Consumer unit wiring diagram: how to read it, with examples for houses and flats
A consumer unit wiring diagram is the graphic representation of all the circuits in a home: it shows how the supply comes in, which protective devices it passes through and where it is distributed to sockets, lighting and special loads. It is read from top to bottom and from left to right, in the order: supply -> main switch -> residual-current protection (30 mA RCD) -> miniature circuit breakers (MCBs) on each circuit -> earthing. A flat usually has 6-10 circuits, while a house has 10-20 or more, depending on the floor area and the loads.
If you are about to set up a new electrical connection (bransament) or to upgrade your installation, understanding the diagram helps you communicate properly with your electrician and avoid costly mistakes. Below we explain, in plain terms, what a consumer unit contains and how to interpret the symbols.
What the consumer unit wiring diagram is used for
The diagram (also known as a single-line diagram) is the “map” of your electrical installation. It documents:
- How many circuits there are and what each one supplies (sockets, lighting, cooker, water heater, boiler, etc.)
- The type and rating of each protective device (MCB, RCD)
- The cable cross-section used on each run
- The way the neutral (N) and earth (PE) are connected
A correct diagram is mandatory at the handover of the works and useful for any subsequent intervention. Without it, troubleshooting a fault becomes a guessing game, and extensions (a new circuit, a high-power load) are done “by guesswork”.
The main components of a consumer unit
Before reading a diagram, you need to recognise the basic elements you will encounter in almost any domestic consumer unit.
The main switch
This is the device from which everything starts: it disconnects the entire home with a single lever. It is fitted immediately after the meter (or after the metering block) and has a rated current sized according to the power approved through the technical connection approval (aviz tehnic de racordare).
The residual-current protection (RCD / the “diferential”)
The RCD, popularly called the “diferential” or “residual-current protection”, compares the current that comes in with the current that goes out of the circuit. If a leakage appears (for example, through a person’s body), it trips within a few milliseconds. For the protection of persons, the standard value is 30 mA. There is also the RCBO version, which combines residual-current protection and overload protection in a single device.
The miniature circuit breakers (MCBs)
These protect the cables against overload and short circuit. Each circuit has its own breaker, sized according to the cable cross-section and the expected consumption:
- Lighting: usually 10 A
- Standard sockets: 16 A
- High-power loads (electric cooker, water heater, hob): 20-25 A
The neutral bar (N) and the earth bar (PE)
In the consumer unit there are two busbars: one for the neutral conductor (blue) and one for the protective / earth conductor (yellow-green). An installation without a correct earth electrode (priza de pamant) does not provide the protection promised by the devices mentioned above.
How to read the diagram: the logical order
Reading a single-line diagram follows the path of the current:
- The supply comes in at the top (from the meter / electrical connection).
- The main switch is the first protective device and allows the whole home to be switched off.
- The 30 mA RCD protects groups of circuits (or the entire consumer unit).
- The MCBs are lined up horizontally, each labelled with the circuit it serves.
- The N and PE bars collect the neutral and earth from all the circuits.
Each branch has marked: the breaker rating (e.g. C16), the cable cross-section (e.g. 2.5 sq mm) and the destination (e.g. “Kitchen sockets”).
Common symbols in a consumer unit diagram
| Symbol / notation | Meaning | Notes |
|---|---|---|
| Square with a lever | Main switch | Disconnects the whole home |
| Rectangle with “I∆n 30 mA” | RCD | Protection of persons, trips at 30 mA |
| ”C16”, “B10” | MCB | Letter = tripping curve, number = rated current (A) |
| RCBO | RCD + MCB | Combines both protections in one device |
| N (blue) | Neutral bar / conductor | Working conductor |
| PE (yellow-green) | Protective conductor | Earthing |
| 1.5 / 2.5 / 4 sq mm | Cable cross-section | Chosen according to current and length |
The letter on the breaker (B, C, D) indicates the sensitivity to inrush currents: B for resistive circuits (lighting), C for general domestic use (sockets, small motors).
Example of a diagram for a flat
A 2-3 room flat typically has 6-10 circuits. A common configuration:
- Main switch
- One general 30 mA RCD
- C10 - living-area lighting
- C10 - bedroom/hallway lighting
- C16 - room sockets
- C16 - kitchen sockets (preferably a separate circuit)
- C16 - bathroom sockets (with additional protection)
- C20 - electric cooker/hob (if present)
- C16 - washing machine
The kitchen and the bathroom are the areas with the highest consumption and the highest risk, which is why they deserve dedicated circuits.
Example of a diagram for a house
A detached house (single storey or ground floor + upper floor) easily reaches 10-20 circuits, sometimes more. In addition to the flat circuits, the following appear:
- Separate circuits per level (ground floor / upper floor)
- Electric water heater (C20-C25)
- Boiler (dedicated circuit)
- Outdoor sockets and garden lighting (with their own RCD)
- Pump, automatic gate, garage
- Possibly a line for vehicle charging or for a photovoltaic system
In houses with high consumption or with several special loads, splitting across several RCDs is recommended, so that a fault on one circuit does not leave the whole house without power. If you install solar panels, the diagram also includes the prosumer connection part, with a bidirectional meter fitted by the operator and dedicated documentation.
Common mistakes with consumer units
- A single RCD for the whole house (on a fault, everything goes off)
- A breaker oversized relative to the cable cross-section (the cable can overheat without the breaker tripping)
- The absence of a separate circuit for the kitchen or bathroom
- The absence of a correct earth electrode or non-compliant values at testing
- The absence of circuit labelling (makes troubleshooting difficult)
Checking the installation and the earthing is done through PRAM measurements (masuratori PRAM), and the resulting test report confirms that the values are within the parameters.
When you need an ANRE-certified electrician
The design and installation of the consumer unit, the sizing of the protective devices and the correct earthing are not “DIY” jobs. For a new electrical connection (bransament), an upgrade or a power upgrade (spor de putere), the installation must be carried out by an ANRE-certified company, with the compliant documentation required by the distribution operator.
SUN WATT SRL (bransam.ro), an ANRE-certified company for installation and design, builds consumer units, complete installations, electrical connections and grid connections (racordari) in Bucharest, Ilfov and Giurgiu. We help you from the design and the connection dossier through to energisation (punere sub tensiune).
See our services for electrical installations, earth electrode and PRAM measurements. For a new connection, check the grid connection page, and if you want to increase the available power, power upgrade. For an assessment of the works and a personalised quote, write to us via the contact page or call 0733 097 440.