# Cable Size Calculator

Design current from load, then the conductor size to carry it.

- Page: https://jianzhujisuan.com/en/wire-size
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/wire-size.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Load 5000 W, Voltage 220 V, Power factor 0.9, Phase Single phase 220 V, Installation method In conduit (×0.8)): **Phase conductor size 4.0 mm²**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Load | number | 5000 | W |  |
| Voltage | number | 220 | V |  |
| Power factor | number | 0.9 | — | 1.0 resistive, 0.8-0.85 motors |
| Phase | select | Single phase 220 V | — |  |
| Installation method | select | In conduit (×0.8) | — |  |

## 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

## Worked example

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.

## Questions

### 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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Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.