Consumer Unit Guide: Replacement, Regulations & Costs

A consumer unit (often called a fuse box or fuse board) is the main distribution point for electrical circuits in a domestic property. It houses the main switch, circuit breakers (MCBs), and residual current devices (RCDs or RCBOs) that protect the individual circuits in the home. The consumer unit is where the electricity supply enters the property and is distributed to lighting, power, cooking, and other circuits.

The consumer unit is a critical safety component. It provides overcurrent protection (preventing cables from overheating and causing fires) and earth leakage protection (disconnecting the supply if current flows to earth, indicating a fault that could cause electric shock). A well-specified and correctly installed consumer unit is fundamental to the safety of the entire electrical installation.

There are three main configurations of consumer unit used in domestic installations. Each offers different levels of protection and flexibility.

Main Switch Only

The simplest type, with just a main switch (main isolator) and individual MCBs for each circuit. This type provides overcurrent protection but no RCD protection. It is now outdated for new installations because BS 7671 requires RCD protection for most circuits. You will still encounter this type in older properties during periodic inspections.

Dual RCD (Split-Load)

The most common type in domestic installations over the past two decades. The consumer unit is split into two banks of circuits, each protected by its own RCD. Typically, one RCD covers the lighting circuits and the other covers the power circuits. This provides RCD protection for all circuits, but if one RCD trips, all circuits on that bank are lost.

• ✓Two RCDs, each protecting a bank of MCBs
• ✓Cost-effective way to provide RCD protection across all circuits
• ✓Drawback: a fault on one circuit trips the RCD, losing all circuits on that bank
• ✓Circuits should be distributed between the two banks to minimise disruption
• ✓Still widely installed and acceptable under BS 7671

RCBO Board (Full RCBO)

The premium option, where every circuit has its own RCBO (Residual Current Breaker with Overcurrent protection). Each circuit has independent RCD and overcurrent protection. If a fault occurs on one circuit, only that circuit is disconnected — all other circuits remain live.

• ✓Each circuit has individual RCBO protection
• ✓A fault on one circuit does not affect any other circuit
• ✓Eliminates nuisance tripping affecting multiple circuits
• ✓More expensive due to the higher cost of RCBOs compared to MCBs
• ✓Increasingly recommended as best practice, especially for larger installations
• ✓Easier to fault-find because the tripped device identifies the faulty circuit immediately

One of the most significant changes to consumer unit requirements came with Amendment 3 to the 17th Edition (BS 7671:2008+A3:2015), which took effect on 1 January 2016. This requirement has been carried forward into the 18th Edition.

Regulation 421.1.201

This regulation states that within domestic premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439-3 and shall have their enclosure manufactured from non-combustible material. In practice, this means consumer units in domestic premises must be made of steel (metal).

The reason for this requirement is fire safety. There were a number of fires caused by faulty connections within plastic consumer units, where the plastic enclosure contributed to the spread of fire. A metal enclosure contains any fire that starts within the unit and prevents it from spreading to surrounding building materials.

When Does This Apply?

• ✓All new consumer unit installations in domestic premises
• ✓All consumer unit replacements in domestic premises
• ✓Does not retrospectively require existing plastic units to be replaced
• ✓Applies only to domestic premises (houses, flats, maisonettes)
• ✓Does not apply to commercial or industrial installations

Consumer unit replacement is one of the most common jobs for domestic electricians. There are several situations where replacement is necessary or recommended.

Safety Reasons

• ✓The existing unit has rewireable fuses (offering less protection than MCBs)
• ✓There is no RCD protection on circuits that require it
• ✓The unit shows signs of damage, overheating, or deterioration
• ✓The unit cannot accommodate additional circuits needed for the installation
• ✓Loose connections or burnt terminals identified during inspection

Compliance Reasons

• ✓The property is being rewired and a new board is part of the rewire
• ✓Additional circuits are being installed and the existing unit is full
• ✓The installation is being upgraded to meet current standards
• ✓A landlord EICR has identified the consumer unit as a C2 defect

Typical Cost

A consumer unit replacement typically costs between 500 and 800 pounds for a standard domestic installation. This includes the cost of the unit itself (typically 100 to 250 pounds depending on type and brand), labour, testing, certification, and Part P notification. Additional work such as earthing upgrades, bonding, or cable modifications will increase the cost.

Replacing a consumer unit is a high-stakes job that requires careful planning and execution. Here are best practice guidelines.

Before Starting

• ✓Carry out a thorough survey of the existing installation before quoting
• ✓Check the earthing arrangement and main bonding — upgrades may be needed
• ✓Note the number and type of circuits to specify the correct replacement unit
• ✓Check for any non-standard wiring or modifications
• ✓Inform the customer about the scope of work and any additional requirements
• ✓Ensure you have the correct consumer unit and all necessary components

During Installation

• ✓Follow safe isolation procedures before starting work
• ✓Take photographs of the existing connections before disconnecting
• ✓Label all cables clearly as they are disconnected from the old unit
• ✓Install the new unit securely on a flat, non-combustible surface
• ✓Make all connections neatly with correct torque settings
• ✓Ensure correct polarity on every circuit
• ✓Check that circuit breaker ratings match the cable sizes
• ✓Apply circuit identification labels

After Installation

• ✓Carry out full testing in accordance with BS 7671
• ✓Issue an Electrical Installation Certificate (EIC)
• ✓Notify the work through your competent person scheme
• ✓Provide the customer with a copy of the certificate and the circuit chart
• ✓Demonstrate the RCD test button to the customer

The consumer unit houses the protective devices that safeguard the installation. Understanding the different types of protection is essential for specifying the right consumer unit.

MCBs (Miniature Circuit Breakers)

MCBs provide overcurrent protection. They trip when the current exceeds the rated value, protecting cables from overheating. Common ratings in domestic installations are 6A (lighting), 16A (immersion heater), 20A (radial power), 32A (ring final, cooker), 40A (shower), and 50A (large shower or cooker).

RCDs (Residual Current Devices)

RCDs detect earth leakage current — current flowing to earth through an unintended path, such as through a person. Standard domestic RCDs are rated at 30mA and must trip within 40ms at a current of 150mA (five times rated residual operating current). RCDs save lives by detecting faults that MCBs alone cannot.

RCBOs (Residual Current Breakers with Overcurrent Protection)

RCBOs combine both overcurrent and RCD protection in a single device. Each RCBO protects one circuit, providing independent protection without affecting other circuits. They are the most comprehensive protection option.

SPDs (Surge Protection Devices)

The 18th Edition introduced requirements for surge protection devices (SPDs) in certain installations. An SPD protects the installation from transient overvoltages caused by lightning or switching events. Many modern consumer units include provision for SPDs.