Electrical installation in a five-room flat: first fix and second fix

A fixed package means the work is priced for the whole flat, not per socket. For that to work, both sides need to know exactly what is included and what is not. In a five-room flat this matters more than in a small one, because the number of points can vary a lot from one household to the next.

First fix

• Marking cable routes and box positions on the walls according to the agreed plan
• Chasing the walls and drilling holes for back boxes
• Fitting back boxes, junction boxes and, where needed, larger installation boxes
• Running power cables for lighting and socket circuits, the kitchen, bathrooms and dedicated appliances
• Fitting and wiring the new consumer unit
• Connecting to the supply from the distribution board in the communal area, if this is in the agreed scope
• Supplementary bonding in bathrooms where required

Second fix

Second fix comes only after plastering, skimming and painting. The electrician fits sockets, switches and dimmers, connects terminals for the light fittings, fills the consumer unit with protective devices and tests the whole installation. This is when you find out whether the first fix was done accurately. A back box set too deep or crooked is hard to correct at second fix.

What is usually not included

The following are normally handled separately:

• Making good the chases
• Fitting light fittings, especially chandeliers and LED strips
• Low-voltage systems such as data, TV, doorbell and alarm
• Smart home systems
• Replacing the riser cables in the building
• Upgrading the main fuse with the network operator

Customers often choose the accessories (sockets and switches) themselves, so agree in advance who supplies them, you or the electrician. Final inspection and certification is sometimes part of the package and sometimes not, so ask about it explicitly.

The price list has the same job for one-, two-, three- and four-room flats, each on its own page. The principle is the same, but as the flat grows, more changes than the number of sockets. The whole concept of the installation changes too.

• One-room and two-room flats: few circuits, a small consumer unit with one or two rows, and a few days' work for one or two electricians. The kitchen and bathroom are the only demanding areas.
• Three-room flat: the typical family flat in a block. The number of circuits grows and the consumer unit usually needs more rows, but the design stays simple.
• Four-room flat: often two bathrooms or a separate WC with a basin, more appliances and longer cable runs.
• Five-room flat: the largest scope. Typically there are two bathrooms, a large kitchen with an induction hob, a study with plenty of sockets, a walk-in wardrobe and sometimes air conditioning in several rooms. There are so many circuits that the consumer unit has several rows, and the split between RCDs needs real thought.

In practice, this is what the size means in a five-room flat:

• Longer cable runs: the route from the consumer unit to the furthest room can be long. For powerful appliances the electrician has to watch voltage drop and choose a suitable cable size.
• More RCDs: if a large flat hangs on a single RCD, one fault cuts power to everything. Circuits are therefore split across several RCDs, or combined RCD and circuit breaker devices (RCBOs) are used for important circuits.
• Supply capacity: a large flat with an induction hob, an oven, two electric water heaters and air conditioning can hit the limit of the main fuse. Sometimes a three-phase supply or a higher supply capacity is needed.
• A bigger crew: several electricians usually work on the first fix at the same time. Otherwise the job would hold up the other trades for weeks.
• More room for planning errors: across five rooms it is easy to forget a socket by the bed or a switch at the second entrance to the living room. A good plan matters more here than in a small flat.

A plan for sockets and lights

The most valuable thing is a plan drawn up before anyone starts chasing walls. A floor plan of the flat with the furniture drawn in is enough. Mark where the bed, desk, television, kitchen units, washing machine and tumble dryer will go, and only then draw the sockets and switches.

Why this order? Because a socket behind a wardrobe is useless, and nobody finds a switch hidden behind an open door. For the kitchen, ask the kitchen supplier for a drawing with the appliance positions. Without it, boxes often have to be moved later.

Agreement with building management

In a block of flats you are not alone. The supply to your flat comes from the communal areas, the meter belongs to the network operator or supplier, and some walls may be load-bearing. Before work starts, tell the building management or owners' association. Agree when the supply will be switched off and arrange access to the distribution board in the corridor. Many buildings have their own rules on noisy work and removing rubble.

