Cable Size Calculator
Design current from load, then the conductor size to carry it.
Quick answer
With the default inputs: Load 5000 W · Voltage 220 V · Power factor 0.9 etc.Phase conductor size 4.0 mm²
Edit the inputs below to recalculate.Inputs
Result
Phase conductor size
4.0mm²
| Design current | 25.25A |
| Installation factor | 0.80 |
| Required capacity | 31.6A |
| Selected capacity | 34A |
| Neutral size | 4.0mm² |
| Earth size | 4.0mm² |
| Headroom | 7.7% |
| Resistance per 100 m | 0.438Ω |
| Suggested breaker | 32A |
| Breaker vs cable capacity | 94% |
Capacities are typical copper PVC-insulated values at 30°C; aluminium is about 0.77 of these. Long runs must also be checked for voltage drop, normally capped at 5%. Note that sizing is not only about current — mechanical strength sets a 1.5 mm² floor for copper, short-circuit withstand matters, and fire circuits need flame-retardant or fire-resistant cable.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Fix the phase count first. Sockets, lighting and small loads are single-phase 220 V: I = P ÷ (U × cosφ). Air-conditioner outdoor units, pumps and welders are three-phase: I = P ÷ (√3 × U × cosφ). The same appliance wired differently draws 1.732 times the current, so read the nameplate.
- Compute current before picking a size. 5 kW at 220 V with cosφ 0.9 gives 5000 ÷ (220 × 0.9) = 25.3 A. Skipping this step and jumping straight from power to 4 mm² is the common shortcut.
- Apply the installation factor: 1.00 in free air, 0.80 in concealed conduit, 0.85 on tray. A 4 mm² that carries 32 A in air carries only 25.6 A in conduit — right on the 25.3 A limit.
- Look up the smallest size whose derated capacity covers the current. At 25.3 A in conduit, 4 mm² at 25.6 A leaves no margin, so 6 mm² is the practical answer.
- Size the protective earth from the phase conductor: same cross-section up to 16 mm², 16 mm² for 25-35 mm², half the phase conductor above 35 mm².
- For aluminium multiply the tabulated capacity by 0.77 and go up one or two sizes. Copper-to-aluminium joints must use bimetallic terminals.
Where it goes wrong
- Using P ÷ U as the current. Ignoring power factor understates inductive loads — motors, air conditioners, fluorescent fittings — by 10-30%, and the chosen cable then sits exactly at its limit and heats up under full summer load.
- Applying the single-phase formula to a three-phase load. The current comes out 1.732 times too high and you buy two sizes larger than needed. The reverse mistake understates by 42% and the cable burns.
- Choosing cable from ampacity alone, without checking the breaker. A 32 A breaker on 4 mm² in conduit (25.6 A) means the cable fails before the breaker ever trips. Correct order: cable capacity ≥ breaker rating ≥ 1.1 × design current.
- Sizing long runs on ampacity only. Beyond about 50 m the governing limit is voltage drop, not current. 25.3 A over 80 m of 4 mm² drops roughly 7%, over the 5% limit.
Material and cost
Copper BV cable is sold in 100 m coils: 2.5 mm² around 180-260 CNY, 4 mm² around 280-400, 6 mm² around 420-600. Flame-retardant ZR-BV adds 10-15%, low-smoke halogen-free WDZ adds 30-50%. Pulling labour for concealed work is 8-12 CNY/m; chasing walls is extra at 8-15 CNY/m. Cable only — conduit and chasing excluded.
On site
- Typical residential allocation: 1.5 mm² for lighting, 2.5 mm² for general sockets, 4 mm² for air conditioning and kitchen, 6-10 mm² for the service entry. An instantaneous water heater at 6-8 kW needs its own 4-6 mm² circuit — never shared.
- Total conductor cross-section inside a conduit must not exceed 40% of the conduit bore. Three 2.5 mm² conductors fit PVC20; a fourth means moving up to PVC25 or the pull will not go through.
- Never joint copper directly to aluminium. Use a bimetallic terminal or a purpose-made connector, otherwise galvanic corrosion causes a hot joint within six months — the leading cause of fires in older buildings.
- Buy cable by measured length, not by weight. Non-standard coils often deliver 90 m and call it 100 m, and weighing them does not reveal it.
Basis
- GB 50054-2011 Code for design of low voltage distribution, clauses on protective device selection and circuit protection
- GB/T 16895.3-2017 Low-voltage electrical installations — earthing arrangements and protective conductors
- 19DX101-1 Building electrical data handbook, cable current-carrying capacity tables
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Cable Size Calculator. 2026-10-25. https://jianzhujisuan.com/en/wire-size
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.