R(ϑ)
Assumptions and standards
- κ = 56 m/(Ω·mm²) for copper, 35 m/(Ω·mm²) for aluminium at 20 °C.
- Pure resistance. The inductive component is not included and becomes noticeable from about 50 mm² upwards.
- The loop resistance assumes outgoing and return conductors of equal cross-section. With a smaller protective conductor it is larger.
Which conductor temperature should I use?
For PVC insulation the permissible operating temperature is 70 °C, for cross-linked polyethylene 90 °C. Calculating at full load means using those. For the actual state of a lightly loaded cable, 30 to 40 °C is closer.
Why is the difference between 20 °C and 70 °C so large?
Resistance rises by roughly 0.4 percent per kelvin. Over 50 kelvin that is about 20 percent. A voltage drop that just passes at 20 °C exceeds the limit at operating temperature.
What do I need the loop resistance for?
For the disconnection condition. The short-circuit current at the end of the cable follows from the voltage and the impedance of the loop. If it is too small, the protective device will not disconnect within the required time.
Contact
Planning a system right now?
We take on the electrical design, from sizing and EPLAN schematics to the finished documentation. A call or three lines are enough.
Direct
- Phone+49 (0) 5247 4070-66
- Emailinfo@fisekon.com
- AvailabilityMon to Fri, 8 am to 5 pm CET
Or write to us
We usually reply within one working day.