Calculators
Size a motor feeder
Motor cables are usually sized for the operating current. Then people wonder why the motor starts sluggishly on a long run. The starting current drops across the same cable, and a multiple of the current means a multiple of the voltage drop.
Result
- Required cross-section, standard sizeLarger of voltage drop and current-carrying capacity
- —mm²
- Governing condition
- —
- Cross-section by voltage drop
- —mm²
- Rated motor current
- —A
- Calculated by voltage drop
- —mm²
- Voltage drop in operation at this size
- —%
- Voltage drop on the cable at startAdds to the dip in the network.
- —%
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
Motor feeder
Current and cross-section of the motor cable, including the voltage drop at start.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Rated motor power
- 30 kW
- System voltage
- 400 V
- Efficiency
- 92 %
- Power factor cos φ
- 0.86
- Length of the feeder
- 80 m
- Permissible voltage drop in operation
- 3 %
- Starting current ratio I_A / I_N
- 7
- Conductor material
- Copper (κ = 56 m/Ω·mm²)
- Installation method
- C · Multi-core cable directly on a wall
- Ambient temperature
- 30 °C
- Circuits installed together
- this cable only
Result
- Required cross-section, standard size
- 10 mm²
- Governing condition
- Both conditions lead to the same cross-section.
- Cross-section by voltage drop
- 10 mm²
- Rated motor current
- 54.7 A
- Calculated by voltage drop
- 9.7 mm²
- Voltage drop in operation at this size
- 2.91 %
- Voltage drop on the cable at start
- 7.11 %
Formula and standards
Formula
- P_in = P_out / η
- I_N = P_in / (√3 · U · cos φ)
- A = (√3 · L · I_N · cos φ) / (κ · Δu)
- At start Δu_A ≈ (√3 · L · I_A · cos φ_A) / (κ · A)
Assumptions and standards
- The cross-section is the larger of operating voltage drop and current-carrying capacity to IEC 60364-5-52, Annex B (PVC-insulated copper cable, three loaded conductors). Short-circuit protection and the disconnection condition are separate proofs.
- For starting, a power factor of 0.3 is used, because at start the motor is strongly inductive.
- For inverter-fed motors, additional requirements on screening and length apply that are not covered here.
What voltage drop is still acceptable at start?
It adds to the dip in the supplying network. Together they determine what reaches the motor, and starting torque falls with the square of that. If less than eighty percent of nominal voltage remains, starting heavy loads becomes unreliable.
Why is the operating current not enough for sizing?
It is enough for heating, not for starting behaviour. A cable with three percent drop in operation reaches a multiple of that at seven times the current. And that is exactly the moment the motor needs the voltage most.
Does this apply with a variable frequency drive?
The starting part does not. A drive limits the current and ramps up the frequency, so there is no inrush. Other topics arrive instead, such as screening, leakage currents and the permissible cable length per the drive manufacturer.
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