Consumer Unit Wiring Diagram: UK CU Layout, RCD, RCBO and BS 7671 18th Edition Requirements

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A consumer unit wiring diagram shows the UK domestic distribution board layout — main switch, RCD or RCBO protection, and MCBs — as required by BS 7671 18th Edition.

The consumer unit (CU) — commonly known as the fuse board or distribution board in the UK — is the heart of any domestic or light commercial electrical installation. It divides the incoming supply into individual circuits, provides overcurrent protection, and in modern installations delivers the residual current device (RCD) protection mandated by BS 7671:2018 (the 18th Edition of the IET Wiring Regulations).

A modern UK consumer unit contains several key components. The main switch (or main isolator) is a double-pole device that isolates both line and neutral simultaneously from the incoming supply tails. It is rated at the maximum current the installation is designed to carry — typically 63 A or 100 A for domestic premises. In a split-load or high-integrity CU, two or more RCDs sit downstream of the main switch and upstream of the MCBs, dividing circuits into separately protected groups.

BS 7671 Regulation 411.3.4 requires RCD protection (≤ 30 mA) for all socket outlets (with exceptions for certain industrial/commercial contexts), all circuits in bathrooms, and all circuits supplying luminaires in bathrooms and locations of increased shock risk. The 18th Edition additionally requires RCD protection for cable concealed in walls at a depth less than 50 mm unless the cable incorporates an earthed metal sheath or runs in earthed steel conduit.

The primary protection types are: RCDs (residual current devices) which protect a group of circuits but trip the entire group on a fault; RCBOs (residual current circuit breakers with overcurrent protection) which protect individual circuits with both overload and RCD protection in a single device; and AFDDs (arc fault detection devices), required by the 18th Edition Amendment 1 (2020) on bedroom and living area circuits in student accommodation and similar high-risk premises.

MCBs (miniature circuit breakers) are rated by their tripping characteristic: Type B (3–5× rated current) for domestic lighting and general power; Type C (5–10×) for circuits with higher inrush currents; Type D (10–20×) for motor loads with high starting currents. The MCB current rating must coordinate with the conductor ampacity.

How to wire consumer unit wiring diagram

Confirm incoming supply details Verify the incoming supply voltage (230 V AC in UK), phase configuration (single-phase domestic, three-phase commercial), and the Electricity Network Operator's (ENO) maximum permitted installation fuse rating. In the UK, the distributor's cut-out fuse is typically 60 A, 80 A, or 100 A. The consumer unit main switch must not exceed the cut-out fuse rating.
Assess circuit requirements and protective device types List every circuit: lighting, ring final, radial power, cooker, shower, EV charger, solar/battery, etc. Determine which circuits require 30 mA RCD protection per BS 7671. Decide between a split-load CU with two RCDs or a fully RCBO-fitted CU (all individual RCBOs, no group RCDs). The all-RCBO approach provides maximum selectivity — a fault on one circuit trips only that circuit.
Select MCB / RCBO ratings and characteristics For each circuit, select an MCB or RCBO with a current rating equal to or below the conductor ampacity after correction factors. Select the tripping characteristic: Type B for lighting and standard socket circuits; Type C for circuits with motor loads, transformers, or high-inrush luminaires. Verify the MCB's breaking capacity (Icn) is sufficient for the installation's prospective short circuit current (PSCC), measured at the CU.
Prepare the CU and label all positions Before energising, plan the CU busbar positions. Typically: main switch at top left; RCDs or the first RCBO group immediately downstream; critical circuits (boiler, alarm, freezer) assigned to separate RCD groups to maintain resilience. Label every way clearly in permanent ink, matching the circuit schedule.
Connect incoming supply tails Incoming supply tails (from the meter) connect to the line and neutral terminals of the main switch. The meter is the property of the energy supplier — do not disconnect or reconnect meter terminals yourself. Only the supply tails between the meter and the CU are within the consumer's domain. Ensure tail size matches the main switch rating (typically 25 mm² for 100 A).
Connect the earth bar and protective conductors All circuit protective conductors (CPCs, green/yellow) and the main earthing conductor connect to the CU's earth bar. In a TN-S or TN-C-S (PME) system, the earth bar is also bonded to the neutral bar via the main earthing terminal (MET) of the installation. Verify that the main protective bonding conductors (to gas, water, structural steel) connect to the MET.
Test, inspect, and certify Before energising, perform dead tests: insulation resistance between conductors and to earth (> 1 MΩ at 500 V DC per BS 7671 Table 64); continuity of CPCs; polarity verification at each outlet. After energising: verify RCD trip times (≤ 40 ms at 5× rated operating current for 30 mA RCDs); measure and record PSCC; complete an Electrical Installation Certificate (EIC) with Schedule of Inspections and Schedule of Test Results.

