Types of electrical panels: how many modules does a consumer unit need?

Short answer: A home consumer unit (fuse box, distribution board) is either flush-mounted into the wall or surface-mounted on it, with a plastic or metal body. Its size is chosen by modules: 1 module is 17.5 mm. Add up the modules of the RCD, surge protector, every circuit breaker and devices such as a time switch, then add a reserve of around 20–30% as common practice. Flats often need 12–24 modules; detached houses 24 or more, or a main board plus sub-boards.

This guide covers the board itself. For breaker ratings and cable sizes, see which breaker size for each circuit and home circuit breakers and cable sizes.

Flush or surface-mounted?

• Flush-mounted board: The enclosure sits inside the wall and only the door shows. Preferred in new builds and full renovations; wall thickness and box depth must be planned. It is hard to enlarge later, so size it correctly from the start.
• Surface-mounted board: Screwed onto the wall, no chasing needed. Practical when replacing an old board and in basements, garages, plant rooms and workshops. Easy to enlarge or extend with a second box.

Plastic or metal?

• Plastic (thermoplastic): The most common choice in homes. Light, rust-free and non-conductive, so the enclosure itself does not need an earth connection. Transparent or solid door.
• Metal: Tougher; used where impact is likely, in large installations and main distribution boards. The enclosure must be bonded to the protective earth.

IP rating and door

A dry hallway usually only needs a board of around IP40. In garages, basements or on a garden wall, where dust and moisture are present, a gasketed IP65 enclosure is used. A door prevents accidental contact and dust; a lockable door helps in homes with children. See IP ratings explained.

What is a module and how many do you need?

Devices clip onto a DIN rail and their width is measured in modules of 17.5 mm. Example widths from the Legrand products we stock:

• Legrand XS4 single-pole circuit breaker: 1 module
• Legrand XS4 three-pole circuit breaker: 3 modules
• Legrand XC10 residual current device: 2-pole 2 modules, 4-pole 4 modules
• Legrand 4 127 90 analogue time switch: 1 module

Add up the devices, then add a reserve. A single-phase flat, for example:

1. 2-pole RCD: 2 modules
2. Surge protector: width depends on the model, check the product data
3. Three lighting circuits: 3 modules
4. Four socket circuits: 4 modules
5. Five dedicated circuits (oven, washer, dishwasher, air conditioning, water heater): 5 modules
6. Time switch for balcony or garden lights: 1 module

That is 15 modules without the surge protector. With a 20–30% reserve you reach 18–20, so a 24-module board makes sense. Spare ways leave room for a future air conditioner, an EV charging circuit or a second RCD; a full board usually means replacing it.

Splitting circuits across two RCDs needs more modules but stops one fault from blacking out the whole home. See RCD types and 30 mA or 300 mA.

Meter cabinet vs consumer unit

• Meter cabinet: Holds the electricity meter, usually at the building entrance or in a shared area. The meter and everything before it follow the distribution network operator's rules and are sealed. Do not modify it yourself.
• Consumer unit: The board after the meter inside your home, with the circuit breakers, RCDs and other protection. The choices in this guide apply here.

Your supply capacity is set by the network operator. Adding a large load such as an EV charger may need an upgrade or a three-phase supply; see how to tell if you have three-phase.

Main board and sub-boards in a detached house

In detached houses and villas there are more circuits and longer distances, so one board is often not enough:

• Main board: Where the supply from the meter enters, with the main switch, surge protection and the breakers feeding each floor.
• Sub-boards: On each floor or in the garage, garden or outbuilding, distributing that area's lighting and socket circuits.

This layout makes fault finding easier and shortens long runs. Plan a separate section with its own RCD for garden and outdoor lighting, a time switch and spare modules. For how we build a new house installation from scratch, see our villa electrical installation page.

What goes inside the board?

• Residual current device: Protects people against electric shock; 30 mA in homes. See residual current devices.
• Circuit breakers: Protect each circuit's cable against overload and short circuit. See circuit breakers and curve B vs curve C.
• Surge protector: Protects equipment against lightning and mains surges. See what is a surge protector and surge protection devices.
• Time switch: Turns garden, shop window, sign or water heater circuits on and off on schedule. See time switches.
• Busbars and terminals: Neutral and earth conductors are gathered on terminal bars; a comb busbar feeds the breakers neatly. Every conductor needs its own tight terminal.
• Labelling: Write which circuit each breaker feeds ("Kitchen sockets", "Living room lights") so the right circuit can be switched off quickly.

A dedicated circuit for EV charging

An EV charger is never added to an existing socket circuit; it gets its own circuit, breaker and the right type of residual current protection. If you may buy an EV, leave 2–4 spare modules. See type A RCD for EV chargers.

When should the board be replaced?

Old rewireable or screw-in fuses, boards without an RCD, unlabelled boards or any sign of overheating mean it is time to replace it, usually together with a check of the wiring; see rewiring a house.

Board work can be fatal: it must be done by a qualified electrician and followed by earth and RCD tests. Local rules apply (IEC 60364 is the international basis; check your national wiring regulations). ENGN sells Legrand circuit breakers, RCDs, surge protectors and time switches as an authorised dealer at its store in Nilüfer, Bursa, and arranges consumer units within its electrical contracting work in Bursa after a site survey. We do not sell empty enclosures online.

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