Wire Gauge Calculator — Size for Voltage Drop
Finds the conductor size that keeps a cable run inside a voltage-drop limit, and gives the nearest metric and AWG sizes. This is the voltage-drop half of cable sizing — the ampacity half is a code question, and it is not answered here.
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The formula
area = k × ρ × L × I ÷ V_drop k = 2 for DC and single-phase, √3 for balanced three-phase ρ = 0.0172 Ω·mm²/m copper, 0.0282 aluminium
This is Ohm’s law rearranged. A conductor’s resistance is its resistivity times length over area, so fixing the drop you will tolerate fixes the minimum area. The factor of 2 is the return conductor; three-phase uses √3 instead because the return currents cancel.
What to watch out for
Cable is sized against two separate limits, and the larger one wins:
- Voltage drop — calculable from physics, which is what this page does. It dominates on long runs and low voltages.
- Current-carrying capacity — how much current the cable can pass without its insulation overheating. That depends on insulation type, whether the cable is bunched with others, ambient temperature, whether it is buried or in conduit, and it comes from a regulatory table. It dominates on short runs.
A 12 V circuit needs far more copper than a 230 V one for the same power, because the same percentage drop is a much smaller number of volts. This is why low-voltage DC runs get expensive quickly.
Resistivity here is quoted at 20 °C. A hot conductor has roughly 20% more resistance at 70 °C, so a run that is marginal when cold will be worse in service.
Frequently Asked Questions
What size cable do I need for 20 A over 30 m?
For a 230 V single-phase circuit at a 3% drop limit, about 2.99 mm² — so 4 mm² as the next standard size, or 12 AWG. Check that size against your local ampacity tables before using it.
What is an acceptable voltage drop?
3% to a final circuit and 5% overall are the figures most design guidance uses. Beyond that, motors run hot, lamps dim and electronics may misbehave.
Does this tell me if the cable is safe?
No. It only tells you the cable is thick enough to keep the voltage up. Whether it can carry the current safely depends on insulation, grouping, ambient temperature and installation method, all of which come from your wiring regulations — and electrical work should be inspected.
Why does aluminium need to be thicker than copper?
Its resistivity is about 64% higher, so an aluminium conductor needs roughly 1.6 times the cross-sectional area of copper for the same drop. It is used anyway on large feeders because it is far lighter and cheaper.