Home Fuse Box Diagram: Consumer Unit Layout, MCBs, RCDs, and Circuit Assignments

Home Fuse Box Diagram: Consumer Unit Layout, MCBs, RCDs, and Circuit Assignments — interactive diagram. Open it in the editor to customise components and wiring.

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A reference diagram for a domestic consumer unit (fuse box), covering the main switch, RCD protection, MCBs, and circuit assignments for a typical residential installation.

The term 'fuse box' persists in everyday usage, but most modern domestic installations feature a consumer unit (CU) containing miniature circuit breakers (MCBs) and residual current devices (RCDs), rather than rewirable fuses. Understanding the consumer unit layout is fundamental to safe electrical fault diagnosis and safe isolation.

**Main switch:** The largest device in the consumer unit is the main isolating switch. It disconnects the entire consumer unit from the incoming supply. Switching it off makes all the circuits in the unit dead — but the supply cables entering the unit from the energy meter remain live. The main switch does not make the meter tails safe.

**RCD (Residual Current Device):** An RCD monitors the balance of current between the Live and Neutral conductors. If a mismatch greater than the trip threshold (typically 30 mA for personal protection) is detected — indicating current flowing through an unintended path such as a person — the RCD trips in approximately 30 ms, disconnecting the circuit. This is the primary protection against fatal electric shock in domestic installations.

**MCB (Miniature Circuit Breaker):** Each circuit in the home is protected by its own MCB rated to match the cable and load of that circuit. Common domestic MCB ratings: 6 A (lighting), 16 A (immersion heater, some kitchen), 20 A (ring circuit — older; note UK ring circuits are now typically 32 A), 32 A (ring final circuit, shower), 40 A (cooker). MCBs protect against overcurrent (overload and short circuit) but do not protect against earth leakage shock.

**RCBO (Residual Current Breaker with Overcurrent protection):** An RCBO combines the function of an MCB and an RCD in a single device. It provides both overcurrent and earth leakage protection on a single circuit, making fault isolation simpler — when it trips, the affected circuit is immediately identifiable.

**Split-load consumer unit:** The most common modern arrangement divides circuits between a non-RCD-protected bus (for circuits where nuisance tripping of the RCD would cause problems, such as burglar alarms and freezers — often on a time-delay RCD) and RCD-protected buses. BS 7671 requires RCD protection for all socket circuits, certain lighting circuits, and circuits in high-risk locations.

How to wire home fuse box diagram

Identify your consumer unit type before any work Confirm whether the unit contains rewirable fuses, MCBs, or a combination. Note the presence and location of the main switch, any RCDs or RCBOs, and the number of spare ways. Photograph the unit with MCB labels visible. Note which circuits are on which RCD — important for fault isolation.
Isolate safely at the main switch Switch off the main isolating switch to de-energise all circuits in the consumer unit. Verify dead at the circuit terminals inside the unit with a multimeter. Remember: the meter tails (cables from the energy meter to the main switch) remain live even with the main switch off. Never touch these conductors.
Identify circuit assignments from the schedule Most consumer units have a circuit schedule card inside the door. Verify the schedule is accurate by systematically switching off each MCB and confirming which outlets and fittings lose power. An accurate circuit schedule is essential for safe isolation and future fault finding.
Perform RCD tests Press the TEST button on each RCD with power restored. Each RCD should trip immediately. Reset each RCD in turn after testing. If any RCD fails to trip on its test button, it must be replaced — it is not providing protection.
Check MCB ratings match their circuits Verify that each MCB is rated correctly for its circuit cable and intended load. A 32 A MCB on a 1.0 mm² lighting circuit cable is unsafe — the MCB would not trip before the cable reached damaging temperature. Circuit schedules from the installation electrician or a professional inspection report will confirm correct ratings.
Inspect all visible wiring inside the consumer unit Inspect meter tail connections, incoming neutral bar, and earth bar for signs of overheating, discolouration, or loose connections. All terminals should be tight. Any sign of arcing, burning, or melting requires immediate investigation by a qualified electrician.
Engage a qualified electrician for any work inside the unit Work inside the consumer unit itself — adding or changing MCBs, installing RCBOs, or modifying busbars — must be carried out by a qualified electrician. This is notifiable work under Part P (UK) and equivalent regulations in other jurisdictions. The energy meter tails are the DNO's responsibility and must not be disconnected without the energy company's involvement.

