Cable Sizing Calculator (BS 7671)
Getting cable size right is the difference between an installation that passes and one that overheats or trips on voltage drop. This calculator sizes 70°C thermoplastic copper cable to BS 7671, the UK wiring regulations, using the same steps a designer follows on site: work out the design current, pick the protective device, apply correction factors for temperature and grouping, then check voltage drop over the run. Enter your load, cable length and installation conditions below and you'll get a recommended size in millimetres squared, with the voltage drop shown against the 3% or 5% limit.
Auto-filled from the supply type; editable for your design.
One-way route length.
This tool covers 70°C thermoplastic (PVC) insulated copper conductors and applies ambient temperature, grouping and fuse-type correction factors with a voltage drop check to BS 7671 Appendix 4. It does not check earth fault loop impedance (Zs), adiabatic (fault current) sizing, or thermal insulation for cable runs partly surrounded by insulation. Always confirm the final design against the full BS 7671 tables and, where required, have it verified by a qualified electrician.
How to use this calculator
Enter the conditions for the circuit you are sizing, then read the recommended size and voltage drop check.
- Choose single-phase (230 V) or three-phase (400 V).
- Enter the load as kW with a power factor, or type the design current straight in if you already know it.
- Pick the cable type and how it's installed. Clipped direct carries the most current; a cable buried in insulation carries the least.
- Set the run length, the ambient temperature and how many circuits are grouped together.
- Read the recommended size and check the voltage drop badge. If it's red, the run is too long for that size and the tool will already have stepped the cable up.
Practical takeaway: installation method changes the answer more than most people expect. The same 2.5 mm² cable that's fine clipped to a wall can be undersized once it's smothered in loft insulation.
How cable sizing works under BS 7671
BS 7671:2018+A2:2022 sizes a circuit by starting with the design current Ib, choosing a protective device In where In ≥ Ib, and then checking the cable's current-carrying capacity. The ambient temperature factor Ca comes from Table 4B1, grouping factor Cg from Table 4C1, and Cf is 0.725 for a BS 3036 rewireable fuse. The required tabulated capacity is It = In ÷ (Ca × Cg × Cf). Finally, the mV/A/m figure from Appendix 4 checks voltage drop over the run. The correction factors only ever make the required cable bigger, never smaller, so skipping them is how installs end up undersized.
Worked example — a garage supply in Leeds
A homeowner in Leeds wants a submain to a detached garage: a 7.4 kW load (a 32 A supply), single-phase at 230 V, run in twin & earth clipped direct for 28 m, ambient 30°C, one circuit, protected by a 32 A Type B MCB. Design current works out at about 32 A, so the device is 32 A. With Ca, Cg and Cf all at 1.0, the required capacity is 32 A, which 6 mm² twin & earth covers (47 A clipped direct). But over 28 m the voltage drop on 6 mm² is around 6.5 V, close to 2.9% — under the 5% power limit, so 6 mm² holds. Run the same cable in an insulated wall and 6 mm² no longer carries enough, and the tool moves you to 10 mm².
Takeaway: the length is what quietly forces the size up. Short runs are almost always limited by current; long runs are almost always limited by voltage drop.
UK cable sizes and typical uses
| CSA (mm²) | Clipped-direct capacity (A)* | Typical UK use |
|---|---|---|
| 1.0 | 16 | Lighting circuits |
| 1.5 | 20 | Lighting, some radial sockets |
| 2.5 | 27 | Ring final circuits, radials |
| 4.0 | 37 | Cookers, showers (small), radials |
| 6.0 | 47 | Showers, cookers, garage submains |
| 10 | 64 | Large showers, submains |
| 16 | 85 | Submains, larger loads |
*70°C thermoplastic twin & earth, Reference Method C. Capacity falls once cables are grouped, run in insulation or in high ambient temperatures.
Frequently asked questions
What cable size do I need for a 32 A supply?
For a 32 A circuit in twin & earth clipped direct at normal temperature with no grouping, 6 mm² is usually enough on current-carrying capacity. Longer runs can push it to 10 mm² once voltage drop is checked. Always run your own figures, because installation method and length change the answer.
Does this calculator follow BS 7671?
Yes. It uses the design current, protective device, correction factors and voltage drop steps set out in BS 7671:2018+A2:2022 Appendix 4, for 70°C thermoplastic copper conductors. It does not check earth fault loop impedance or adiabatic sizing, which a full design still needs.
What is the voltage drop limit in the UK?
BS 7671 recommends a maximum of 3% of the nominal voltage for lighting and 5% for other uses, measured from the origin of the installation to the far point. On a 230 V supply that's about 6.9 V for lighting and 11.5 V for power.
Why does cable in insulation need to be bigger?
Insulation traps the heat a cable gives off, so the same conductor can safely carry less current before it overheats. BS 7671 reflects this with lower tabulated capacities for cables in or above insulation, which is why a cable that's fine clipped to a wall may be undersized in a loft.
What's the difference between design current and protective device rating?
Design current (Ib) is the current the circuit actually draws. The protective device rating (In) is the fuse or MCB size, chosen so In is at least Ib. The cable then has to be able to carry In after correction factors, not just Ib.
Can I use this for three-phase circuits?
Yes. Select three-phase and the tool uses 400 V and the three-phase voltage drop factor. Design current is calculated using the √3 relationship for balanced three-phase loads.
Does the calculator handle rewireable fuses?
Yes. Choose the BS 3036 rewireable fuse option and it applies the 0.725 correction factor BS 7671 requires, which increases the cable size the circuit needs.
Do I still need an electrician?
For notifiable work under Part P, and any time you're unsure, yes. This tool speeds up sizing but a competent person still has to design, install and certify the circuit. Use a registered electrician such as one on the NICEIC or NAPIT scheme.
Related tools
Professional advice notice: this calculator is an estimating aid for BS 7671 cable sizing and does not replace a full design by a competent person. It does not check earth fault loop impedance or fault-current (adiabatic) sizing. Electrical work in the UK is subject to Part P of the Building Regulations. If in doubt, use a registered electrician (for example NICEIC or NAPIT registered).