Consumer Units: What Every Homeowner Needs to Know Before Second Fix

Your electrician tells you the existing consumer unit is full and can't take the new extension circuits. He quotes £650 to replace it with a modern board. You wonder if that's fair, whether you should buy the unit yourself to save money, and what the difference is between all those acronyms on the quote. You search online, find consumer units at Screwfix for £650, and think he's overcharging. He's not. That £650 buys a pre-populated dual-RCD board with MCBs, not the all-RCBO configuration your electrician will specify for a modern installation. The price also excludes installation labour (4–8 hours), testing, and the certification that building control requires.

What it is and what it's for

A consumer unit is the metal box where your electricity supply splits into individual circuits. Your energy supplier delivers power through a meter, and that single feed enters the consumer unit through a main switch. It then distributes power to separate circuits (lighting, sockets, cooker, shower, and so on), each protected by its own device that trips if something goes wrong.

The older name is "fuse box" because older units used rewirable fuses. Modern consumer units use MCBs (miniature circuit breakers, which are resettable switches that trip on overload), RCDs (residual current devices, which detect earth faults and cut the power in milliseconds), or RCBOs (which combine both functions in a single device). The consumer unit houses all of these on a metal rail inside a steel enclosure.

Every domestic property in the UK has one. If you're building an extension, you either need spare capacity in your existing unit for the new circuits, or a replacement unit with more ways, or a separate sub-consumer unit for the extension. Your electrician decides which approach fits your situation.

Why steel, not plastic

Since 1 January 2016, Regulation 421.1.201 of BS 7671 (the UK wiring standard) requires consumer units in domestic premises to be housed in a non-combustible enclosure. That means steel. Plastic consumer units are no longer compliant for new installations or replacements.

The rule exists because consumer units concentrate all of a property's electrical connections in one place. An arc fault or loose connection generates heat. In a plastic enclosure, that heat can ignite the housing itself. A steel enclosure contains the fault and buys time for the protection devices to trip.

If your existing consumer unit is plastic, it doesn't need replacing just because it's plastic. There's no retrofit obligation. But the moment you add new circuits (for an extension, a rewire, or even a new cooker supply), the work triggers compliance with current regulations, and that means a metal enclosure.

Configuration types

This is where the jargon gets thick. There are three main configurations, and the choice matters because it affects both cost and how your circuits behave when something trips.

The industry consensus has shifted decisively toward all-RCBO boards. The price gap has narrowed to the point where a populated RCBO board often costs less than a dual RCD board plus the separate MCBs you'd need to fill it. Nuisance tripping is the practical reason electricians prefer RCBOs: on a dual RCD board, a faulty appliance in the garage takes out your kitchen lighting and your fridge freezer. With RCBOs, only the garage circuit trips.

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A common misconception

Some electrical retailer websites imply that dual RCD boards are "no longer compliant" with current regulations. This is marketing, not fact. Dual RCD consumer units with Type A RCDs remain fully compliant with BS 7671:2018+A2:2022. The regulations don't require RCBO-only boards. But all-RCBO is better practice, most electricians prefer it, and the cost difference has largely disappeared. If your electrician quotes a dual RCD board for a full replacement, ask why. There may be a good reason (existing wiring configuration, for example), but on a new installation, RCBO is the standard choice.

Type A RCDs: the change you need to know about

Since 27 September 2022 (Amendment 2 of BS 7671), Type AC RCDs are prohibited for socket outlet circuits and circuits feeding equipment with electronic components. Your consumer unit must use Type A RCDs or RCBOs as a minimum.

What does that mean in plain terms? Type AC devices only detect sinusoidal (pure alternating current) faults. Type A devices also detect pulsating DC faults, which is the type of fault produced by modern electronics: phone chargers, LED drivers, washing machines with inverter motors, induction hobs. Almost everything you plug in today contains electronics that could produce a DC fault. Type AC can't see it. Type A can.

Any new consumer unit you buy in 2026 should already come with Type A devices. But check, especially on budget boards. If the specification sheet says "Type AC" anywhere, it's not compliant for new work.

