What is a consumer unit?

A consumer unit (commonly called a fuse box) is the main electrical distribution board in a domestic property. It receives the incoming 100A mains supply from the electricity meter and distributes it to the individual circuits in the building - ring mains, lighting circuits, cooker circuit, immersion heater, etc. - via miniature circuit breakers (MCBs) or fuses. Modern consumer units also incorporate RCD (residual current device) protection to prevent electric shock.

What is the difference between an MCB and an RCD?

An MCB (miniature circuit breaker) protects against overcurrent and short circuits - it trips if more current than its rated capacity flows through it (e.g. 32A for a ring main circuit). An RCD (residual current device) protects against earth faults and electric shock - it monitors the difference between live and neutral currents and trips within 25ms if it detects a leakage to earth of 30mA or more. An RCBO combines both functions in a single device and is used in split-load consumer units.

How many circuits does a consumer unit need?

A typical 3-bedroom house needs a minimum of: two ring main circuits (upstairs and downstairs sockets), two lighting circuits (upstairs and downstairs), one cooker circuit, one shower circuit, one immersion heater or boiler circuit, and potentially an outdoor socket circuit. A 12-way or 16-way consumer unit is standard for most homes to allow for future additions. An electrician will design the number of circuits based on the property's layout and electrical load requirements.

Electrical noun

Consumer Unit

Q: Does a consumer unit need to be changed?

Old-style consumer units with rewirable fuses or MCBs but no RCD protection should be replaced with a modern RCD-protected unit. Since 2016, all new and replacement consumer units in domestic properties must have non-combustible enclosures (metal or composite) under BS 7671 Amendment 3. Consumer unit replacement is notifiable under Part P of the Building Regulations and must be carried out by a registered electrician or notified to Building Control.

/ kənˈsjuːmə ˈjuːnɪt /

Also known as: fuse box, fuseboard, distribution board, CU

Definition

A consumer unit (colloquially a fuse box) is the main electrical distribution board in a domestic property. It receives the incoming mains supply from the electricity meter via the main switch and distributes power to individual circuits - ring mains, lighting, cooker, shower, boiler - each protected by a miniature circuit breaker (MCB) or fuse. Modern consumer units also incorporate RCD (residual current device) protection across all circuits to reduce electric shock risk.

In practice

Modern domestic consumer units use a split-load design with a main isolating switch, one or two RCDs protecting groups of circuits, and individual MCBs for each circuit. Some designs use RCBOs (combined MCB and RCD in one device) on each circuit to prevent a single fault tripping all the circuits on one RCD. Since 2016 (BS 7671 Amendment 3), all new or replacement consumer units must have non-combustible enclosures - metal or composite - to reduce fire risk from arcing faults within the unit.

The consumer unit must be positioned to allow easy access to the main switch and all circuit breakers in an emergency, at a height between 1350mm and 1450mm above floor level (to suit wheelchair users while remaining reachable standing). It must be protected from physical damage and moisture. In bathrooms, a consumer unit is not permitted unless it is specifically designed for the purpose. The meter and consumer unit are typically adjacent, with a 25mm steel conduit or trunking running between them.

Building Regulations

Relevant PartPart P - Electrical safety in dwellings

Wiring standardBS 7671:2018+A2:2022 (IET Wiring Regulations 18th Edition)

Enclosure (since 2016)Non-combustible (metal or composite) - BS 7671 Amendment 3

NotificationRegistered electrician self-certifies, or notify Building Control

Approved Document P requires that electrical installation work in dwellings is either carried out by a Competent Person Scheme (CPS) registered electrician who self-certifies, or notified to Building Control before work starts. Consumer unit installation or replacement is specifically listed as notifiable work. An Electrical Installation Certificate (EIC) must be issued on completion and copies retained for any future sale of the property.

Full Building Regulations guidance

Consumer Unit

Just so you know...