Building regulations

Whether a rewire needs notifying or approving depends on local building regulations and on the scope of the work. This is especially true if partition walls are removed at the same time or load-bearing walls are affected. In England and Wales, for example, a full rewire is notifiable work under the Building Regulations. Electricians registered with a competent person scheme can usually self-certify it. Do not assume nothing is needed. Check with your local building authority before the work begins.

The network operator

You may plan a higher supply capacity, for example for induction cooking, a heat pump water heater or charging an electric car in the building's garage. In that case, the request to change the main fuse or the supply goes to the distribution network operator in your area. It takes time, so deal with it alongside the planning, not after the work is finished.

Condition of the flat

The flat should be empty. Old tiles and loose plaster along the chase routes should already be removed. Any partitions being knocked down or built should be finished. The electrician needs to know where the final walls will be, otherwise the routes are drawn blind. Arrange a temporary power source for tools, such as a site distribution board or an agreed temporary circuit.

1. Disconnect and strip out the old installation. In concrete-panel blocks, old aluminium cables are often left disconnected in the wall if pulling them out would damage the plaster. What matters is that nothing stays live.
2. Walk through the flat with the customer and mark all boxes and routes on the walls. In a five-room flat this often takes a full day, and it is time well spent.
3. Chase the walls and drill the box holes. A wall chaser with dust extraction cuts down the dust considerably. Routes run vertically and horizontally in agreed zones, never diagonally across a wall.
4. Set the back boxes in plaster or fast-setting mortar. They must sit flush with the future wall surface, so the thickness of plaster or tiles has to be allowed for.
5. Pull the cables. Each circuit is labelled at its end in the consumer unit, so it stays clear later what goes where.
6. Fit the consumer unit and connect the supply. The supply from the corridor is connected only after agreeing it with building management, and only while it is switched off.
7. Carry out test measurements on the first fix and photograph all cable routes before they are covered. Photos of the walls with cables showing are invaluable years later, when you want to drill into a wall.
8. Make good the chases, then plaster, paint and tile. Other trades do this part.
9. Second fix: fit the accessories, equip the consumer unit and connect the lighting terminals.
10. Final inspection and testing by a qualified electrician, followed by the certificate for the installation. In the UK this is an Electrical Installation Certificate.

Cables

Flats are wired with copper installation cable. In the UK this is usually flat twin and earth, while many other countries use round multi-core cable. Lighting circuits are usually wired in 1.0 or 1.5 mm², socket circuits in 2.5 mm². The hob gets its own dedicated cable, sized for a single-phase, two-phase or three-phase connection and for the hob manufacturer's requirements. On long runs to powerful appliances in a five-room flat, the electrician may choose a larger cable size to limit voltage drop.

Consumer unit

A large flat needs a multi-row consumer unit with spare ways. The spare space pays off: if you add air conditioning or a car charger later, you will not need a new enclosure. A flush-mounted unit looks neater but needs a thick enough wall. In a concrete-panel block, cutting a large opening into a load-bearing panel is not advisable. In that case, choose a surface-mounted unit or a different position.

Protection

• 30 mA RCD: protects people from electric shock on contact. It is now standard on socket circuits and in bathrooms.
• RCBO (combined RCD and circuit breaker): provides both functions for a single circuit. When a fault occurs, only that one circuit trips, not half the flat. With five rooms this is worth having, for example on the fridge or computer circuit.
• Surge protection: protects electronics against surges from the grid. The electrician will judge whether it is suitable and which type fits the building's installation.

Boxes and accessories

Plasterboard walls take boxes designed for cavity walls, while masonry and concrete take standard back boxes. Deeper boxes make wiring easier, especially when they hold several cables or a dimmer. With accessories, the difference between cheap and good-quality products lies mainly in how firm the terminals are and how the switch mechanism feels. A five-room flat has a lot of them, and saving on each one often shows later as loose sockets.

