Find the cable size for your load. Enter the load in kW (or the current in amps), the supply, the cable length and the voltage drop you can accept. The calculator checks both the current rating and the voltage drop, then gives the smaller standard size that passes both, with a breaker size to match.
Guide only, based on typical PVC insulated cable ratings and a simple resistance voltage drop. Always confirm with your local code (IEC, NEC or national rules) and a qualified electrician before buying or installing cable.
Formulas and assumptions
- Design current, single phase: I = kW times 1000, divided by (volts times power factor)
- Design current, three phase: I = kW times 1000, divided by (1.732 times line volts times power factor)
- Voltage drop: single phase = 2 times I times length times resistivity, divided by cable area. Three phase = 1.732 times I times length times resistivity, divided by cable area
- Resistivity used: 0.0225 ohm mm2 per metre for copper and 0.036 for aluminium, at normal operating temperature
- Current rating: typical PVC insulated copper values, reduced for ambient temperature and installation. Aluminium is taken as 78 percent of copper and starts at 16 mm2
- Breaker: next standard size at or above the design current. The cable must carry at least the breaker rating
Example
A 5 kW single phase load at 230 V and a power factor of 0.9 draws about 24.2 A, so a 25 A breaker is needed. On a 30 metre copper run with a 3 percent limit, 4 mm2 would carry the current but lose too much voltage, so the answer is 6 mm2, with a drop of 5.43 V (2.36 percent).
Frequently asked questions
How do I choose the right cable size?
Work out the load current, then pick a cable whose rating (after derating for temperature and installation) is at least the breaker rating. Then check that the voltage drop over the full length stays within about 3 to 5 percent.
What voltage drop is acceptable?
Many codes allow around 3 percent for lighting and final circuits and up to 5 percent overall from the supply to the load. Check the rule that applies to you.
Is copper or aluminium better?
Copper carries more current for the same size and is easier to terminate. Aluminium is lighter and cheaper for large cables but needs a larger size and correct connectors.
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