Same resistance method as the voltage-drop tool, rearranged. Enter a size to get a maximum length, a length to get a minimum size, or both. Single-phase uses 2 × I × R. Three-phase uses √3 × I × R.
Resistance only. Larger cables also have reactance, so BS 7671 Appendix 4 (mV/A/m) is the figure to design with. The 3% and 5% limits are from the origin to the equipment, not a pass mark for one cable on its own. This does not check current-carrying capacity.
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.
All tools