Cable size by voltage drop

What “smallest size that fits” means here

The calculator walks the chosen catalogue from the smallest size upward, computes the voltage drop for each one under the conditions you gave, and stops at the first size whose drop is inside your percentage limit. That is a search over a discrete list, not a formula rearranged, which is why the answer is always a real size you can order rather than a theoretical area you then have to round.

Two catalogues are available. The metric series runs from 0.5 mm² to 1000 mm². The North American series runs from 36 AWG up through 4/0 and on into kcmil sizes to 2000 kcmil. They are kept separate on purpose: a value from one series is never silently converted into the other, because 4 AWG is not 25 mm² and treating them as interchangeable is how errors get in.

Why voltage drop alone is not a cable size

This is the important part, and most free calculators leave it out. The size this page returns satisfies exactly one constraint. A conductor you can actually install has to satisfy several, and any one of them can demand something larger:

• Current carrying capacity, corrected for ambient temperature, for grouping with other circuits, for the installation method, and for thermal insulation. These values are in your national wiring rules and are not computed here.
• Coordination with the protective device, so that the device protects the conductor it is feeding.
• Fault loop impedance or short circuit withstand, so the protection operates within the required time and the conductor survives until it does.
• Mechanical and practical constraints: minimum sizes for the type of circuit, terminal capacity, conduit or tray fill, bending radius.

So treat the result as a lower bound from one direction. If the voltage drop calculation says 10 mm² and your ampacity check says 16 mm², the answer is 16 mm². The reverse case, where voltage drop is the binding constraint, is common on long runs, and that is exactly where this page earns its keep.

A worked example

40 A, three phase 400 V, 60 m of copper at 70 °C, power factor 0.9, limit 3 %. Walking the metric series: 6 mm² gives 3.28 % and fails, 10 mm² gives 1.97 % and passes. The answer is 10 mm² with 1.97 % achieved, and the sheet records both the candidate that failed and the one that was selected.

Reactance is left out of this one

The size search runs with reactance at zero, because the reactance of a cable depends on its construction and spacing, and there is no single defensible value to assume across a whole catalogue. On conductors above roughly 35 mm² that omission is not conservative, so once the search has narrowed the size, put it into the voltage drop calculator with the reactance figure for the cable you are actually using and check it again.

The sheet, and sharing a calculation

Each result comes with a sheet listing the formula and every input at each step, including the selection step itself. Copy it, print it, or attach it to your own documentation. The link button copies a URL that carries all your inputs, so the person you send it to opens the identical calculation rather than a screenshot of it.

The constraints this page cannot check are the reason the app exists: it resolves current capacity with derating alongside the voltage drop, returns one size that passes everything, and tells you which check was binding. See what it does.