How To Wire A Consumer Unit

Contents:

• Key takeaways
• What is a consumer unit & how does it work?
• Wiring a consumer unit: Part P, certification & who can do the work
• How a consumer unit is wired: the professional process
• Testing and certification
• Plastic vs metal consumer units: which do you need?
• Common mistakes to avoid
• FAQs

Wiring a consumer unit is not a task that should be done yourself, and under UK regulations, it is a job that requires a professional electrician. 

This is what’s called notifiable electrical work, and it means that wiring a consumer unit falls under a list of jobs that cannot be deemed officially complete without being carried out, checked, and tested by a registered electrician.

In this guide, we’ll cover exactly how a consumer unit is professionally installed, including what each component is, the installation process itself, and any necessary safety precautions.

• Electrician required – Wiring a consumer unit is notifiable Part P work and, therefore, must be certified by a registered electrician or supervised by one at the very least.

• The process – Installation follows a set process of isolating the power, connecting the supply, and fitting protective devices before wiring in circuits, testing, and certifying.

• RCBO vs RCD – RCBOs are now recommended per circuit over one shared RCD.

• Enclosure material – As per the latest wiring regulations, metal enclosures are required for all consumer unit installations.

A domestic consumer unit, also known as a fuse box, fuse board, or consumer board, is the control centre that takes the incoming power supply and splits it into separate circuits. 

Domestic consumer units differ from commercial distribution boards, as they handle only the needs of residential properties, whereas commercial boards are used in areas with large machinery, for example.

Consumer units manage power safety using different switch setups: main switch units protect every circuit individually to prevent total blackouts, while dual RCD units group circuits together to save on costs. High-integrity boards combine both to keep critical appliances running safely, whereas single-protection RCD incomers and mini garage units are reserved for simple outbuildings.

Even once you understand how to wire a consumer unit, you’ll need to be aware of the necessary safety precautions:

• Isolate the supply at the main switch (or contact the DNO/network operator for a full supply disconnection if working on the incoming tails).

• Prove a voltage tester works on a known live source before and after testing.

• Wear appropriate PPE.

• Never work on a consumer unit in wet conditions.

• Lock off or tag the isolated supply so it can't be switched back on by someone else.

When it comes to consumer unit wiring, there are a few tools and materials needed to get the job done properly:

Tools: 

• VDE insulated screwdriver – Protects the user from shocks when tightening terminal screws.

• Wire strippers – Removes a wire’s plastic insulation without harming the copper beneath.

• Torque screwdriver – Tightens terminal screws to the exact required rotational force.

Materials: 

• MCBs – Protect individual circuits from overloads and short circuits.

• RCBOs – Combines overcurrent protection with earth leakage shock protection for a single circuit.

• Twin and earth cable (sized to each circuit) – Carries current safely to and from appliances.

• Cable clips – To secure cables to walls or joists.

• Cable ties – To bundle and organise wiring.

• Circuit labels – To identify which part of the building each component controls.

After the wiring of the consumer unit is complete, the electrician will test and certify their work. 

At this point, the electrician will label every pathway on the circuit chart and run multiple tests before issuing any certification. The tests conducted are:

• Insulation resistance.

• Earth fault loop impedance.

• RCD trip times.

• Polarity checks.

Once these have all been passed, the electrician will issue an Electrical Installation Certificate (EIC) before the consumer unit can be used. The EIC must be issued by the registered electrician who did the work or as part of a competent person scheme self-certification submitted to a local authority.

If you skip this step and continue without a certificate, you will be left both legally exposed and without proof of safety, which may prove detrimental if it is needed for insurance purposes or when selling your property. 

Since Amendment 3 of the 17th edition, all new domestic consumer units must be made from non-combustible material, i.e., metal. This is because plastic units were linked to house fires spreading from electrical faults. Older plastic units do not need to be replaced, though, so long as they are safe, but any new installation will mean a metal consumer unit is fitted.

There are a few common mistakes that can occur when wiring a consumer unit. This is one of the reasons why it is necessary to have a registered electrician carry out or supervise the work:

• Sharing a neutral across two circuits – This keeps a wire dangerously live even after you turn off the fuse, causing unexpected electric shocks and instant fuse board tripping.

• Fitting a Type AC RCD on modern electronic loads that need Type A – This will fail to turn off the power during an electrical fault.

• Overfilling in ways beyond the manufacturer's module rating – This traps heat and can cause parts to melt, starting house fires.

• Skipping the dead-test-before-and-after-proving step – This means you could touch a live wire thinking it’s safe.

• Leaving a consumer unit uncertified after DIY work – This leaves you exposed legally and without proof of safety for insurance and property-selling purposes.

• Mixing old rewireable fuses with new incompatible MCBs – This will cause your circuit to break or fail.

Any work on the incoming supply side, RCD/RCBO type selection, and all testing and certification must be done by a qualified and registered electrician. 

Though there are some aspects of this job you can do, the majority of it must be carried out or supervised by a professional. For those with minimal experience, it’s best not to attempt any of it yourself.

Additionally, a full rewire often happens once a new consumer unit is installed, so an electrician could also help with this. Take a look at our guide to rewiring a house.