Electrical Calculators

All electrical installation work in the UK must comply with BS 7671, also known as the IET Wiring Regulations. The current edition (18th Edition, Amendment 2) sets out the requirements for the design, erection, and verification of electrical installations. These calculators are designed to help qualified electricians and competent persons perform the calculations required during the design stage of domestic and small commercial installations.

Under Part P of the Building Regulations (England and Wales), most electrical work in dwellings is notifiable and must either be carried out by a registered competent person (e.g., NICEIC, NAPIT, ELECSA) or inspected and approved by Building Control. Notifiable work includes new circuits, consumer unit replacements, and any work in special locations such as bathrooms and kitchens. Our cable sizing calculator helps ensure that cable selection meets the current-carrying capacity and voltage drop requirements of BS 7671 Table 4Ab and Appendix 4.

Voltage drop is a critical compliance requirement. BS 7671 Regulation 525 states that the voltage drop from the origin of the installation to any point of utilisation must not exceed 3% for lighting circuits and 5% for other circuits. On a 230V supply, that means a maximum of 6.9V for lighting and 11.5V for power circuits. The voltage drop calculator checks your proposed cable run against these limits, helping you decide whether to upsize the cable.

The UK domestic electrical market is dominated by brands such as Hager, Schneider Electric (formerly MK), British General, and Wylex for consumer units and protective devices. Cable is typically supplied by Prysmian (6491X singles and 6242Y twin and earth) and Doncaster Cables. For EV charger installations, which have become increasingly common since the 2022 Building Regulations update requiring EV charge points on new-build dwellings, correct cable sizing is essential due to the sustained high-current draw of 7.4kW or 22kW chargers.

Whether you are designing a complete house rewire, adding a new ring final circuit, sizing cables for an electric vehicle charger, or planning a lighting layout to meet recommended lux levels, these calculators provide the technical foundation. Always verify your designs against the full requirements of BS 7671 and consult the IET On-Site Guide or Electrician's Guide to the Building Regulations where necessary.