Can a homeowner wire their own consumer unit?

Consumer unit installation or replacement is notifiable work under Building Regulations Part P. A homeowner may carry out the work themselves, but they must notify their local building control authority before starting and arrange for the work to be inspected and certified on completion. In practice, most people use an electrician registered with a competent person scheme such as NICEIC, NAPIT or ELECSA, who can self-certify and notify Building Control without an inspector's visit.

What cable size are UK meter tails?

For a standard 100A domestic supply, meter tails are typically 25 mm squared PVC insulated single-core copper cable. The neutral is usually brown and the line cable is grey in older installations, or brown and blue in more recent work. The tails between the meter and the consumer unit are the homeowner's responsibility, but the tails between the cutout and the meter belong to the DNO and may only be touched by authorised persons.

What is a split-load consumer unit?

A split-load consumer unit has two RCDs. One protects one group of circuits (often upstairs), the other protects a second group (often downstairs). This improves selectivity: if one RCD trips due to a fault, half the house still has power. All-RCBO boards take this further, giving each individual circuit its own RCBO so a single fault does not take out any other circuits at all.

What tests must be done before energising a new consumer unit?

Before the meter tails are connected, the electrician must carry out continuity of ring final circuit conductors, continuity of circuit protective conductors, insulation resistance between live conductors and earth, and polarity checks on every circuit. These dead tests confirm the wiring is safe before any voltage is applied. Results are recorded on the Schedule of Test Results that accompanies the Electrical Installation Certificate.

How to Wire Up a Consumer Unit: Dressing Cables and Making Connections (UK)

Helpful video reference. This guide uses the SparkyHelp tutorial "Wiring Up a Consumer Unit" as its video reference. SparkyHelp is a UK-based electrical education channel aimed at apprentices, students and those refreshing their knowledge of BS 7671. The video demonstrates dressing and connecting PVC single-insulated cables into a domestic consumer unit, which is the practical core of what this guide covers.

Before you start. Consumer unit replacement is notifiable work under Part P of Building Regulations. The meter tails between the cutout and the meter are owned by the DNO and may not be touched except by authorised persons. If you are reading this as a homeowner, this guide explains what your electrician will do. If you are a qualified electrician, ensure the main isolator is off and the meter tails are isolated at the meter before working inside the enclosure.

1. Prepare the consumer unit enclosure

The enclosure must be steel, a requirement introduced to prevent fire spreading through plastic boards. Fix it to the wall at the correct height, typically with the main isolator at the top and circuit breakers or RCBOs below. Most modern consumer units use a 35 mm DIN rail. Fit the main isolator, RCD block (if a split-load design), neutral bar and earth bar before any cables are brought in.

Plan the MCB positions on paper first. Circuits that are likely to trip (immersion heater, electric shower, cooker) should be easy to identify at a glance. Lighting circuits and ring circuits should be clearly separated.

2. Feed the circuit cables into the enclosure

Bring each circuit cable up from the consumer unit position through a knockout at the base of the enclosure. Use a rubber grommet or cable bushing where the sheath passes through the metal edge to prevent chafing. Leave plenty of slack inside the enclosure so you can dress cables neatly to each termination point without stretching.

3. Identify every circuit before connecting anything

This step saves hours later. Before disconnecting anything from the old board, switch each circuit on and off and note which appliances and lights go off. Label each cable clearly with masking tape. When you bring it into the new board, you know exactly where it goes. An unlabelled cable at the bottom of the enclosure is a problem you do not want to solve after the power is on.

4. Dress the cables neatly inside the enclosure

Group cables and route them to their termination side: earths and neutrals to their bars, line conductors to the MCB or RCBO they will terminate in. Cable tie everything at regular intervals. The goal is a board where you can trace any conductor by eye from the cable entry to the terminal. A tidy board is much easier to test, fault-find and modify in future.

5. Connect the circuit protective conductors to the earth bar

Strip the outer sheath from each cable and sleeve the bare CPC in green-and-yellow sleeving. Cut the sleeving so it covers the copper right up to the terminal. Terminate each CPC in the earth bar and tighten the screw. The earth bar on modern boards is usually at the top left, bonded back to the main protective bonding conductors for gas, water and structural steel.

6. Connect neutral conductors to the neutral bar

Strip the blue conductor from each cable and terminate it in the neutral bar. Keep each neutral aligned with its circuit. On a split-load board, there are usually two neutral bars, one for each RCD section. Make sure each neutral goes to the bar that corresponds to the RCD protecting its circuit, or the RCD will not operate correctly in a fault condition.

7. Connect line conductors to the MCBs or RCBOs

Strip the brown conductor for each circuit and terminate it in the correct MCB or RCBO. Follow the manufacturer's torque setting for the terminal screw. Too loose and the connection will run hot under load. Too tight and you crush the conductor strands, creating a weak point. Most domestic MCB terminals accept single-core cable and specify between 1 and 2.5 Nm depending on the conductor size.

8. Dead test every circuit before energising

With the main isolator off and the meter tails disconnected from the supply, carry out all the required dead tests on a multifunction tester:

Continuity of ring circuit conductors (end-to-end resistance for each leg).
Continuity of CPCs (to confirm each earth path is complete).
Insulation resistance (between L and N, and L-and-N combined to earth) at 500 V DC. The reading must exceed 1 MΩ per circuit.
Polarity (confirm brown is connected to the switched terminal, not the neutral bar).

Record every result. Any reading that falls outside the acceptable range means something is wrong in the wiring. Do not energise until it is resolved.

9. Energise and carry out live tests

The DNO or an authorised person connects the meter tails. The electrician then closes the main isolator and tests live. This covers voltage confirmation at each MCB, RCD operating current and trip time (30 mA RCDs must trip within 40 ms at rated current), and earth fault loop impedance on each circuit. The Zs reading for each circuit must be within the limit for the protective device, as set out in BS 7671 Table 41.4.

10. Label and certify

Label every MCB and RCBO with the circuit it protects, using permanent labels rather than handwriting on tape. Fit the Schedule of Circuit Details into the door pocket. The completed Electrical Installation Certificate with Schedules of Inspection and Test Results goes to the homeowner. The job is notified to Building Control, either through the competent person scheme or directly if using local authority building control.

Stop and call an electrician if: the meter tails are energised and you are not authorised to work on them, any dead test result is outside limits before you have identified why, the insulation resistance reading is below 1 MΩ on any circuit, or polarity tests show line and neutral are transposed at any point.

When to call us

Consumer unit replacement is never a straight DIY job under Part P. If your board is old, has mixed fuses and MCBs, no RCD protection, or a plastic enclosure, a replacement is the right call. Richard covers Sandwich and east Kent and issues an Electrical Installation Certificate with every consumer unit change.

Need a new consumer unit in Sandwich?

Richard replaces consumer units to the 18th Edition, issues all certificates and notifies Building Control. Clean, tidy work.

Contact Richard

How to wire up a consumer unit: dressing cables and making connections