Voltage drop calculator
This is arithmetic on the current, length and resistance you provide, with copper and aluminium resistivity taken as physical constants. It does not decide the drop limit your job must meet — that is set by your code and your authority having jurisdiction. Confirm the conductor and the acceptable drop before you rely on it.
Enter the load current, the one-way run length and the supply voltage, then either type the resistance in ohms per kilometre or let the tool derive it from the conductor's cross-section and whether it is copper or aluminium. The result is the volts lost, the percentage, and the voltage that reaches the far end.
Worked example
A 20 A single-phase load on a 50 m run of conductor at 8.6 Ω/km loses 2 × 20 × (8.6 × 50 ÷ 1000) = 17.2 V. On a 240 V supply that is a 7.2% drop, leaving about 222.8 V at the load — enough to send you looking at a larger conductor.
The formula
drop = k × I × (R/km × length ÷ 1000) · k = 2 (1φ) or √3 (3φ)
Resistance over the run is the ohms-per-km times the length in kilometres. Multiply by the current and the phase factor for the volts lost. When derived from size, the resistance is R = ρ × length ÷ area, with ρ the resistivity of the metal — a measured property, not a tabulated code value.
Questions
- How do I calculate voltage drop on a wire run?
- Multiply the current by the conductor’s resistance over the run, and by two for a single-phase circuit (the current travels out and back) or by √3 for three phase. The result is the volts lost along the way. Divide that by the supply voltage for the percentage drop, and subtract it from the source to see the voltage that actually arrives.
- Where does the resistance value come from?
- You either type the ohms-per-kilometre yourself, from a source you trust, or let the tool derive it from physics: R = ρ × length ÷ area, using the measured resistivity of copper or aluminium and the conductor cross-section you enter. Resistivity is a property of the metal, not a code table, so this stays clean and never goes stale.
- What is an acceptable voltage drop?
- As general guidance, many installers aim to keep the drop on a branch circuit within a few percent and the total from the source modest, so equipment sees close to its rated voltage. The exact limit that applies to your work is set by your code and authority — this tool reports the drop and leaves the limit to you.
- Why multiply the length by two for single phase?
- Current has to return, so a single-phase run is really twice the one-way distance of conductor carrying current. The tool takes the one-way run length you enter and applies the ×2 itself. Three-phase circuits use the √3 factor instead, from the geometry of the three line voltages.
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