This cable size calculator UK tool works out the minimum cable size you need from the voltage drop, current load, cable run length and installation method, following BS 7671 (IET Wiring Regulations 18th Edition). Supports Twin & Earth and SWA cables.
This cable size calculator UK tool takes the electrical load, cable run length and how the cable is installed, then works out the smallest cable that safely carries the current and keeps the voltage drop within the limits set by BS 7671 (the IET Wiring Regulations 18th Edition). It is the same two-part check a qualified electrician makes: current-carrying capacity first, then voltage drop.
| Cable size | Typical current rating* | Common use |
|---|---|---|
| 1.0 mm² | ~11–16 A | Lighting circuits |
| 1.5 mm² | ~14–20 A | Lighting, immersion heater |
| 2.5 mm² | ~18–27 A | Ring final (socket) circuits |
| 4.0 mm² | ~25–37 A | Cooker (small), radial sockets |
| 6.0 mm² | ~32–47 A | Showers, small cookers |
| 10 mm² | ~43–64 A | Large cookers, 9–10 kW showers |
| 16 mm² | ~57–85 A | Sub-mains, EV chargers |
*Current rating varies with installation method, grouping and ambient temperature – always use the calculator with your own figures rather than relying on the range above.
A 9.5 kW electric shower on a 230 V supply draws about 41 A (9500 ÷ 230). The cable is Twin & Earth clipped direct, run 18 m from the consumer unit, ambient 30 °C, on its own. A 6 mm² cable is rated around 47 A so it passes the current check, but over 18 m its voltage drop (7.3 mV/A/m × 41 A × 18 m ≈ 5.4 V, about 2.3%) is within the 5% limit – so 6 mm² is suitable. Increase the length to 30 m and voltage drop climbs to about 9 V (3.9%), still under 5%, but a longer run or a bigger shower can push you up to 10 mm². This is exactly the trade-off the calculator resolves for you.
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Yes. This cable size calculator UK tool is based on BS 7671 (IET Wiring Regulations 18th Edition) and UK cable data for Twin & Earth and SWA cables, using a 230 V single-phase supply and the 3% / 5% voltage drop limits used in UK domestic and light commercial work.
Voltage drop is the reduction in voltage as electricity travels along a cable. Excessive voltage drop means appliances receive less than their rated voltage, which can cause poor performance, dimming lights, or overheating. BS 7671 limits it to 3% for lighting and 5% for other circuits.
A typical 9–10 kW electric shower or cooker on a 230 V supply draws around 40–45 A, usually requiring 10 mm² Twin & Earth cable clipped direct, though the actual size depends on cable run length, installation method, and grouping.
Yes – BS 7671 requires correction for ambient temperature above 30 °C and for cables grouped together. This calculator applies both automatically based on your inputs.
This tool gives a useful starting estimate but electrical work in the UK must comply with Part P of the Building Regulations. Always have cable sizing verified by a qualified electrician before installation.
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