| Size | CCC | CCC โ | Volt drop | VD โ |
|---|
Selecting a cable involves two checks that must both pass. The cable must carry the design current without overheating, and the voltage lost along its length must stay within BS 7671 limits. A cable that passes on current but fails on volt drop is undersized for the run; one that passes on volt drop but fails on current is a fire risk.
CCC is the maximum current a cable can carry continuously without exceeding its rated temperature. Values come from BS 7671 Appendix 4 for copper conductors and vary by installation method โ a cable clipped direct to a surface can carry more than the same cable bunched in conduit, because the heat has somewhere to go. The grouping factor further reduces CCC when multiple cables run together and share heat. The tool applies both corrections automatically.
Every cable has resistance. Current flowing through that resistance causes a voltage drop along the cable's length. BS 7671 limits this to 3% of supply voltage for lighting circuits (6.9V on a 230V supply) and 5% for power circuits (11.5V). The longer the run or the smaller the cable, the greater the volt drop โ which is why a short, high-current circuit and a long, low-current circuit can end up needing the same cable size for completely different reasons.
When cables run together in a bunch or conduit, each cable's heat has less space to dissipate. A grouping factor is applied to reduce the effective CCC โ two circuits together are derated to 80% of their solo rating, three to 70%, and so on. This is one of the most commonly overlooked steps in cable selection. If you're running several circuits through the same trunking, always apply a grouping factor.
BS 7671 (the IET Wiring Regulations) is the UK standard for electrical installations. The cable selection methodology in this tool follows its Appendix 4 tables for CCC by installation method and Table B1 resistance values for volt drop. The tool covers copper conductors from 1.5mmยฒ to 185mmยฒ. For aluminium conductors, armoured cables, or installations outside standard BS 7671 conditions, consult a qualified electrician.
The tool identifies the smallest cable that passes both checks โ but minimum compliance isn't always the right answer. Specifying one size up gives thermal headroom for future load growth, reduces volt drop further, and lowers resistive losses over the cable's lifetime. For long runs or high-current circuits, the table of all sizes lets you weigh the trade-offs directly.