Cable calculator
Cable size calculator (BS 7671)
Find the minimum copper conductor size for a circuit from its design current, installation method and run length. It also checks volt drop against the 3% and 5% limits.
Guide only. Copper, 70°C thermoplastic (such as 6242Y twin and earth), single phase 230 V, from BS 7671 Tables 4D5 and 4D2B.
How to size a cable to BS 7671
A cable has to pass several checks before it is the right size, and the current rating is only the first. Start with the design current of the circuit (Ib). The protective device must be rated at or above it (In at least Ib), and the cable's current-carrying capacity must be at or above the device rating once you allow for the conditions it runs in.
Rating factors that reduce capacity
- Ca: ambient temperature above 30°C.
- Cg: grouping with other cables.
- Ci: thermal insulation, the common one in modern lofts and walls.
- 0.725: circuits protected by a BS 3036 rewireable fuse.
Multiply the factors together and divide the tabulated capacity by the result. That is what the correction field above does. The installation method matters as much as the current. Pick the wrong one and the answer can be a full size out.
Then volt drop, then the rest
A cable that carries the current can still be too small on a long run, because volt drop has to stay within 3% for lighting or 5% for other circuits. After capacity and volt drop, a full design also needs a disconnection-time check (max Zs) and an adiabatic check on the protective conductor. Use the max Zs checker and volt drop calculator alongside this one. This tool covers the first two checks as a guide. The full design stays with you.
FAQs
What size cable do I need for a 32A circuit?
For a 32A design current on a clipped-direct run, 4 mm2 copper twin and earth (47A tabulated) is typical. The right size depends on the installation method, grouping, ambient temperature and run length for volt drop. Use the calculator and check against BS 7671.
How do correction factors change the cable size?
Grouping, high ambient temperature and thermal insulation all reduce how much current a cable can carry. You divide the tabulated capacity by the combined factor, so a cable in loft insulation may need to be one or two sizes larger than the same cable clipped in free air.
Does this calculator replace a full BS 7671 design?
No. It checks current capacity and volt drop as a guide. A full design also confirms the disconnection time (max Zs) and the adiabatic limit for the protective conductor, and applies every correction factor for the actual route. You remain the designer.