Two things decide the size
A cable has to be thick enough to carry the current without overheating, and thick enough that the voltage still arrives at the far end. Short runs are usually decided by current; long runs are almost always decided by voltage drop.
Step 1: find the current
If you know the load in watts, divide by the voltage. For a 3 kW heater on 230 V that is 3000 ÷ 230 = 13 A. On a three-phase supply, divide by √3 × voltage × power factor instead.
Step 2: check the current rating
Every cable size has a current-carrying capacity, and it is not a single number. The same copper conductor is rated lower when it is buried in insulation, run in a bundle with other cables, or working in a hot roof space. Typical copper figures used for house wiring are:
- 1.5 mm² (about 15 AWG): lighting circuits, roughly 14 to 20 A depending on how it is installed.
- 2.5 mm² (about 13 AWG): socket circuits, roughly 18 to 27 A.
- 6 mm² (about 9 AWG): cookers and showers, roughly 34 to 47 A.
Those spans are why wiring rules publish tables by installation method rather than one value per size. Take the rating for how your cable will actually be run, then apply the derating factors for ambient temperature and grouping.
Step 3: check the voltage drop
Resistance rises with length, so a long run loses volts along the way:
drop = 2 × length × current × ρ ÷ area
ρ is the resistivity of the conductor, about 0.0175 Ω·mm²/m for copper and 0.028 for aluminium. The 2 accounts for the current going out and coming back; on a balanced three-phase run use √3 instead.
A 20 A load 30 m away on 2.5 mm² copper drops 2 × 30 × 20 × 0.0175 ÷ 2.5 = 8.4 V, which is 3.7% of 230 V. Moving up to 4 mm² brings it to 5.25 V, or 2.3%.
How much drop is allowed
Wiring rules put a limit on it, commonly in the range of 3% to 5% of the nominal voltage, and lighting is often held tighter than power. The exact figure and how it is split between the supply and the final circuit depend on the standard you work to, so check the one in force where you are.
Why it matters beyond the rules
Low voltage at the far end makes motors run hot and draw more current, dims lights, and wastes the missing volts as heat in the cable. On a long outbuilding or pump run, the next size up often costs little and solves the problem permanently.
Things that make a cable smaller than it looks
- Insulation: cable covered by loft insulation cannot shed heat and is derated sharply.
- Grouping: several cables in one conduit or tray each carry less.
- Ambient temperature: ratings assume around 30 °C; a hot plant room or roof needs a factor applied.
- Insulation type: a 90 °C rated cable carries more than a 70 °C one, but only if every terminal it lands in is rated for that too.
Tools
The Cable Size Calculator works out a size from the load, length and allowed drop, and the Voltage Drop Calculator checks a size you already have. The Wire Gauge Chart converts between AWG and mm², and the kW to Amps Calculator turns a load in kilowatts into the current you need to size for.