Calculators
Greatest cable length for the disconnection condition
Voltage drop limits cable length, but often it is not the first limit. As length grows the fault current at the far end falls, and at some point it is no longer enough to operate the device. This calculation shows where that limit lies.
Result
- Greatest permissible cable length
- —m
- Greatest permissible loop impedance
- —Ω
- Available for the cableWhat remains after deducting the upstream impedance.
- —Ω
- Length at the next size upWith the protective conductor growing along.
- —m
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.
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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.
Method and questions
Length and disconnection
How long the cable may become for the protective device to still operate.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- System voltage (line to line)
- 400 V
- Voltage factor c
- 0.95
- Operating current of the protective device I_a
- 160 A
- Upstream impedance
- 0.05 Ω
- Line conductor cross-section
- 16 mm²
- Protective conductor cross-section
- 16 mm²
- Conductor material
- Copper (κ = 56 m/Ω·mm²)
- Conductor temperature under fault
- 80 °C
Result
- Greatest permissible cable length
- 479 m
- Greatest permissible loop impedance
- 1.37 Ω
- Available for the cable
- 1.32 Ω
- Length at the next size up
- 748 m
Formula and standards
Formula
- Z_max = c · U₀ / I_a with U₀ = U / √3
- available for the cable is Z_max − Z_upstream
- Loop resistance per metre = (1/(κ·A_L) + 1/(κ·A_PE)) · [1 + α · (ϑ − 20 °C)]
- L_max = available impedance / resistance per metre
Assumptions and standards
- The calculation uses a heated conductor, because the loop impedance is then highest.
- The inductive component of the cable is not included. At small cross-sections that is acceptable. From about 50 mm² it shortens the permissible length further.
- The upstream impedance is subtracted arithmetically. That is on the safe side, because the actual loop is geometrically somewhat smaller and the permissible length therefore slightly greater than shown.
- Voltage drop is a separate criterion and limits the length independently. The smaller of the two always governs.
Which limit bites first, voltage drop or the disconnection condition?
It depends on the protective device. At small rated currents with a slow characteristic, voltage drop is usually the tighter limit. At large rated currents or a fast characteristic, meaning a high operating current, it is the disconnection condition. Calculate both and take the smaller value.
Why is the protective conductor included?
Because the fault current returns through it. A protective conductor half the size shortens the permissible length by a third. That is why it is a separate field here rather than an assumption.
What if no value is produced?
Then the upstream impedance alone already exceeds the permissible loop impedance. The device would not operate even with zero metres of cable. In that case the device is wrongly chosen, not the cable too long.
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