Conventional switches or smart control

If you are considering smart control of lights and blinds, decide before the first fix. Some systems need a neutral conductor at every switch box, others a separate bus cable or deeper boxes. Adding this to a finished flat is difficult.

The amount of work in a five-room flat does not depend only on the number of rooms. Two flats of the same size can be very different jobs.

The job is easier with:

• A finished, agreed plan that does not change during the work
• An empty flat and completed partition walls
• Plasterboard linings and suspended ceilings, where cables run without chasing
• Brick partitions, which are easier to chase than concrete
• An accessible, roomy distribution board in the corridor and cooperative building management

The job is harder with:

• Concrete panels, where chasing is slow and limited in load-bearing walls
• Ceilings without a suspended ceiling, where cables to the lights must run under plaster or in chases in the slab
• Changes to the plan during the work, which mean new chases and moved boxes
• Many special points: underfloor heating in two bathrooms, air conditioning, electric blinds, pop-up sockets in the kitchen
• Joining two flats into one, where two original supplies have to be dealt with and possibly one meter removed

One point is important with load-bearing walls: deep horizontal chases can weaken them. If you are not sure which walls are load-bearing, check the building's drawings with the management or ask a structural engineer.

The electrician comes to the flat twice, and other trades work in between. First fix follows demolition and partition building, but comes before plastering, screeds and tiling. It has to be coordinated with the plumber. In the bathroom and kitchen, cables and water pipes should keep clear of each other by agreement, not by luck. With underfloor heating, everyone needs to know where the thermostat and floor sensor will go.

Each of the other trades needs something from the electrician:

• Tiler: needs to know where the boxes are, to cut them neatly into the tiles, ideally in the centre of a tile or on a joint.
• Plasterboard fitter: needs to know where the downlights go before closing the ceiling.
• Decorator: should paint before second fix, not after, otherwise there will be paint on the switch plates.

With a well-coordinated crew, first fix in a five-room flat usually takes one to two weeks, depending on the wall material and the number of points. Second fix is shorter, typically a few days. Between them comes plastering, drying and painting, which can take several weeks. Plaster around the boxes sets quickly, but the filler in the chases needs to dry according to the manufacturer's instructions before skimming.

• Too few sockets in the study and by the beds: extension leads across the floor appear straight after moving in. In a five-room flat this happens most in rooms that had "no purpose yet" during planning.
• Boxes set too deep or sticking out: the plates do not sit flat and sockets wobble.
• Everything on one RCD: a fault in one appliance cuts power to the whole flat, including the fridge.
• Unlabelled circuits in the consumer unit: when something goes wrong, nobody knows what to switch off.
• Diagonal cable routes: sooner or later a drill hits one while someone hangs a shelf.
• An undersized kitchen supply: the induction hob trips or cannot be connected as the manufacturer requires.
• Forgotten low-voltage cabling: after decorating, data cables to the rooms can only run in surface trunking.
• No photos of the routes: later changes mean searching for cables with a detector, with all the risk that involves.

A safety note: electrical installation work in a flat may only be carried out by a suitably qualified electrician. It must be inspected and tested before use, so no part of this job is suitable for DIY. If you smell gas while work is going on in the flat, do not touch any electrical switches. Leave the flat and call your local emergency number or 112.

Set aside enough time for the handover and go through the flat room by room. Switch on every light and try every socket, for example with an inexpensive plug-in socket tester. Press the TEST button on each RCD. Ask the electrician to explain the consumer unit to you.

• The installation certificate and test results are handed over and signed
• Every circuit in the consumer unit is labelled clearly and matches reality
• Every RCD trips when its TEST button is pressed
• All sockets and switches are firm and level, and the plates sit flat on the wall
• Every light switches on from the agreed positions, and two-way circuits work from both ends
• The kitchen supply for the hob and oven matches the manufacturers' requirements
• Supplementary bonding in the bathrooms is complete and the accessories suit the zones around the bath and shower
• No bare cable ends or uncovered empty boxes are left anywhere
• You have photos of the routes before covering and an updated as-built plan
• Any extra or omitted points are recorded in the handover document