BS 7671 allows a smaller CPC in some cases, by table or by the adiabatic equation. This screen does not offer that reduction. It suggests a CPC the same size as the line. If you enter fault current, time and k, it also shows the adiabatic size so you can see whether a reduction might be available.
To use a CPC smaller than the line, confirm it with the adiabatic calculation and with Tables 54.2 to 54.7 in your copy of BS 7671. Same-size is the cautious choice, not the smallest permitted choice.
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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