Cable Voltage Drop Calculator
Estimate the voltage drop along a copper or aluminium cable run — to check it stays within the IEC / BS 7671 limit and pick the right conductor size for your circuit.
A quick resistance-based voltage-drop estimate. For a full cable schedule with derating and reactance, send us your single-line diagram.
Keeping voltage drop within limits
The 4% rule of thumb
IEC / BS 7671 practice keeps total voltage drop to about 4% (roughly 3% for lighting, up to 5% for power). Too much drop dims lighting and overheats motors.
Bigger cable or shorter run
If the drop is too high, increase the conductor cross-section or shorten the route. Aluminium needs a larger size than copper for the same drop.
We wire it right
Our panels and sub-mains are built with correctly-sized, correctly-derated cabling — factoring length, current and installation method.
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Voltage drop
How is cable voltage drop calculated?
Approximately, drop (V) = k × ρ × length × current ÷ cross-section, where k is 2 for single-phase and √3 for three-phase and ρ is the conductor resistivity. This tool computes the volts and the percentage.
What is the maximum allowed voltage drop?
IEC / BS 7671 practice limits total drop to about 4% of nominal voltage — near 3% for lighting circuits and up to 5% for power. Local and DEWA regulations may differ.
Does this include cable reactance?
No — this quick estimate uses conductor resistance only, which is accurate for small and medium cables. For large cables or long runs we include reactance in the detailed design.
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