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About this tool
Estimate voltage loss along a wire run from its length, current, and conductor type or gauge.
The Voltage Drop & Wire Gauge Calculator works out how many volts a cable run loses under load, using R = ρ × k × L / A with a resistivity of 0.0175 Ω·mm²/m for copper and 0.0282 for aluminium, a path factor k of 2 for DC and single-phase two-wire circuits and √3 for balanced three-phase, then drop = I × R. It is for electricians, solar and battery installers, and anyone running a long cable who needs to know whether the far end still sees usable voltage. Alongside the drop in volts and percent, it back-solves the minimum conductor area that would meet your target percentage.
Open Voltage Drop & Wire Gauge Calculator on AltFTool — it loads instantly in your browser.
Choose Circuit — "DC / single-phase 2-wire" or "Balanced three-phase" — and Conductor: Copper or Aluminium.
Enter Nominal voltage (V), Current (A), One-way length (m), Conductor area (mm²) and Target maximum drop (%), or load the "230 V copper" example.
Read the volt drop with its percentage and the row "Minimum area for target (resistance only)" in mm², then use Copy or Download.
It reports the minimum conductor area in mm² that would hit your target percentage, so you get the answer you actually need when specifying cable.
Two-wire runs are multiplied by 2 for the return path and balanced three-phase by √3, rather than applying one blanket factor to both.
Switching conductor material swaps in the correct resistivity, so the roughly 60% higher resistance of aluminium shows up in the result immediately.
3% for a branch circuit is the usual working target and is the default here, with around 5% commonly treated as the ceiling for the whole run from supply to load. These are the values used in common practice; check the specific figure your local wiring code or client specification requires, because it is not identical everywhere.
0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminium, which are room-temperature values. Conductor resistance rises with temperature, so a cable running hot in an insulated wall will drop slightly more than the calculated figure.
One-way length — the distance from source to load. The calculator applies the return path itself, multiplying by 2 for DC and single-phase two-wire circuits and by √3 for balanced three-phase.
No. This is a resistance-only estimate and does not consider ampacity, insulation temperature rating, grouping and derating, fault current, motor starting current, reactance or terminal ratings. Voltage drop is one of several checks — have the final design confirmed by a qualified electrician against your local code.
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