Installing and connecting a consumer unit in a flat

In a block of flats, the electricity supply usually passes through a meter in a shared area, such as a corridor cupboard or a meter room on your floor. Before the meter there is a main fuse or main breaker and a sealed section that belongs to the network operator. After the meter, a supply cable runs into your flat and ends in your consumer unit. Everything from that point on is your own installation.

Typical contents of a modern consumer unit

• A main switch that turns off the whole flat at once, for example before a long trip or during repairs.
• Residual current devices (RCDs) that check whether current is leaking out of the circuit, for example through a person or through damaged insulation. Socket circuits are usually protected by RCDs with a sensitivity of 30 mA.
• Miniature circuit breakers (MCBs) for each circuit. They protect the cables from overload and short circuits. Their rating must match the size of the cable they protect.
• RCBOs, which combine an RCD and a circuit breaker in one device for a single circuit. When a fault occurs on that circuit, only that circuit switches off, not half the flat.
• Surge protection (SPD), which limits voltage spikes from the grid or after a lightning strike nearby. In flats, a Type 2 device is the usual choice.
• Terminal bars for the protective earth conductors (green and yellow) and the neutral conductors (blue), clearly grouped by RCD.
• Where needed, a contactor for a water heater on an off-peak tariff, a doorbell transformer, a time relay or space for smart home modules.

Why not simply put the whole flat on one RCD? Because when it trips, you are left in the dark with the fridge off. Splitting circuits across several RCDs, or using RCBOs for important circuits, means a fault in the washing machine does not switch off your lights or freezer.

Flush or surface-mounted

A flush-mounted unit sits inside the wall, with only the frame and door visible. It looks tidy and suits a full renovation, when walls are being chased and plastered anyway. It needs a wall thick enough to take it, though. In many concrete blocks of flats, a lot of walls are load-bearing, and cutting a deep recess into them is not a minor job. Discuss it in advance with the building management or owners' association, and check with your local building authority whether the work counts as a structural alteration that needs approval. Do not assume it is enough to simply let someone know.

A surface-mounted unit is fixed onto the wall. It goes in faster and does not touch the structure. It suits replacing a board in a flat where people are living, or a cellar, garage or wall that cannot be chased. The downside is that it sticks out and is more visible.

There are also enclosures made specifically for plasterboard partitions. They are lighter and fixed differently, so they need to be planned in while the partition is being built.

Plastic or metal

Most flats use plastic consumer units. They are light, do not corrode and the casing does not conduct electricity. Metal enclosures are more common in workshops, garages or anywhere the unit could be knocked or damaged. A metal enclosure must be properly connected to the protective earth.

Size and spare space

Size is given in rows and modules. One module is the width of one standard circuit breaker. The unit is sized by the number of circuits, always with room to spare. In practice, leaving roughly a fifth to a quarter of the spaces free is a sensible rule. In a few years you may add air conditioning, a tumble dryer, an induction hob or a balcony socket, and with a full board the only option left is replacing the whole enclosure.

Single-phase or three-phase supply

Older flats often have a single-phase supply. If you want a high-power induction hob or plan to run several powerful appliances at once, a three-phase supply may be worth considering. Changing the main fuse rating or adding phases is arranged with the network operator and the building management, not just by swapping the board in your flat. The electrician can tell you what the building's rising main can support.

The procedure depends on whether it is a new installation or a replacement in a flat where people live. The basic steps are similar, though.

1. Survey and design. The electrician checks the supply, the condition of the wiring, how many circuits are needed and where the unit should go. They prepare a simple schedule showing which circuit goes to which breaker and RCD.
2. Arranging the isolation. The supply to the flat is switched off at the meter cupboard in the corridor. If the main fuse before the meter is sealed, only the network operator may work on it. Arrange any shutdown well in advance, especially if it affects other flats.
3. Removing the old board, if it is being replaced. Every conductor is labelled before disconnection so it is clear where it belongs.
4. Fixing the enclosure. For a flush unit, a recess is cut, the enclosure is levelled and set in plaster or mortar. A surface unit is screwed to the wall with plugs. The height is chosen so the devices are easy to operate, usually with the switches roughly between eye and shoulder height of an adult.
5. Bringing in the cables. Cables enter through glands or grommets, with enough spare length but without loose loops that clutter the enclosure.
6. Mounting the devices on the DIN rail according to the schedule: main switch, surge protection, RCDs, breakers and any other devices.
7. Wiring. Conductors are stripped to the correct length, and stranded conductors get ferrules. Line conductors go to the breakers, neutrals to the neutral bar of the correct RCD, and earth conductors to the common earth bar. Devices are linked with busbars or conductors of a suitable size.
8. Tightening the terminals to the torque specified by the device manufacturer. A loose terminal heats up over time and is one of the common causes of faults.
9. Labelling the circuits. Each breaker gets a label, so a year later you still know what you are switching off.
10. Switching on and checking. After power-up, the circuits, RCDs and connections are checked. Then comes testing and certification, described in a separate section below.

Good preparation saves time and avoids the job stopping halfway through the day.

• A list of appliances and where they go. Induction hob, oven, dishwasher, washing machine, tumble dryer, water heater, air conditioning, electric underfloor heating. Each of these may need its own circuit.
• A location for the unit. It should be easy to reach, for example in the hallway. Avoid placing it behind a door that opens against it, inside a fitted wardrobe without proper access, in the bathroom, or directly above a water or heat source.
• Information from the building management about where the meter cupboard is, who holds the key and how to arrange the shutdown.
• Old documents, if you have them: a circuit schedule, the previous inspection report, details of any earlier rewiring.
• Clear space around the work area, and protection for floors and furniture against dust if chasing is needed.
• A plan for a day without power. Empty the fridge and freezer or arrange a temporary supply, and charge phones and laptops.

