Wire size by voltage drop

What “smallest size that fits” means

The calculator goes up through the sizes, starting from the smallest, works out the drop for each one with your inputs, and stops at the first one inside your limit. So the answer is always a size you can buy, not a theoretical area you’d have to round up.

There are two size ranges. Metric runs from 0.5 mm² to 1000 mm². North American runs from 36 AWG up to 4/0, then in kcmil up to 2000 kcmil. They’re kept separate on purpose: 4 AWG isn’t 25 mm², and mixing the two up is how mistakes get in.

Voltage drop alone doesn’t give you a cable size

Most free calculators leave this out. The size you get here passes one check. A cable you can install has to pass several, and any of them can need a bigger size:

• Current-carrying capacity, corrected for ambient temperature, grouping with other circuits, installation method and thermal insulation. Those values are in your national wiring rules and aren’t on this page.
• The breaker or fuse, which has to protect the cable it feeds.
• Fault loop impedance or short-circuit withstand, so the protection trips in time and the cable survives until it does.
• Practical limits: minimum sizes for the type of circuit, what the terminals take, conduit or tray fill, bending radius.

So treat the result as a minimum. If voltage drop says 10 mm² and your ampacity check says 16 mm², you need 16 mm². On long runs it’s often the other way round and voltage drop decides the size, which is where this page is most useful.

A worked example

40 A, three phase 400 V, 60 m of copper at 70 °C, power factor 0.9, limit 3 %. Going up the metric sizes: 6 mm² gives 3.28 % and fails, 10 mm² gives 1.97 % and passes. The answer is 10 mm² at 1.97 %, and the sheet shows both the size that failed and the one that was picked.

This one ignores reactance

The search sets reactance to zero, because reactance depends on how a cable is built and laid, and no single value fits every size. Above about 35 mm² that makes the result a little optimistic. Once you have a size, put it into the voltage drop calculator with the reactance of the cable you’re using and check it again.

The sheet, and sharing a calculation

Each result comes with a sheet listing the formula and the inputs at every step, including how the size was picked. Copy it, print it or attach it to your paperwork. The link button copies a URL with all your inputs in it, so whoever you send it to opens the same calculation, not a screenshot.

The app does the checks this page can’t: current capacity with derating, alongside voltage drop. You get one size that passes everything, and it tells you which check decided it. See the app.