How much cable is needed to rewire a 3-bed semi?

A typical 3-bed semi-detached house requires approximately 150m of 2.5mm T&E cable (for ring mains/sockets), 100m of 1.5mm T&E (for lighting), 15m of 6mm T&E (for the cooker), and 15m of 10mm T&E (for the electric shower). These are estimates and actual quantities depend on the layout, cable routes and number of accessories.

How many circuits does a house need?

A standard 3-bed house typically needs around 8-12 circuits: 2 ring mains (ground floor and first floor sockets), 2-3 lighting circuits (one per floor), 1 cooker circuit, 1 shower circuit, plus dedicated circuits for any high-power appliances like an electric vehicle charger. BS 7671 does not specify exact numbers but requires adequate provision.

What size consumer unit do I need for a rewire?

For most 3-bed houses, a 12-way or 16-way consumer unit (split-load with dual RCDs or RCBO board) is standard. Always include at least 2 spare ways for future additions. Modern RCBO boards provide individual circuit protection and are increasingly preferred. The consumer unit must be metal-clad (Amendment 3 to BS 7671).

How many smoke detectors are required for a rewire?

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), plus a heat alarm in the kitchen. Most installers also fit smoke alarms in the living room. All alarms must be interlinked.

Do I need Building Regulations approval for a rewire?

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.

House Rewiring Calculator — Circuits, Cable & Consumer Unit UK

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.

Property Type

Select the closest match to your property

Number of Bedrooms

Total bedrooms in the property

Number of Floors

Including ground floor

Additional Requirements

Electric Shower

Dedicated 40/45A circuit (10mm T&E)

EV Charger

Dedicated 32A circuit (16mm cable)

Underfloor Heating

Dedicated radial circuit per zone

Smart Home / Network Hub

Dedicated circuit for smart home equipment

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 + landing)
Cooker circuit: 1 (6mm T&E, 32A MCB)
Shower circuit: 1 (10mm T&E, 40A/45A MCB)
Consumer unit: ~12-way with spare capacity
2.5mm cable: ~150m (sockets)
1.5mm cable: ~100m (lighting)

Adjustments

Additional bedrooms and floors increase circuit count and cable length
Electric shower adds a dedicated 10mm circuit if not already included
EV charger adds a dedicated 32A circuit with 16mm cable
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 hallway/landing, plus a heat alarm in the kitchen. One detector per floor as an absolute minimum.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

House Rewiring Calculator — Circuits, Cable & Consumer Unit

Additional Requirements

How We Calculate This

Base specification (3-bed semi)

Adjustments

Smoke detectors (BS 5839-6)

Frequently Asked Questions

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