This picks a cable construction from how the run is installed, then a conductor size from the load and a resistance-only voltage drop. It does not use the current-carrying tables in BS 7671 Appendix 4. A result here is a shopping guide. Confirm the size, the protective device and the installation method in the current book before you buy or install.
Voltage drop is copper resistance at 70°C, with no reactance. Single-phase and two-phase use a go-and-return path. Three-phase uses √3. A ring final is treated as a load at the mid-point, shared both ways, so that drop is lower than the same length run as a radial. Lighting is compared with 3%. Other uses are compared with 5%, from the origin of the installation to the equipment, not for this cable on its own. Current-carrying capacity is not taken from Appendix 4.
These tools are quick guides for a ballpark figure. They are not certified design or test software, and a result on this page is not a design, a certificate, or a substitute for the current edition of BS 7671. Check the working and cross-reference it before you apply it to a real installation. Thanet Electrical Ltd does not accept liability for loss or cost arising from reliance on these figures, except where the law does not allow that liability to be excluded, including death or personal injury caused by negligence.
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