Cable Size Calculator
Design current from load, then the conductor size to carry it.
Inputs
Result
Formula
| Single phase | I = P ÷ (U × cosφ) |
| Three phase | I = P ÷ (√3 × U × cosφ) |
| Required capacity | I ÷ installation factor |
| Selection | smallest size whose capacity ≥ requirement |
| Earth conductor | same up to 16 mm², 16 mm² for 25-35 |
| Aluminium | capacity × 0.77 |
5000 W at 220 V with a 0.9 power factor in conduit: 25.25 A design current, 31.6 A required, so 4 mm² (34 A) with 4 mm² neutral and earth and a 32 A breaker.
Copper cable capacity and load at 30°C in air
| Size mm² | Capacity A | Single-phase kW | Three-phase kW | Typical use |
|---|---|---|---|---|
| 1.5 | 19 | 4.2 | 12.5 | Lighting circuits |
| 2.5 | 26 | 5.7 | 17.1 | General sockets |
| 4 | 34 | 7.5 | 22.4 | Air conditioning, kitchen |
| 6 | 44 | 9.7 | 29.0 | Small dwelling supply |
| 10 | 61 | 13.4 | 40.1 | Medium dwelling supply |
| 16 | 82 | 18.0 | 54.0 | Large dwelling supply |
| 25 | 108 | 23.8 | 71.1 | Unit supply |
| 35 | 135 | 29.7 | 88.8 | Floor distribution |
FAQ
How much can 1.5, 2.5 and 4 mm² carry?
At 220 V and unity power factor roughly: 1.5 mm² about 4.2 kW, 2.5 mm² 5.7 kW, 4 mm² 7.5 kW and 6 mm² 9.7 kW. Those are clipped-direct ceilings; in conduit take 80%, and derate further for continuous full load. Domestic practice: 1.5 for lighting, 2.5 for general sockets, 4 for air conditioning and kitchen circuits, 10-16 for the incoming supply.
Why the derating in conduit?
Because heat cannot escape. A conductor sheds heat by convection from its surface, and inside a conduit the air is still while several cables warm each other, so the same size runs much hotter. Code factors for bunched cables run 0.7-0.85, falling with the number of circuits — which is why 2.5 mm² carries 26 A clipped direct but only about 20 A in conduit.
Can the earth be smaller than the phase?
Yes, within limits: the same size up to 16 mm², 16 mm² for 25-35 mm² phases, and half the phase above 35. The earth provides a low-impedance fault path so protection operates; too small and the fault current is insufficient to trip the breaker, which is lethal in a shock. Never use the earth as a neutral.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.