Specifications

UK supply voltage (single-phase)	230 V AC ±10 % / 50 Hz (BS EN 50160)
Standard domestic main switch rating	63 A or 100 A double-pole
RCD trip threshold for socket/residential protection	30 mA (BS 7671 411.3.4)
RCD maximum disconnection time at rated current (30 mA type)	300 ms at 1× IΔn; 40 ms at 5× IΔn (BS EN 61008)
MCB breaking capacity (typical domestic)	6 kA (Icn) minimum; verify PSCC ≤ Icn
Minimum insulation resistance (between conductors, each to earth)	> 1 MΩ at 500 V DC (BS 7671 Table 64)
Consumer unit enclosure material requirement (UK, new domestic)	Metal (non-plastic) enclosure required for fire containment
Wiring standard (UK)	BS 7671:2018 + Amendment 1 (2020) + Amendment 2 (2022)

Safety warnings

Consumer unit work in the UK is notifiable under Part P of the Building Regulations (England and Wales) and equivalent provisions in Scotland and Northern Ireland. Work must be carried out by a competent person registered with an approved Competent Person Scheme (such as NICEIC, NAPIT, or ELECSA) or notified to local authority building control. Unnotified work is a criminal offence and may invalidate buildings insurance and prevent property sale.
The supply tails between the electricity meter and the consumer unit are at mains voltage and cannot be isolated by the main switch alone — they remain live until the energy supplier's cut-out fuse is removed. Never work on the incoming tails or the main switch terminals with the cut-out fuse in place unless you are an authorised person working under a permit. Contact your Distribution Network Operator (DNO) to arrange cut-out removal.
BS 7671:2018 (18th Edition) including Amendment 2 (2022) is the current standard. Always work to the edition adopted by your local authority. Verify that the consumer unit enclosure is rated as a dual-RCD, high-integrity, or all-RCBO type to maintain compliance — plastic (non-metallic) enclosures are no longer permitted for new domestic consumer unit installations per Amendment 3 of the 17th Edition and subsequent requirements.
Test insulation resistance before reconnecting supply. A reading below 1 MΩ between any conductor and earth (at 500 V DC test voltage) indicates a fault that must be rectified before energising the installation. Energising a fault can result in electric shock, fire, or damage to distribution equipment.
Ensure the prospective short circuit current (PSCC) at the consumer unit is measured and documented. Every MCB and RCBO must have a breaking capacity (Icn or Ics) equal to or greater than the measured PSCC. Undersized breaking capacity can result in catastrophic device failure during a fault.

Tools needed

Multifunction installation tester (insulation resistance, continuity, loop impedance, RCD test)
Clamp meter and true-RMS multimeter
Torque screwdriver (for tightening terminals to manufacturer specifications)
DIN-rail cutters and cable-end ferrule crimping tool
Non-contact voltage tester (verify dead before every contact)
Circuit labelling system and permanent marker
Cable strippers appropriate for 16–35 mm² supply tails

