Motor current and cable size calculator
Enter the supply, motor power, power factor, efficiency, cable length and installation method. The calculator gives the rated and starting current of the AC motor and the smallest copper cable that meets both the voltage-drop limit and the current-carrying capacity of IEC 60364-5-52.
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What it calculates
- Rated current for 3-phase and single-phase motors
- Starting current for direct-on-line, star-delta and soft starter / VFD
- Voltage drop in volts and percent for each cable size
- Minimum cable cross-section from IEC 60364-5-52 installation methods (PVC, copper)
Formulas
3-phase: I = P / (√3·U·cos φ·η) 1-phase: I = P / (U·cos φ·η) ΔU (3-ph) = √3·I·L·cos φ / (κ·S) ΔU (1-ph) = 2·I·L·cos φ / (κ·S) κ (Cu) = 56 m/(Ω·mm²) Ib ≤ In ≤ Iz
Worked example
A 7.5 kW motor on 400 V three-phase with cos φ 0.85 and efficiency 0.9 draws 14.2 A. Over 50 m of 2.5 mm² copper the voltage drop is 7.4 V (1.9 %).
How to use it
- Create a free account with Google or e-mail.
- Open the calculator and enter your values; units can be metric or imperial.
- Read the result with its formula and source, and export to PDF, DXF or CSV where available.
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Frequently asked questions
How do I calculate motor current?
For three-phase: I = P / (√3 × U × cos φ × η). A 7.5 kW, 400 V motor with cos φ 0.85 and η 0.9 draws about 14 A.
What voltage drop is allowed?
IEC 60364-5-52 recommends about 4 % for general circuits; many installations use 3 % for motors and 5 % in total from the origin.
Why is the cable sometimes larger than the current suggests?
On long runs the voltage-drop limit, not the current-carrying capacity, decides the cross-section.
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Results are engineering estimates for planning. They do not replace the applicable standard, a qualified procedure or a check by a qualified engineer.