How many ways do I need in my consumer unit?

A typical 3-bed UK house needs 10-12 ways: 2 lighting circuits, 2 ring mains, cooker, shower, smoke detectors, and 2-3 spare ways. Always allow at least 20% spare capacity for future additions like EV chargers or extensions.

What size consumer unit for a 3-bed house?

A 3-bed house typically requires a 12 or 16 way consumer unit. This accommodates 2 lighting circuits, 2 ring mains, cooker circuit, shower circuit, smoke detector circuit, and leaves 3-5 spare ways for future needs.

Do I need a dual RCD or RCBO consumer unit?

Since Amendment 2 of BS 7671, RCBO boards (where each circuit has its own RCD protection) are increasingly recommended. They prevent a fault on one circuit from tripping others. Dual RCD boards are cheaper but a fault can affect half your circuits. Many electricians now fit RCBO boards as standard.

How many spare ways should I leave?

The 18th Edition Wiring Regulations (BS 7671) don't specify a minimum, but best practice is to leave at least 20% spare ways. With EV chargers, home offices, and smart home systems becoming common, having spare capacity avoids costly upgrades later.

Can I install a consumer unit myself?

No. Under Part P of the Building Regulations (England and Wales), replacing a consumer unit is notifiable work that must be done by a Part P registered electrician or building control must be notified. The work must comply with BS 7671 and an Electrical Installation Certificate (EIC) must be issued.

Consumer Unit Sizing Calculator — How Many Ways Do I Need?

Calculate the right consumer unit size for your property based on BS 7671 18th Edition. Includes spare ways for future expansion.

Property Type

Preset loads typical circuits automatically

Spare Ways (%)

20% recommended for future expansion

Cooker circuit

32A dedicated circuit for electric cooker

Electric shower circuit

40A dedicated circuit for electric shower

Immersion heater

16A dedicated circuit

EV charger

32A dedicated circuit for 7kW charger

Intruder alarm / CCTV

6A dedicated circuit

Outdoor socket / lighting

16A dedicated circuit

Underfloor heating

16A dedicated circuit

Smoke / heat detectors

6A dedicated circuit — required by Building Regs

How We Calculate This

This calculator determines the consumer unit size you need by counting all required circuits for your property and adding spare ways for future expansion, following BS 7671 18th Edition guidelines.

The formula

Total ways = Number of circuits + Spare ways (min 20% or 2 ways)

The calculator then rounds up to the nearest standard consumer unit size available in the UK: 6, 10, 12, 16, 18, or 21 ways.

Standard UK circuit MCB ratings

Lighting circuits: 6A Type B MCB
Ring main (sockets): 32A Type B MCB
Cooker circuit: 32A Type B MCB
Electric shower: 40A Type B MCB (for up to 9.5kW)
Immersion heater: 16A Type B MCB
EV charger (7kW): 32A Type B MCB
Smoke detectors: 6A Type B MCB

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.