Wire Size Calculator
Home & ConstructionCable size from current and run length, checked against both the ampacity and the voltage drop.
Runs entirely in your browser — nothing is uploaded
A cable has to satisfy two independent limits, and the larger one governs. It must carry the current without overheating, and it must not drop so much voltage that things on the end of it misbehave. The second is the one people miss — nothing gets hot, and the symptom is indirect: a motor that will not start, or lights that dim when the pump kicks in.
What people do next
Features
- Both limits computed, with the governing one named.
- Voltage drop at operating temperature, not at 20 °C.
- Copper or aluminium, with the ampacity difference applied.
- Derating for how the cable is installed — conduit, bunched, buried, in insulation.
- Separate drop limits for lighting, power and motor circuits.
- Runs entirely in your browser — nothing is uploaded.
How to use the Wire Size Calculator
- 1Enter the load in watts, or the current directly.
- 2Give the one-way length of the cable run.
- 3Choose the conductor material and how it is installed.
- 4Read the recommended size and which limit decided it.
Frequently asked questions
How do I calculate cable size?
Two calculations, and you take the larger answer. For current: pick the smallest cable whose rating, derated for how it is installed, covers the load. For voltage drop: area = (k × resistivity × length × current) ÷ permitted drop, where k is 2 for single phase because current goes out and comes back, and √3 for a balanced three-phase circuit. A short run is usually governed by current and a long one by voltage drop.
How much voltage drop is acceptable?
IS 732 allows 3% for lighting and 5% for power, measured from the origin of the installation. Lighting is tighter because the drop shows up directly as dimming. Motor circuits are often held to 2%, since a motor starting on low voltage draws even more current and may stall.
What size wire do I need for a 3 kW load?
At 230 V and 0.9 power factor that is about 14.5 A. Over a short run 2.5 mm² copper carries it; over 40 m or more the voltage drop pushes you to 4 mm², and beyond about 70 m to 6 mm². The current is the same in all three cases — only the length changes the answer, which is exactly the point.
Is aluminium wiring acceptable?
It is used widely for service cables and larger feeders because it costs less, but it is about 60% more resistive than copper, so an aluminium cable is always a size or two larger for the same duty. It also creeps under a terminal screw and needs connections designed for it. For final circuits inside a house, copper is the norm.
Why does the installation method change the answer?
Because what limits a cable is heat, not current in itself. A cable clipped in open air sheds heat freely and carries more; bunched in a conduit with other circuits it runs hotter and carries less; buried in thermal insulation it can lose a third of its rating. The ratings in any table come with an installation method attached, and using them without it is how cables end up undersized.