Question 1

What size consumer unit do I need for a typical house?

Accepted Answer

A typical 3-bedroom semi-detached house usually needs a 14 to 18-way consumer unit. This allows for 2 lighting circuits, 2 ring finals, cooker, shower, smoke alarm, and 2-3 spare ways plus an SPD. Larger detached properties with EV chargers, heat pumps, or underfloor heating may need 20 or 22 ways. Always include at least 2 spare ways for future additions.

Question 2

What is the difference between a dual RCD board and an RCBO board?

Accepted Answer

A dual RCD (split-load) board groups circuits under two RCDs, typically 30mA. If a fault occurs on any circuit in a group, all circuits in that group lose supply. An RCBO board gives each circuit its own combined overcurrent and RCD protection, so a fault on one circuit only trips that circuit. RCBO boards cost more (typically 500-800 pounds vs 200-300 for dual RCD) but provide much better selectivity and are increasingly the standard for new installations.

Question 3

Is an SPD (surge protection device) required in a consumer unit?

Accepted Answer

Under BS 7671 Amendment 2 (Regulation 443.4), SPD Type 2 surge protection is required for most new domestic installations and rewires. It protects electronic equipment from voltage transients caused by lightning, switching surges, and supply disturbances. An SPD occupies one way in the consumer unit and should be installed at the origin of the installation, as close to the main switch as possible.

Question 4

Should the smoke alarm circuit be on an RCD?

Accepted Answer

A smoke alarm circuit should ideally NOT be on a shared RCD with other circuits. If the RCD trips due to a fault on another circuit, the smoke alarm would lose power. Best practice is to use a dedicated RCBO for the smoke alarm circuit, or connect it to a non-RCD protected way with a dedicated MCB. Some consumer units have specific non-RCD ways for this purpose.

Question 5

What main switch size do I need?

Accepted Answer

For a standard domestic property, a 63A main switch is usually sufficient. Larger properties with electric heating, EV chargers, and heat pumps may need 80A or 100A. The main switch rating should exceed the expected maximum demand after applying diversity factors. Your DNO (distribution network operator) supply fuse is typically 60A or 80A for single-phase domestic, which effectively limits the maximum demand regardless of main switch size.

Question 6

Do I need a Type A RCD for an EV charger?

Accepted Answer

Yes. BS 7671 and IET guidance recommend Type A RCD (or Type B if specified by the charger manufacturer) for EV charging circuits. Standard Type AC RCDs may not detect DC fault components that can occur with the rectifier circuits in EV chargers. Some dedicated EV chargers have built-in DC fault detection, allowing a Type A RCD, but always check the manufacturer's installation instructions.

Question 7

How many circuits can I put on one RCD?

Accepted Answer

There is no hard regulatory limit, but best practice is a maximum of 6 circuits per RCD to reduce the impact of nuisance tripping. The more circuits on a single RCD, the greater the cumulative earth leakage current, which increases the risk of unwanted trips. For reliability, balance the circuits across both RCDs in a dual-RCD board, and consider RCBO protection for critical circuits.

Question 8

What is an AFDD and do I need one?

Accepted Answer

An AFDD (Arc Fault Detection Device) detects dangerous electrical arcs caused by damaged cables, loose connections, or deteriorated insulation. Amendment 2 to BS 7671 recommends AFDDs for locations with sleeping accommodation, HMOs, premises with combustible construction, and locations with irreplaceable goods. They are not yet mandatory for all circuits but are increasingly specified. An AFDD occupies one way per protected circuit.

Board Size	Typical Property	Typical Circuits
6-way	Small flat, bedsit	1 lighting, 1 ring, cooker, spare
10-way	Flat, mid-terrace	2 lighting, 2 ring, cooker, shower, spares
12-way	Terrace, small semi	As 10-way + immersion, smoke alarm
14-way	Semi-detached	Standard domestic + outdoor + spares
18-way	Detached	Full domestic + EV charger or heat pump
20-22 way	Large detached, commercial	Full domestic + EV + heat pump + UFH + spares

Consumer Unit Configurator

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Quick Reference -- Standard Board Sizes

Practical Notes

Frequently Asked Questions