This uses conductor resistance only. It is a fair check on smaller final circuits. Larger cables also have reactance, so BS 7671 Appendix 4 (mV/A/m) is the figure to design with.
The 3% and 5% figures are the recommended maximum from the origin of the installation to the current-using equipment, not a pass mark for one cable on its own. Resistivity is adjusted from 20°C. Reactance, grouping and power factor are not included.
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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