Common mistakes

Fitting a plastic (non-metallic) consumer unit enclosure in a new installation — not permitted for new domestic installations; a metal enclosure must be used to provide fire containment.
Connecting circuits requiring 30 mA RCD protection to an MCB downstream of a main switch but without RCD protection — a common error when adding circuits to an older CU, creating non-compliant installations.
Tightening busbar and terminal screws by hand feel rather than to the manufacturer's specified torque — loose connections overheat, causing insulation damage, circuit breaker failure, and potentially fire.
Assigning a critical circuit (boiler, alarm, freezer) to the same RCD group as fault-prone circuits (outdoor sockets, garden equipment) — when the outdoor circuit trips the RCD, the boiler also loses supply, which in winter can cause frozen pipes.
Not measuring and recording the PSCC at the board — leaving the installation without verification that the protective device breaking capacity is adequate for the fault energy available.

Troubleshooting

RCD trips immediately when reset: Cause: An earth fault is present on one or more circuits protected by that RCD — current is flowing from line to earth outside the normal load path Fix: With the RCD off, disconnect all circuits from that RCD by switching off every downstream MCB. Reset the RCD. Switch MCBs on one at a time. When the RCD trips, the last MCB switched on feeds the faulty circuit. Isolate that circuit and test insulation resistance to identify the specific fault.
MCB trips repeatedly on a specific circuit: Cause: Overload (load current exceeds MCB rating continuously) or short circuit on that circuit's wiring or a connected appliance Fix: Disconnect all loads on the circuit. If the MCB holds with no loads, reintroduce loads one by one — the last reconnected load is the overloading or faulty device. If the MCB trips immediately with all loads disconnected, the fault is in the fixed wiring — test insulation resistance on the cable.
RCD fails to trip during testing at rated tripping current: Cause: RCD has developed an internal fault or mechanical degradation; or test equipment calibration is off; or the test current path has high impedance interrupting the test current Fix: Test the RCD with a calibrated RCD tester through the test terminals. If it fails at 1× rated current (should trip within 300 ms for standard type) or 5× rated current (should trip within 40 ms), the RCD must be replaced. Do not use a non-tripping RCD as protection — it provides no protection.

Frequently asked questions

What is the difference between an RCD and an RCBO?

An RCD (Residual Current Device) monitors a group of circuits for earth leakage current and disconnects the entire group if the leakage exceeds the trip threshold (typically 30 mA). An RCBO (Residual Current Breaker with Overcurrent protection) combines RCD and MCB functions in a single device, protecting one circuit for both earth leakage and overcurrent without tripping adjacent circuits.

What does BS 7671 18th Edition require for RCD protection?

The 18th Edition (2018) requires 30 mA RCD protection for: all socket outlets in residential installations (with exceptions for specific industrial use); all circuits in rooms containing a bath or shower; all circuits supplying mobile equipment outdoors; and cables concealed in walls at less than 50 mm depth without additional mechanical or earthed metallic protection. Amendment 1 (2020) also requires AFDDs on certain circuits in higher-risk premises.

Why are two RCDs used instead of one in a split-load consumer unit?

Splitting circuits across two RCDs means that a fault on one circuit trips only half the installation's circuits. A single whole-house RCD would disconnect everything simultaneously — including freezers, alarms, and medical equipment — when any circuit develops a fault. The split-load arrangement maintains supply continuity to critical circuits while the faulty circuit is identified and repaired.

What is an AFDD and when is it required?

An AFDD (Arc Fault Detection Device) detects the characteristic electrical signature of arcing faults — such as a damaged cable being stapled through, a loose terminal, or deteriorated insulation. BS 7671 Amendment 1 (2020) makes AFDDs mandatory on final circuits in specific premises including houses in multiple occupation (HMOs), student accommodation, and care homes, installed in bedrooms and living areas.

Who is permitted to work on a consumer unit in the UK?

In England and Wales, consumer unit work is notifiable under Part P of the Building Regulations. Competent Person Scheme members (registered with NAPIT, NICEIC, ELECSA, or similar approved bodies) can self-certify their work. Non-registered persons must notify the local authority building control before starting and have the work inspected. In Scotland, similar provisions apply under the Scottish Building Standards.

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