Cable Sizing Calculator UK (BS 7671)
Getting cable size right is the difference between a circuit that runs cool for thirty years and one that cooks itself inside a wall. This calculator follows the method set out in BS 7671: work out the design current, pick the protective device, apply the correction factors for how and where the cable is actually installed, then check the result against both current-carrying capacity and voltage drop.
It covers twin and earth up to 16mm² and steel wire armoured up to 50mm², which between them handle most domestic and light commercial final circuits. Enter your load and route below.
1.0 for heaters and showers, ~0.85 for motors
Cable route length — measure the actual path, not a straight line
Leave blank to skip. Typical UK: 0.35Ω TN-C-S, 0.8Ω TN-S
CPC adiabatic check — optional
Advisory only. Uses your supplied fault-current figure.
Recommended cable
10mm² Twin & Earth
Protected by 50A MCB Type B
CPC / Earth: 4mm²
Working
| Design current (Ib) | 41.3 A |
| Device rating (In) | 50 A |
| Correction factors | Ca 1.00 × Cg 1.00 × Ci 1.00 × Cc 1.000 = 1.000 |
| Minimum tabulated capacity (It) | 50.0 A |
| Tabulated capacity of 10mm² | 64 A |
| Capacity in situ (Iz) | 64.0 A |
Checks
Current-carrying capacity: Iz 64.0A ≥ In 50A PASS
Voltage drop: 3.27V (1.42%) against a 5% limit (11.5V) PASS
CPC adiabatic: advisory only, enter fault current to check
Zs: not checked, Ze not entered
This is a design aid, not an electrical design.
Results are based on BS 7671 tabulated values and standard correction factors, and cover current-carrying capacity and voltage drop only. They do not constitute a full circuit design. Earth fault loop impedance across the installation, prospective fault current, discrimination, diversity and thermal constraints under fault conditions all need separate assessment.
Electrical installation work in dwellings in England and Wales is notifiable under Part P of the Building Regulations. Design, installation, inspection and testing should be carried out by a qualified and competent person, and verified against the current edition of BS 7671 and the IET On-Site Guide. Always confirm figures against the published standard before installing anything.
Build by Jai accepts no liability for work carried out on the basis of this calculator.
How to use this calculator
Work through it in the order the fields appear, because each one feeds the next.
Start with the load, not the cable. If you know the appliance rating, enter it in watts or kilowatts and leave power factor at 1.0 for anything resistive like a shower or an immersion heater. Motors and some LED drivers pull a lower power factor, around 0.85, which raises the current for the same power.
Be honest about the installation method. This is where most cable sizing goes wrong, and it costs more than any other input. A 2.5mm² twin and earth clipped direct carries 27A. The same cable buried in loft insulation and not touching the wall carries 13.5A, roughly half. If part of the run is in insulation, size the whole run for the worst section.
Measure the route, not the straight line. Cable length means the path the cable actually takes, up and over joists, round door frames, down to the consumer unit. On long runs this is what decides the cable size, not the current.
Takeaway: if you only get one input right, make it the installation method.
The BS 7671 method in plain terms
Four currents matter, and they have to line up in order:
- Ib is the design current, what the circuit will actually draw.
- In is the rating of the fuse or breaker protecting it. It has to be at least Ib.
- It is the minimum capacity the cable needs from the tables before any deratings.
- Iz is what that cable can actually carry once installed, after correction factors.
The rule is Ib ≤ In ≤ Iz. If that chain holds and voltage drop is inside limits, the cable is correctly sized.
Correction factors are where the tabulated number meets reality. Ambient temperature (Ca) derates the cable if it's running somewhere hot. Grouping (Cg) derates it if it's bunched with other circuits all generating heat. Thermal insulation (Ci) derates it if the heat can't escape. A rewireable fuse to BS 3036 carries its own 0.725 factor because of how imprecisely it blows.