The difficulty, and therefore the amount of work, varies a lot from flat to flat. This explains why two jobs that look the same can differ in time and scope.

What makes it easier

• New copper wiring with an earth conductor, already prepared and labelled during the renovation
• Surface mounting or a recess prepared in advance
• A clear circuit schedule and decisions about appliances made before work starts
• An empty flat with nothing that has to be kept running
• Easy access to the meter cupboard and the possibility of switching off the supply

What makes it harder

• Old aluminium wiring, which is brittle, breaks easily and behaves differently from copper
• Two-wire installations without a separate earth conductor, where an RCD cannot fully do its job and a decision is needed on how to proceed
• Unlabelled cables, where each circuit has to be traced with test equipment
• Recessing into hard concrete or into a wall that carries other services
• Many circuits and extra devices, such as a water heater contactor, smart home modules or a three-phase supply
• Replacement in an occupied flat, where some circuits must be restored quickly

A common question is whether it is worth replacing only the consumer unit and keeping the old wiring. Technically it is possible, but a new board with RCDs sometimes exposes hidden faults in old cables and starts tripping. The electrician should tell you in advance what condition the wiring is in and what risks come with keeping it.

The consumer unit is the point where every cable in the flat meets, so the timing matters.

• First fix wiring must be finished first, so that every cable can be brought to the unit's position.
• Plasterers make good around a flush enclosure. When fixing it, allow for the thickness of the future plaster or tiles, so the frame does not end up too deep or sticking out.
• Drylining contractors need to know about the enclosure in advance if it goes into a partition or lining wall.
• The kitchen supplier should provide appliance power ratings and connection positions before the board is wired permanently.
• The plumber and heating engineer deal with the water heater, electric boiler or pump, which need their own circuit and sometimes off-peak control.
• The decorator comes after the frame is fitted. If devices are already installed before painting, they must be covered.

In a renovation, the usual order is this: the enclosure goes in during first fix, then plastering and painting follow, and the devices are wired at second fix, when there is no more risk of dust and moisture.

• A small board with no spare space. You notice it with the first new appliance, when there is nowhere to put a breaker. The fix is replacing the whole enclosure.
• One RCD for the whole flat. Any fault switches everything off, including the fridge and the lights.
• Mixed neutrals between RCDs. The neutral from a circuit under one RCD is connected under another. The RCDs then trip for no obvious reason, and finding the cause takes a long time.
• A breaker that does not match the cable. A breaker too large for a thin cable means the cable can overheat before the breaker reacts.
• Loose terminals. Over time you notice a burning smell, darkened plastic around a terminal or a circuit that cuts out now and then.
• Missing labels. When there is a fault or a light fitting needs changing, nobody knows which breaker to switch off, so the whole flat goes off.
• A poor location. A unit hidden by furniture or inside a cupboard is hard to find and reach in the dark during a fault.
• No testing and certificate. Without a test report you have no proof the installation is safe. That can cause problems when selling the flat, when making an insurance claim or at final sign-off of a renovation.

Replacing a consumer unit in a typical flat with prepared and labelled wiring usually takes one working day. If the enclosure has to be recessed into the wall, unlabelled circuits have to be traced or old aluminium conductors dealt with, the job can take longer. With a flush unit, allow time for the plaster or mortar to set, usually from a few hours to a day depending on the material, plus further days for plastering and painting.

Inspection and testing

Once finished, a new or altered installation must be inspected and tested by a qualified electrician who is authorised to certify the work. They check the connections and measure insulation resistance, earth fault loop impedance, continuity of the protective conductors and RCD operation. The result is a test certificate or report, which you should keep with your flat's documents. Requirements differ from country to country, so ask your electrician which certificate applies where you live.

Installing and connecting a consumer unit is work only for a person qualified to work on electrical installations. It is not a job for a layperson, not even under supervision. A fault in the board may not show up straight away, but when an appliance fails it can put lives at risk or start a fire. For this reason, this page contains no DIY guide for installing it yourself.

What you can check yourself

Even without technical knowledge, you do not have to accept a finished consumer unit blindly. You can check several things on your own.

• Appearance. The enclosure is level, the door opens and closes smoothly, empty spaces are closed with blanking plates and you cannot reach any live parts with a finger.
• Tidiness inside. Conductors are routed neatly, not in a tangled bundle. No bare conductor ends stick out of the terminals.
• Labels. Every breaker and RCD has a clear label, such as "kitchen sockets" or "bathroom lighting".
• RCD function. Each RCD has a test button. When you press it, the RCD must trip. Manufacturers recommend doing this regularly, roughly every six months.
• Circuits. Switch off the breakers one by one and check that the labels match what actually goes off.
• Documents. Ask for the circuit schedule and the test certificate.

Handover checklist

• The unit is installed in the agreed, easily accessible location
• The enclosure is level, firmly fixed and the door closes without difficulty
• The number of circuits matches the agreed schedule and spare space remains
• Socket circuits are protected by an RCD
• Important circuits such as the fridge and lighting are split so one fault does not switch off the whole flat
• All devices are labelled and the labels match reality
• Empty spaces are closed with blanking plates and no live parts are visible
• The RCD test buttons work
• All lights and sockets work after switching on
• The wall around a flush enclosure has been made good, or it is agreed who will do it
• You have the circuit schedule
• You have the test certificate signed by the qualified electrician