Cable Size UK: BS 7671 sizing

Cable Size UK - The Calculator That Shows Its Working Free calculators give you a number. Cable Size UK gives you the chain of reasoning: design current, protective device check, correction factors, the minimum tabulated capacity your cable must meet, and volt drop as a value and as a percentage of nominal. The intermediate values are what an inspector asks for, so a result can be transcribed onto a design or a certificate and defended during an inspection. HOW IT WORKS Enter the circuit details: the load and supply, the protective device rating, the correction factor values that apply to the installation, and the cable's volt drop figure and run length. The app works through the calculation in the order the method describes and shows every step, so you can check the working rather than trust a black box. WHAT IT CALCULATES - Design current (Ib) for single and three phase loads, entered as power or as stated current, with power factor, efficiency and diversity - Protective device check: the rating you choose is verified against the design current (Ib not exceeding In) - Required tabulated capacity (It): applies the correction factors you enter for ambient temperature, grouping, thermal insulation and rewireable fuses, and shows the combined factor - Volt drop from the cable's millivolt per amp per metre figure, in volts and as a percentage of nominal, checked against the 3 percent lighting and 5 percent other-uses limits - The full working for every result, step by labelled step WORKS WITH YOUR OWN TABLES This app reproduces no tabulated data. You read the factor values and cable figures from your own copy of the tables, exactly as you would on paper; the app applies the method, keeps the arithmetic honest and shows the working. Where a case falls outside what it covers, it says so rather than guessing. WHO IT IS FOR UK electricians, electrical designers and competent-person scheme members, and anyone learning cable sizing who wants to see the method, not just the answer. A DESIGN AID, NOT A DESIGNER Cable Size UK is a calculation aid. It does not replace the judgement of a competent person, and every result should be verified by one before an installation is energised. Calculations follow the methods of BS 7671:2018 Amendment 4:2026; the app is not certified, approved or endorsed by BSI or the IET. PRIVACY No sign-in, no ads, no tracking. Works offline. Your calculations stay on your device.