Surge protection (SPDs)

Amendment 2 also made surge protection devices (SPDs) a default requirement for all new electrical installations and consumer unit replacements. An SPD sits inside the consumer unit (taking up one or two module positions) and absorbs voltage spikes from the supply, typically caused by lightning strikes on the distribution network or switching surges.

The regulatory position is precise: your electrician must fit an SPD unless you, the customer, sign a written declaration declining it. Even then, safety services (smoke alarms, fire detection systems) must have surge protection regardless of your preference. In practice, almost nobody declines. The SPD costs £20 – £40 as a component and is fitted during the board change at negligible extra labour cost. Retrofitting one later means opening up the consumer unit again.

How many ways do you need?

A "way" is a slot for one protective device (one MCB or RCBO) protecting one circuit. But the number of ways advertised on the box doesn't equal the number of usable circuit slots. A "14-way" unit with a double-pole main switch taking 2 modules and an SPD taking 2 modules leaves you 10 usable circuit positions. Always check how many are actually available for circuits.

For a typical kitchen extension, your electrician will need circuits for:

• 1x 32A ring main for general sockets (kitchen worktop sockets, utility sockets)
• 1x 6A lighting circuit (extension downlights and feature lighting)
• 1x 32A radial for the oven
• 1x 32A radial for the hob (if electric or induction)
• 1x 16A radial for a dedicated fridge/freezer (recommended, so a fault elsewhere doesn't defrost your food)

That's five circuits minimum. Add a separate radial for a dishwasher, a circuit for an outside light, and a spur for an extractor fan, and you're at eight. Your existing house already uses 6-10 circuits. Put those together and a 16-way board is the sensible minimum for a house with an extension. If you're adding an EV charger or solar panels in the next few years, go larger.

What about a sub-consumer unit?

If your existing board is modern, metal-clad, and has capacity, your electrician may run new extension circuits from it. But if the main board is old, full, or a long distance from the extension, a separate sub-consumer unit for the extension is often the cleaner solution.

The sub-consumer unit sits in the extension and receives a single feed from the main board via SWA (steel wire armoured) cable or heavy twin and earth. It has its own main switch and RCBOs for the extension circuits only. The upstream protection at the main board is a switch-fuse or MCB, not an RCD. Putting an RCD at both ends of the sub-main feed is a fault isolation problem: both devices could see the same fault and both trip, making diagnosis harder.

A sub-consumer unit typically costs £200 – £350 installed (supply, labour, and SWA cable for runs up to 20m), which is significantly less than replacing the main board entirely. Your electrician will advise which approach makes sense for your installation.

Cost and where to buy

You can buy a consumer unit yourself, but your electrician must install and certify it. Some electricians prefer to supply their own (they get trade prices and know exactly what they're fitting). Others are happy for you to supply. Ask before ordering.

Supply-only prices (March 2026, inc. VAT)

Bare enclosures (the metal box without any protection devices) start from about £37 – £44 for a FuseBox 12-way, or £37 – £44 for a Hager 12-way. You then buy RCBOs or MCBs separately. Electricians sometimes prefer this approach because they select the exact device ratings for each circuit. Individual RCBOs cost £37 – £44 each depending on brand and rating, so populating a 10-circuit board adds £37 – £44 to the bare enclosure cost.

Installed costs

The installed price includes the consumer unit, all protective devices, SPD, labour (typically 4-8 hours), testing, and certification.

London and the South East add roughly 20% to these figures. If your meter tails (the thick cables between the meter and consumer unit) need replacing, add £60 – £120. Your electrician may recommend an EICR (Electrical Installation Condition Report) before the work, to check the existing wiring is sound before connecting it to a new board. This isn't legally mandatory but is strongly recommended. Discovering a wiring fault after the new board is fitted means opening it all up again.

Brands

Electricians have strong brand preferences. Here are the main players in the UK market.

Hager is the electrician's default. A French manufacturer with decades in the UK market, Hager boards are widely stocked at electrical wholesalers (CEF, Edmundson, Rexel) and have excellent backward compatibility. Hager RCBOs fit boards up to 30 years old, which matters when your electrician needs to add a device to an existing installation. More expensive than budget brands, but spares availability is guaranteed.

Wylex is another established name, part of the Electrium group. Widely available at Screwfix and Toolstation. Solid mid-tier choice. The populated boards with MCBs represent decent value.

