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House Rewiring Calculator — Circuits, Cable & Consumer Unit
Estimate the number of circuits, cable lengths, consumer unit size and accessory counts for a full house rewire based on BS 7671.
Total bedrooms in the property
Including ground floor
Additional Requirements
Adds a dedicated 40/45A circuit and 15m of 10mm² T&E
Dedicated 32A radial. 6mm² T&E up to ~30m; the result steps to 10mm² for longer runs (volt drop plus derating in insulation, BS 7671 Section 722 / Table 4D5)
Dedicated radial circuit per zone
Dedicated circuit for smart home equipment
Spare ways in consumer unit (default 2)
Typical cable run to EV charger (default 25m)
Socket outlets added per extra bedroom (default 4)
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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How We Calculate This
This calculator estimates the electrical requirements for a full house rewire based on property type, size and additional requirements. It uses typical UK domestic installation practices and BS 7671 18th Edition guidelines.
Base specification (3-bed semi)
- Ring mains: 2 (ground floor + first floor)
- Lighting circuits: 3 (one per floor plus the landing)
- Cooker circuit: 1 (6mm² T&E, 32A MCB)
- Smoke-alarm supply: 1 dedicated way for the interlinked BS 5839-6 system
- Consumer unit: circuits + your chosen spare ways (default 2); a bare 3-bed comes to ~9 ways, rising to 11-13 once an electric shower, EV charger or other dedicated circuits are added
- 2.5mm² cable: ~150m (sockets)
- 1.5mm² cable: ~100m (lighting)
Adjustments
- Additional bedrooms and floors increase circuit count and cable length
- Electric shower adds a dedicated 40/45A circuit and ~15m of 10mm² T&E
- EV charger adds a dedicated 32A radial (standard 7kW home charger). The cable is 6mm² T&E up to roughly 30m; the result steps up to 10mm² for longer runs. The 30m step-up reflects the more onerous of two BS 7671 limits — voltage drop (6mm² = 7.3 mV/A/m, Table 4D5; the 5% / 11.5V limit is reached near 49m but good practice keeps an EV charger nearer 3%) and current-carrying capacity once the cable runs through wall voids or thermal insulation (Reference Method 101/102, Section 722), where 6mm² derates well below its 47A clipped-direct rating
- Underfloor heating and smart home systems add dedicated circuits
Smoke detectors (BS 5839-6)
A rewire triggers minimum Grade D LD2 requirements: mains-powered, interlinked smoke alarms in every circulation space (hallways and landings), a heat alarm in the kitchen, and an alarm in the principal habitable room. The detector count shown here is an indicative minimum based on floor count — confirm it against the actual circulation spaces and high-risk rooms in your property, as LD2 is driven by layout, not a fixed multiple of floors.
Frequently Asked Questions
A typical 3-bed semi-detached house requires approximately 150m of 2.5mm² T&E cable (for ring mains/sockets), 100m of 1.5mm² T&E (for lighting), 15m of 6mm² T&E (for the cooker), and — if an electric shower is fitted — about 15m of 10mm² T&E for it. These are estimates: actual quantities depend on the layout, cable routes and number of accessories, and some installers allow 200-300m of 2.5mm² for a heavily-socketed rewire.
A standard 3-bed house typically needs around 8-12 circuits: 2 ring mains (ground floor and first floor sockets), 2-3 lighting circuits (commonly one per floor plus the landing), 1 cooker circuit, an electric-shower circuit if fitted, a dedicated supply for the interlinked smoke alarms, plus dedicated circuits for any high-power appliances such as an EV charger or underfloor heating. BS 7671 does not specify exact numbers but requires adequate provision and allows for future additions.
Size the board to the circuits you actually have plus spare ways for future additions. A typical 3-bed rewire lands around 8-12 circuits, so a 12-way or 16-way unit (split-load with dual RCDs, or an all-RCBO board) is the common choice once spares are included — a 10-way is usually considered too small for a 3-bed once shower, EV and dedicated circuits are added. Always allow at least 2 spare ways. RCBO boards give individual circuit protection and are increasingly preferred. The enclosure must be non-combustible (typically steel): this was introduced by Amendment 3 to the 17th Edition (BS 7671:2008, Regulation 421.1.201, in force 1 January 2016) and is retained in the 18th Edition. Use our Consumer Unit Sizing Calculator to determine the exact number of ways needed.
Under BS 5839-6, a full rewire triggers the requirement for a Grade D LD2 fire detection system as a minimum. This means mains-powered interlinked smoke alarms in every circulation space (hallways and landings on every floor), a heat alarm in the kitchen, and a smoke alarm in the principal habitable room (usually the living room). All alarms must be interlinked, and the count depends on your actual layout rather than a fixed number per floor.
Yes. A full house rewire is notifiable work under Part P of the Building Regulations. It must be carried out by a registered competent person (registered with NICEIC, NAPIT, ELECSA etc.) who can self-certify, or you must apply for Building Control approval before starting work. An Electrical Installation Certificate (EIC) must be issued on completion.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.