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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.
20% recommended for future expansion
32A dedicated circuit for electric cooker
45A dedicated circuit for an electric shower up to 9.5kW (10mm² T&E)
16A dedicated circuit
32A dedicated circuit for a 7.4kW charger, normally a Type A RCBO (BS 7671 Section 722 / IET Code of Practice for EV Charging)
6A dedicated circuit
16A dedicated circuit
16A dedicated circuit
6A dedicated circuit — required by Building Regs
Minimum number of spare ways regardless of percentage (default 2)
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 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 single-row domestic consumer unit size sold in the UK: 6, 8, 10, 14, 16, or 20 ways. Above 20 ways you move to a split-load or dual-row board.
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 (up to 9.5kW): 45A Type B MCB with 10mm² cable (9.5kW ÷ 230V = 41.3A; 40A only suits showers up to ~8.5kW)
- Immersion heater: 16A Type B MCB
- EV charger (7.4kW): 32A device, normally a Type A RCBO (BS 7671 Section 722)
- Smoke detectors: 6A Type B MCB
Working figures for this are collected in our UK Consumer Unit Sizes & Types reference table.
Frequently Asked Questions
A typical 3-bed UK house needs around 10 ways: 2 lighting circuits, 2 ring mains, cooker, shower, smoke detectors, and 2-3 spare ways (the next standard board up is 14-way). Always allow at least 20% spare capacity for future additions like EV chargers or extensions. Use our Circuit Breaker Sizing Calculator to determine the correct MCB rating for each circuit, and our Electrical Load Calculator to check total demand.
A 3-bed house typically requires a 10 or 14 way consumer unit. This accommodates 2 lighting circuits, 2 ring mains, cooker circuit, shower circuit, smoke detector circuit, and leaves a few spare ways for future needs. 10 and 14 way are standard off-the-shelf single-row board sizes (there is no 12-way single-row board in most ranges).
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. Use our RCBO Selection Calculator to choose the right RCBO type for each circuit.
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.
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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.