Extension Electrical Calculator — Circuits, Cable Sizing & MCBs

A typical extension needs 2–4 circuits: one lighting circuit (1.5mm² cable, 6A MCB) and one ring final for sockets (2.5mm² cable, 32A MCB). Add a dedicated circuit for cookers (6mm², 32A) or electric showers (10mm², 40A/45A/50A depending on rating — e.g. a 9.5kW shower draws 41.3A and needs a 45A MCB, a 10.5kW shower draws 45.7A and needs 50A). Each circuit needs its own MCB or RCBO in the consumer unit.

Not necessarily. If your existing consumer unit has enough spare ways for the new circuits, you can connect directly. If not, you may need a new consumer unit or a sub-board (small secondary consumer unit) in the extension. New consumer units in dwellings must have a non-combustible enclosure (steel or other ferrous metal is the common example) per regulation 421.1.201 of BS 7671:2018+A2:2022 — a rule introduced by Amendment 3 to the 17th Edition, effective 1 January 2016.

Detached buildings require SWA (steel wire armoured) cable buried at minimum 450mm depth (600mm under driveways) per BS 7671 reg 522.8.10. The cable size depends on the load and run length: a small outbuilding on a 32A supply is typically served by 6mm² or 10mm² 3-core SWA, with 16mm² reserved for larger workshop loads or long runs. Size it by design current and the 5% voltage-drop limit (reg 525.202). An earth rod and separate earth electrode may be required at the outbuilding (a TT earthing arrangement is often used where the supply is PME).

Yes. New circuits in an extension are notifiable work under Part P of the Building Regulations. You must either use a registered competent person (e.g. NICEIC, NAPIT) who can self-certify, or notify Building Control before starting work and have it inspected. Non-compliance can cause problems when selling the property.

There is no minimum number specified in regulations, but good practice is one double socket per 4m² of floor area, or at least 2 double sockets per room. Kitchens need more — typically 6–8 double sockets plus dedicated appliance connections. It is much cheaper to install extra sockets during the build than to add them later.