Wire Gauge & Breaker Calculator
Enter the appliance power, the cable run and how it is installed to get the working current, the copper wire cross-section, the breaker rating and the voltage drop at the far end.
Wire size is decided by current, not by habit. Undersized cable heats up, the breaker trips on a hot day, and the light at the far end dims when the kettle goes on. Enter the appliance, the run and how the cable is installed; the page returns the current, the cross-section, the breaker and the voltage drop.
Circuit details
Single phase 220 V. Results update as you type.
Working current
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Copper wire
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Breaker
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Voltage drop
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Copper cable capacity (in conduit)
| Cross-section | Current | Power at 220 V |
|---|---|---|
| 1.5 mm² | 14 A | about 3.1 kW — lighting, small sockets |
| 2.5 mm² | 19 A | — |
| 4 mm² | 25 A | about 5.5 kW — kitchen, water heater |
| 6 mm² | 32 A | — |
How to read the result
- The breaker protects the cable, not the appliance. A 32 A breaker on 2.5 mm² cable lets the wire overheat before the breaker ever trips.
- Derating matters: the same cable carries less in conduit (×0.8) and less again under insulation (×0.7).
- Voltage drop is about length: 2.5 mm² is fine at 10 m and marginal at 40 m for the same load.
- For fixed appliances above 3 kW, run a dedicated circuit back to the distribution board instead of sharing a ring.
- Aluminium cable needs a larger cross-section than copper; this calculator is for copper only.
Practical notes
- Air conditioners and motors draw a starting surge of 3–5 times the running current; use the inductive option and do not size to the wire limit.
- Never run a 16 A load on a 10 A socket. The plug and the socket have their own rating, separate from the cable.
- Keep the breaker rating at or below the cable capacity after derating, and above the appliance current.
- For long garden or garage runs, check the drop first — it often decides the cable size before the current does.
Four common mistakes
- Choosing cable by the breaker instead of the load, so the protection never matches the wire.
- Ignoring derating and using the free-air ampacity for a cable buried under insulation.
- Sharing one 2.5 mm² circuit between a kitchen, a heater and a washing machine.
- Joining cables with tape inside a wall; joints belong in accessible boxes.
Guidance for planning only. Local wiring regulations, cable type and installation conditions govern the final choice; have fixed wiring installed and inspected by a licensed electrician.