Takeaway: the tables give you a starting number, and the correction factors tell you what you've really got.
Worked example: 9.5kW shower on an 18m run
A 9.5kW electric shower, twin and earth, clipped direct along a joist run of 18 metres, one circuit on its own, loft at 30°C, protected by a Type B MCB.
Design current: 9,500W / 230V = 41.3A
Device rating: the next standard size up from 41.3A is 50A.
Correction factors: ambient 30°C gives Ca = 1.00. Single circuit gives Cg = 1.00. No insulation gives Ci = 1.00. Type B MCB gives Cc = 1.00. Combined, Ct = 1.00.
Minimum tabulated capacity: 50 / 1.00 = 50A.
Cable selection: 6mm² clipped direct carries 47A, short of 50. 10mm² carries 64A. So 10mm².
Voltage drop: 10mm² drops 4.4 mV/A/m. That's (4.4 × 41.3 × 18) / 1000 = 3.27V, or 1.42% of 230V. The limit for a non-lighting circuit is 5%, so it passes comfortably.
Result: 10mm² twin and earth, 50A Type B MCB, 4mm² CPC. Which is exactly what you'll find on the shelf at CEF or Screwfix for a shower of that size, because the sums land there.
Takeaway: for showers, the current decides the cable and voltage drop rarely gets a look in. That changes fast on longer runs.
When voltage drop takes over
Take a 6kW garage supply, 45 metres of twin and earth clipped direct, Type B MCB. Design current is 6,000 / 230 = 26.1A, so a 32A device. On capacity alone, 4mm² carries 37A and sails through. Then check voltage drop: (11 × 26.1 × 45) / 1000 = 12.9V, which is 5.6% of 230V. The limit is 5%. It fails.
Step up to 6mm²: (7.3 × 26.1 × 45) / 1000 = 8.57V, or 3.7%. That passes. The cable is 6mm² not because of the current, but because of the distance. Nothing about the load changed.
Takeaway: past roughly 30 metres on a domestic circuit, assume voltage drop is driving the decision until you've proved otherwise.
UK cable and device reference
| Cable | Common UK sizes | Typical use |
|---|---|---|
| Twin & Earth 6242Y | 1.0 to 16mm² | Lighting, sockets, showers, cookers |
| SWA 2-core XLPE | 1.5 to 50mm² | Garages, outbuildings, buried supplies |
| SWA 3/4-core XLPE | 1.5 to 50mm² | Three-phase supplies, sub-mains |
| Singles 6491X | 1.0 to 25mm² | Conduit and trunking installations |
| Device | Trip multiple | Common UK application |
|---|---|---|
| MCB Type B | 3 to 5 × In | Domestic circuits, resistive loads |
| MCB Type C | 5 to 10 × In | Small motors, fluorescent banks |
| MCB Type D | 10 to 20 × In | High inrush, transformers, welders |
| BS 3036 rewireable | varies | Legacy installations, 0.725 factor applies |
Doncaster Cables and Prysmian both publish full ratings for their own cable ranges, and those are worth checking against anything specialist.
Takeaway: Type B covers almost everything domestic. Reach for Type C only when inrush is genuinely tripping a correctly sized Type B.
Voltage drop limits
BS 7671 Appendix 4 gives 3% for lighting circuits and 5% for everything else, measured from the origin of the installation to the point of use, on a supply from a public distribution network. On 230V that's 6.9V and 11.5V.
Here's the bit that catches people out: those percentages sit in Appendix 4, which is informative guidance, not a regulation in itself. The actual requirement is Regulation 525.202, which says the voltage at the terminals of equipment has to be suitable for that equipment. The 3% and 5% figures are how you demonstrate compliance in practice, and every UK inspector will expect them, but a manufacturer specifying tighter limits for their kit overrides the rule of thumb.
Takeaway: treat 3% and 5% as the working limits, and check the equipment data sheet when anything sensitive is on the end of the run.
Frequently asked questions
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