Domestic Consumer Unit Replacement: Step-by-Step

Why Consumer Unit Replacements Matter

A consumer unit (CU) replacement is one of the most common notifiable jobs domestic electricians carry out. It's also one of the most scrutinised. Since it falls squarely under Part P of the Building Regulations, getting the process right — from design through to certification — is non-negotiable.

Whether you're upgrading an old rewireable fuseboard to a modern RCBO board, or fitting an amendment 2 compliant metal unit, this guide walks through the full process as it applies in practice.

Part P and Building Control Notification

A consumer unit replacement in a domestic dwelling is notifiable work under Part P of the Building Regulations (England and Wales). You have two routes to compliance:

• Competent Person Scheme (CPS): If you're registered with a scheme such as NAPIT, NICEIC, or ELECSA, you self-certify the work and your scheme provider notifies Building Control on your behalf.
• Building Control notification: If you're not CPS-registered, the homeowner must apply for Building Control approval before work starts. An inspector will need to check the completed installation.

Initial Assessment and Survey

Before you touch anything, carry out a thorough survey of the existing installation. This isn't just about sizing up the CU — it's about understanding what you're inheriting.

Key checks at survey stage

• Earthing arrangement: Confirm the earthing system (TN-C-S, TN-S, or TT). Check the distributor's earthing terminal is present and in good condition. For TT systems, verify the earth electrode resistance.
• Main bonding: Check existing main protective bonding conductors to gas, water, and oil pipes. These must comply with BS 7671 Section 544 (minimum 10mm² Cu for TN systems, 16mm² for TT).
• Tails and meter arrangement: Assess the condition and size of meter tails. If they need upgrading, you'll require a DNO appointment to pull the main fuse — plan ahead, as this can add weeks to the job.
• Existing circuits: Identify all circuits, their cable sizes, and their protective devices. Note any non-standard arrangements or previous bodge jobs.
• Condition of existing wiring: If the property has old T&E without a CPC, rubber-insulated cables, or other legacy wiring, a CU swap alone may not bring the installation up to a safe standard. Be upfront with the customer about this.

Design Considerations

The design stage is where you make the decisions that determine whether your installation is compliant and practical.

Selecting the consumer unit

Since BS 7671 Amendment 3 (2015), all consumer units in domestic premises must be constructed of non-combustible material — in practice, this means a metal enclosure. Regulation 421.1.201 is the specific clause. Plastic CUs are no longer acceptable in domestic installations.

Choose between:

• Dual RCD split-load boards: Cost-effective, but you lose half the board if one RCD trips. Ensure essential circuits (lighting, freezer) are split across both RCDs.
• RCBO boards: Each circuit has individual RCD and overcurrent protection. More expensive but eliminates nuisance tripping affecting multiple circuits. This is increasingly the professional standard.
• High-integrity boards: A hybrid with RCDs for some circuits and RCBOs for others. A good middle-ground option.

RCD protection requirements

Under BS 7671 Regulation 411.3.4, additional RCD protection (rated at 30mA) is required for:

• Socket outlets rated up to 32A
• Mobile equipment rated up to 32A for outdoor use
• Cables installed in walls at a depth less than 50mm without mechanical protection

In practice, for a domestic CU replacement, virtually every circuit needs 30mA RCD protection. The few exceptions (e.g., a dedicated freezer circuit on a time-delayed RCD, or fire alarm circuits) should be designed with care and justified in your documentation.

Circuit protective device selection

Match your MCB or RCBO ratings to the cable sizes and intended load. Remember that Regulation 433.1.1 requires the protective device rating (In) to be less than or equal to the cable's current-carrying capacity (Iz), accounting for grouping, thermal insulation, and ambient temperature correction factors from Appendix 4 of BS 7671.

Installation Process

Preparation

1. Confirm the customer is prepared for a power outage — typically 4 to 8 hours. Suggest they plan accordingly.
2. Lay out dust sheets and set up temporary lighting if needed.
3. If a DNO fuse pull is required, coordinate the timing with the DNO engineer.

Safe isolation

Follow the GS38 safe isolation procedure to the letter. Use a proving unit to confirm your voltage tester is working, test for voltage, then prove the tester again. Isolate at the main switch, lock off where possible, and verify dead at every outgoing terminal before disconnecting.

Physical installation

1. Remove the existing board carefully, labelling every circuit conductor as you disconnect.
2. Mount the new consumer unit. Ensure cable entries use appropriate grommets or glands to protect against abrasion.
3. Connect the meter tails — 25mm² is the standard minimum for single-phase domestic supplies up to 100A.
4. Connect main earth and bonding conductors.
5. Dress and terminate each circuit conductor to its designated protective device, ensuring correct torque settings per the manufacturer's instructions.
6. Fit blanking plates to unused ways.
7. Apply clear, permanent circuit labelling using an appropriate marker or labelling system — not handwritten biro on masking tape.

Testing Sequence

Testing must follow the sequence laid out in BS 7671 Chapter 64 and Guidance Note 3. The order matters because each test validates conditions assumed by the next.

1. Continuity of protective conductors (R1+R2) — confirms the CPC is continuous on every circuit.
2. Continuity of ring final circuit conductors — for ring circuits, confirm the ring is complete and no interconnections or spurs are broken.
3. Insulation resistance — minimum 1MΩ per circuit at 500V DC between live conductors and earth. Disconnect any sensitive electronics first.
4. Polarity — confirm at the board, at each socket outlet, and at each switching device.
5. Earth fault loop impedance (Zs) — measured or calculated values must not exceed the maximum Zs for the protective device, as per Table 41.2, 41.3, or 41.4 of BS 7671.
6. RCD testing — test each RCD at 1×, ½×, and 5× rated residual current. Confirm tripping times are within limits (300ms at 1×IΔn, 40ms at 5×IΔn for general type). Also test the RCD test button.
7. Prospective fault current (PSCC) — measure at the origin. Ensure all protective devices have adequate breaking capacity.

Certification and Paperwork

A consumer unit replacement requires an Electrical Installation Certificate (EIC), not a Minor Works Certificate. This is because it involves work at the origin of the installation and new design decisions.

The EIC must include:

• Design details: Earthing arrangement, supply characteristics (Uo, PSCC at origin), and the method used for fault protection.
• Schedule of Inspections: Completed for the relevant parts of the installation you've worked on.
• Schedule of Test Results: Full test results for every circuit in the consumer unit.
• Next inspection date: Recommend an interval — typically 10 years for a domestic installation, though you may set a shorter interval if the existing wiring condition warrants it.

Observations and recommendations for other parts of the installation (e.g., old wiring that wasn't part of your scope) should be recorded as departures or observations, coded appropriately (C1, C2, C3, or FI).

Common Pitfalls to Avoid

• Issuing a Minor Works instead of an EIC — this is the single most common paperwork error. A CU replacement always needs an EIC.
• Not testing every circuit — you've disturbed every connection. Test everything.
• Ignoring existing defects — if you find a C1 (danger present) defect on existing circuits, you have a duty of care. Document it clearly and discuss it with the customer. You cannot simply sign off a new board connected to dangerous wiring.
• Forgetting to notify — even experienced electricians occasionally forget to submit their notification through their CPS portal. Set a reminder or do it on site before you leave.

Further Reading

For the full regulatory detail, refer to:

• BS 7671:2018+A2:2022 — IET Wiring Regulations
• Approved Document P — Electrical Safety in Dwellings
• HSE GS38 — Electrical Test Equipment for Use by Electricians

Published 2026-03-10. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.