Question 1

How do I calculate the right cable size for a load?

Accepted Answer

First find the current: for single phase, current = watts ÷ (voltage × power factor); for three phase, divide by (1.732 × voltage × power factor). Add about 25% headroom to get the design current, then pick the smallest cable whose rated capacity exceeds it. Finally, check that the voltage drop over the cable length stays within 3%. This calculator does all three steps automatically.

Question 2

What size cable do I need for a 3 kW load?

Accepted Answer

A 3 kW single phase load at 230 V and 0.85 power factor draws about 15 A, or roughly 19 A with headroom. Over a short run, 2.5 mm² copper is usually enough, but for a long run the cable often has to be upsized to 4 mm² to keep the voltage drop under 3%. Enter your exact length above to see the recommended size.

Question 3

What is voltage drop and why does it matter?

Accepted Answer

Voltage drop is the loss of voltage along a cable due to its resistance. If a cable is too thin or too long, the appliance at the far end receives less than the rated voltage, which causes dim lights, weak motors, overheating and wasted energy. The recommended limit is about 3% for final circuits, so long runs often need a thicker cable than the current alone would suggest.

Question 4

Should I use copper or aluminium cable?

Accepted Answer

Copper carries more current for a given size, resists corrosion and makes more reliable connections, so it is preferred for house wiring and smaller circuits. Aluminium is cheaper and lighter, which makes it common for larger service cables and long feeders, but it needs a bigger size for the same load. For the same current, aluminium cable is usually one or two sizes larger than copper.

Question 5

Is this cable size calculator accurate enough for wiring my house?

Accepted Answer

It gives a solid first estimate using standard ampacity values, 25% headroom and a 3% voltage-drop check. However, real installations also depend on ambient temperature, how cables are grouped, the insulation type and the installation method, all of which apply derating factors. Use this as a planning tool and always have a licensed electrician confirm the final cable size against IS 3961 and the load schedule.

Size (mm²)	Approx. Capacity	Common Use
1.5	~14 A	Lighting circuits
2.5	~18 A	Power sockets
4	~25 A	Geysers, small AC
6	~32 A	Large AC, cooking
10	~44 A	Sub-mains
16	~60 A	Service mains

Cable Size Calculator

Cable Size Calculator

The Current Formula

Voltage Drop Formula

Typical Copper Cable Ratings

Important Caveats