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Free Calculator
Wire Size Calculator
Enter the load, the run length from your distribution board and the supply type. The calculator sizes the cable on current capacity and then re-checks it against voltage drop, which is what catches out long runs to an outbuilding, a pump or a top-floor air conditioner.
Frequently asked questions
How do I calculate the wire size for a circuit?
Convert the load to amperes, choose the smallest cable whose current rating exceeds that figure, then check voltage drop over the actual run length and step up a size if the drop exceeds 3%. Long runs are frequently governed by voltage drop rather than current, which is why a short 16A circuit and a 30-metre 16A circuit can need different cable.
What wire size is needed for a 1.5 ton AC?
Typically 4.0 sq mm with a 20A Curve C MCB for a normal run length. If the distribution board is more than about 20 metres away, run the calculator with your actual distance, because voltage drop may push the requirement to 6.0 sq mm.
What is an acceptable voltage drop in house wiring?
3% is the recommended design limit for a domestic circuit and 5% is generally treated as the permitted maximum from the supply point to the furthest outlet. Excessive drop causes dim lighting, poor motor starting and, in pumps, overheating.
Does cable in a hot roof space need a larger size?
Yes. The current ratings used here assume around 40°C ambient with three loaded conductors in conduit. Cables bunched together, run through hot roof voids or buried directly need derating, which usually means going up one size. Send us the layout and we will check it.
Can I use this calculator for a three-phase circuit?
Yes — select three phase and the calculator uses 415V, divides by the square root of three for current and applies the line-to-line factor for voltage drop. It covers domestic and light commercial sizes up to 25 sq mm.
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