Cable size and voltage drop
From watts or amps, the length of the run and the installation method to the smallest cable that carries the current and stays under 4%, plus the maximum length.
The power factor (cos φ) is 1 for heaters, ovens, water heaters and filament lamps, around 0.9 for a mixed household load and around 0.8 for a motor. On direct current it does not exist and drops out of the maths.
The length is the distance from the board to the appliance, measured along the real route of the cable (the drops and the corners count). The return path is added by the calculation, so do not double it yourself.
The K factor derates the ampacity when conditions are worse than the table assumes (several circuits in one conduit, hot room, dry soil). Leave it at 1 if the cable runs on its own at normal temperature. The 4% limit is the one recommended by IEC 60364-5-52 and by the Italian CEI 64-8/5 art. 525.
The calculation runs in your browser and no data is sent anywhere. This is a first sizing to get the order of magnitude right, not a design: fixed wiring has to be installed and certified by a qualified electrician.
How to use it
Enter what the load draws (in watts or in amps, whichever you know), the voltage, how many metres of cable it really takes to get there and how the cable will be installed. The tool works out the voltage drop and the ampacity for every commercial size from 1.5 to 95 mm² and points at the smallest one that passes both checks. The table under the result shows the margin, that is how close to the limit you are and how far you could still go.
The formula, and why the return path counts
On single phase and on direct current the drop is 2 · ρ · L · I · cos φ ÷ S, and on three phase the 2 becomes the square root of 3 because the drop is measured between two phases. That 2 is why a 30 metre run is 60 metres of copper: the current goes and comes back. The resistivity ρ of copper is 0.018 ohm·mm²/m at 20 °C and rises with temperature as ρ(T) = ρ₂₀ · (1 + 0.00393 · (T − 20)), so at 70 °C it becomes 0.0215, a fifth more. Aluminium starts at 0.028, which is why it needs a bigger size for the same current.
Where the 4% comes from
IEC 60364-5-52 (and the Italian CEI 64-8/5 art. 525) recommends that the drop between the origin of the installation and any point of use stays under 4% of the nominal voltage, that is 9.2 V out of 230 V. It is a recommendation with exceptions, not a ban, but under that figure motors start properly, leds do not flicker and a heating element delivers the power written on its label. On a 12 V system a tighter 3% is common, because 4% of 12 V is less than half a volt: the limit is a field you can change.
Ampacity, the K factor and the breaker
The ampacity Iz is how much current the cable can carry without pushing its insulation past its rated temperature. It depends almost entirely on how the cable sheds heat, not on the copper itself: the same 2.5 mm² carries 24 A in a conduit on a wall and 30 A in free air. The figures used here come from the CEI-UNEL 35024/1 tables for a single circuit, with air at 30 °C and soil at 20 °C; several circuits in one conduit or a warmer room cut it down, and that is what the K factor is for (say 0.7 with three circuits in the same conduit). The breaker suggested follows the Ib ≤ In ≤ Iz rule, which protects the cable rather than the appliance.
What this tool does not do
It is a first sizing, meant to get the order of magnitude right and stop you buying the wrong reel, not a design. It does not check short circuit withstand, protection against indirect contact, the size of the protective or neutral conductor or motor inrush, and it ignores cable reactance, which above 25 mm² makes the real drop slightly higher than the figure shown. For aluminium the ampacity is estimated at 78% of copper. Sizes are metric mm², the ones used outside North America (2.5 mm² sits between AWG 14 and 13, 6 mm² is close to AWG 10, 16 mm² close to AWG 6), and the tables are the IEC ones, not the NEC. Fixed wiring has to be installed and certified by a qualified electrician.