How to use it
- Choose the Supply: single-phase or three-phase, then type the Supply voltage (V) or the Line voltage (V) of that supply.
- Choose Enter: Power (kW) with the Power factor, or Current (A) if you know the current.
- Type the Cable length (m), the Allowed voltage drop (%) and the Conductor material.
- Read Cable cross-section (standard size) and the drop it gives. Ask a licensed electrician to confirm the size against the current rating and the protection.
The formulas
The current comes from the power. The voltage drop comes from the resistance of the cable, which is the resistivity times the length divided by the cross-section. Two conductors carry the current out and back in a single-phase circuit; a balanced three-phase circuit uses the √3 factor for the line-to-line drop. The resistivity of copper and aluminium is taken at the conductor temperature.
Single-phase: I = P ÷ (V × PF), V = supply voltage
Three-phase: I = P ÷ (√3 × V × PF), V = line voltage
Resistivity: ρ(T) = ρ20 × (1 + α × (T − 20))
copper: ρ20 = 1.72 × 10⁻⁸ Ω·m, α = 3.93 × 10⁻³ /K
aluminium: ρ20 = 2.82 × 10⁻⁸ Ω·m, α = 3.90 × 10⁻³ /K
Single-phase: A = 2 × I × ρ × L ÷ ΔV
Three-phase: A = √3 × I × ρ × L ÷ ΔV
ΔV = allowed drop: nominal voltage × allowed %- The cross-section is rounded up to the next IEC 60228 standard size (0.5 to 2500 mm²; 1400 and 1800 mm² are non-preferred and not used).
- The drop is calculated from the resistance only. The reactance of the cable, which matters for long runs and larger sizes, is not included.
- Resistivity values are for annealed copper and aluminium at 20 °C (Wikipedia, Electrical resistivity and conductivity), scaled to the temperature you enter.
Why the current rating is not checked here
A cable must also carry its current without overheating. That depends on the installation method, the number of cables together, the insulation and the ambient temperature, which are set out in the IEC 60364-5-52 tables. Those tables could not be used for this calculator, so it gives only the size for the voltage drop. A size that passes the voltage-drop check can still be too small for the current, so the size finally chosen must be checked against those tables and the protective device by a licensed electrician.
The Turkish rules
The Elektrik İç Tesisleri Proje Hazırlama Yönetmeliği (Resmî Gazete 3 December 2003, no. 25305) requires the final project to include voltage-drop calculations (article 10, paragraph (c)). The 3 % and 5 % defaults in this calculator are design values. They are not quoted from the regulation, which should be checked for the limits that apply to your installation.
Worked example: 3.5 kW on 230 V, 25 m, 3 % drop
The current is 3500 W ÷ (230 V × 0.9) = 16.91 A at a power factor of 0.9. Copper at 70 °C has a resistivity of 0.0206 Ω·mm²/m.
The allowed drop is 230 V × 3 % = 6.9 V, so the minimum cross-section is 2 × 16.91 A × 0.0206 Ω·mm²/m × 25 m ÷ 6.9 V = 2.52 mm².
The next standard size is 4 mm². It gives a drop of 4.35 V, or 1.89 %. The current rating of 4 mm² still has to be checked by an electrician.
Frequently asked questions
Who decides the cable size?
A licensed electrician does, using the installation method, the current rating and the protection of the circuit. This calculator gives the voltage-drop minimum only, which is one of the checks.
Why does a longer cable need a bigger size?
Its resistance grows with length, so the same current causes a larger voltage drop. A bigger cross-section lowers the resistance and brings the drop back within the limit.
Is aluminium cable as good as copper?
It has a higher resistivity, so the cross-section must be larger for the same drop. Its use in building installations depends on the rules and the connections, so ask an electrician.
What power factor should I use?
Use 1 for resistive loads such as heaters and most lighting. Motors and many appliances have a lower power factor, often between 0.8 and 0.9; the nameplate or the datasheet gives the figure.
Why does the temperature matter?
A conductor's resistance rises by about 0.39 % per degree Celsius for copper, so a cable at 70 °C has about 20 % more resistance than at 20 °C. The calculator includes this rise.
Sources
- Wikipedia: Electrical resistivity and conductivity (copper and aluminium at 20 °C)
- Wikipedia: IEC 60228 (standard conductor cross-sections)
- mevzuat.gov.tr: Elektrik İç Tesisleri Proje Hazırlama Yönetmeliği (voltage-drop calculations in the project)
Figures and text last checked on Friday, November 13, 2026.