BG (British General) sells the Fortress range through Screwfix. Budget to mid-tier pricing, reasonable quality. Watch the RCD ratings on BG boards: some use 63A or 80A RCDs with a 100A main switch, which may not comply with BEAMA guidance on RCD rating matching.

FuseBox disrupted the market with aggressive pricing and pre-populated boards including SPDs as standard. Good value. The concern is longevity: FuseBox is a relatively new brand, and if the company ceased trading, sourcing replacement devices could become difficult. Established brands like Hager carry less of this risk.

MK Sentry (now part of Honeywell) remains available and is a solid mid-tier option. Some electricians say MK has shifted manufacturing, but the Sentry range continues to sell through Screwfix and wholesalers.

Who can install a consumer unit?

Nobody except a registered electrician. Consumer unit installation and replacement is notifiable work under Part P of the Building Regulations in England and Wales (and equivalent regulations in Scotland and Northern Ireland). "Notifiable" means either:

1. The work is done by an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA, or similar), who self-certifies the work and notifies building control on your behalf. This is the normal route.
2. You notify your local authority building control before the work starts, and they inspect it. This route is slower and more expensive (building control charges a fee), and you still need a qualified person to do the work and test it.

After installation, you must receive two documents: an Electrical Installation Certificate (EIC) confirming the work meets BS 7671, and a Building Regulations Compliance Certificate confirming Part P has been satisfied. Keep both. You'll need them when selling the property and for your building control sign-off if the consumer unit work is part of a larger extension project.

Alternatives

Keeping your existing board is an option if it's metal-clad, has spare ways, and uses Type A protection devices. Your electrician runs the new extension circuits from the existing board. This is the cheapest route but depends entirely on the existing board's condition and capacity.

A sub-consumer unit for the extension (covered above) is the middle ground. The main board stays untouched, and the extension gets its own dedicated board. Good when the main board is modern but full.

Full rewire with new consumer unit is the most expensive option but sometimes necessary. If the existing wiring is pre-2006 (old colour codes, no RCD protection), adding an extension may be the trigger to rewire the whole house. Your electrician's EICR will tell you if the existing installation is sound enough to extend, or if it needs replacing first.

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Where you'll need this

• Second fix electrics - consumer unit installed, tested, and certified during second fix
• Electrical layout planning - circuit planning determines the size and configuration of your consumer unit
• Utility meter relocation - the distance between meter and consumer unit affects meter tail lengths and may require relocation

The consumer unit is one of the last things installed but one of the first things planned. Your electrician needs to know how many circuits the extension requires before specifying the board, and that means the kitchen layout, appliance list, and lighting plan must all be agreed before the consumer unit is ordered.

Common mistakes

Buying a board without checking usable ways. A "14-way" consumer unit sounds generous until you subtract 2 modules for the main switch and 2 for the SPD. That leaves 10 usable circuit slots. Check the datasheet for "usable ways," not total modules.

Ordering the wrong brand of MCBs to "top up" an existing board. MCBs from Brand A won't safely work in Brand B's consumer unit, even if they physically clip in. The devices haven't been tested in combination. This is one of the most common mistakes in forum threads. Match the brand. Every time.

Undersizing for future needs. You're adding an extension now. In three years, you want an EV charger. Two years after that, solar panels. Each needs a circuit. If you replace the consumer unit today with the exact number of ways you need today, you'll pay for another replacement when the future arrives. Specify at least two spare ways. Three is better.

Assuming dual RCD is non-compliant. It's not. Dual RCD with Type A devices is perfectly legal. But all-RCBO is better in almost every measurable way, and the cost difference has largely vanished. Don't let marketing scare you into thinking your electrician did something wrong if they fitted a dual RCD board. But if you're specifying a new board from scratch, ask for RCBO.

Skipping the EICR before a board change. Your electrician connects new, tested protection devices to your existing wiring. If that wiring has faults (degraded insulation, missing earths, incorrect connections from decades of amateur additions), the new board won't fix them. An EICR before the board change catches these problems before they're hidden behind a shiny new consumer unit. Budget £60 – £120 for this. It's money well spent.