Specifications

Main switch rating (UK domestic)	100 A, double-pole
RCD trip sensitivity (personal protection)	30 mA
RCD trip time (30 mA, BS EN 61008)	Less than 300 ms (max); less than 40 ms at 5× rated current
Ring final circuit MCB	32 A, Type B
Lighting circuit MCB	6 A, Type B
Cooker circuit MCB	40–45 A, Type B or C
Consumer unit standard (UK)	BS EN 60439-3 / BS EN 61439-3
Wiring standard (UK)	BS 7671 (IET Wiring Regulations)

Safety warnings

Work inside the consumer unit — including adding, replacing, or adjusting MCBs, RCDs, or connections — must only be performed by a qualified and competent electrician. In the UK, this is notifiable work under Part P of the Building Regulations for dwellings.
The meter tails (cables between the energy meter and the consumer unit main switch) are live at all times, even with the main switch off. These cables belong to the energy supplier (Distribution Network Operator). Never touch, disconnect, or work near the meter tails without prior arrangement with the energy supplier.
RCDs must be tested regularly using the TEST button. An RCD that does not trip on test provides no protection. Replace faulty RCDs immediately.
Never insert a higher-rated MCB than specified for a circuit's cable. The MCB must be the weakest link — it protects the cable, not just the load. Upsizing an MCB creates a risk of cable overheating and fire.
This diagram is for reference and educational purposes only. Consumer unit work must comply with BS 7671 (UK), NEC/NFPA 70 (US), AS/NZS 3000, or IEC 60364 as applicable. Always engage a qualified electrician.

Tools needed

Non-contact voltage tester
Multimeter (voltage, resistance, continuity)
RCD tester (measures trip time and current — for professional use)
Insulation resistance tester (megohmmeter, for circuit testing)
Loop impedance tester (for Ze and Zs testing — professional)
Flat-blade and cross-head screwdrivers (insulated)
Torque screwdriver (for terminal torque compliance)

Common mistakes

Uprating an MCB to stop nuisance tripping without investigating the cause — the original rating was correct for the cable; the fault must be found, not the protection increased.
Assuming the main switch makes all conductors in the consumer unit safe — the meter tails remain live regardless.
Not testing RCDs regularly — an untested RCD may have mechanically seized contacts and provide zero protection.
Connecting more than the maximum number of conductors to a single terminal on the neutral or earth bar — a terminal rated for one conductor carrying two creates a high-resistance joint.
Selecting the wrong MCB type for a load — Type C MCBs are required for loads with high inrush current (motors, transformers). Using Type B on these loads causes nuisance tripping at startup.

Troubleshooting

RCD trips and will not reset: Cause: Persistent earth leakage fault on the protected circuits Fix: Switch off all MCBs on the affected RCD. Attempt to reset RCD — if it holds, switch MCBs on one at a time. The MCB that causes re-tripping indicates the faulty circuit. Test appliances on that circuit individually to find the earth leakage source.
MCB trips immediately when reset: Cause: Short circuit or sustained overload on the circuit Fix: Switch off the circuit at the MCB and disconnect all loads. Reset the MCB. If it holds, reconnect loads one at a time to find the overloaded device. If the MCB trips with all loads disconnected, the fault is in the fixed wiring itself — test insulation resistance between Live, Neutral, and Earth.
Entire consumer unit loses power (main switch has not tripped): Cause: Failure of incoming supply or fault at the energy meter or service fuse Fix: Check whether adjacent properties also have no power — this indicates a supply network fault, not a fault in your installation. If supply failure is isolated to your property, the main fuse (service fuse, DNO equipment) may have operated — contact your energy supplier. Do not attempt to access or replace the service fuse.

Frequently asked questions

Why does my RCD keep tripping but my MCBs are fine?

The RCD trips when it detects earth leakage current — typically from a faulty appliance, damaged cable insulation, or moisture in a fitting. The MCBs only detect overcurrent. To identify the fault: isolate circuits one by one by switching off MCBs on the affected RCD, then reset the RCD. The circuit that causes re-tripping when its MCB is switched back on is the fault circuit.

What is the difference between a fuse box and a consumer unit?

A traditional fuse box contains rewirable or cartridge fuses that must be replaced after a fault. A modern consumer unit contains MCBs and RCDs, which reset without replacement. Consumer units provide significantly better protection. Rewirable fuse boards are outdated and considered inadequate under current BS 7671 standards, particularly lacking RCD protection.

Can I replace a tripped MCB myself without calling an electrician?

Resetting a tripped MCB by switching it back to the on position is not electrical work and is safe for a householder. However, if the MCB trips again immediately or repeatedly, there is a genuine fault on the circuit. Do not repeatedly reset a tripping MCB — find the fault cause first. Persistent tripping without finding the cause warrants calling a qualified electrician.

How do I test if my RCD is working correctly?

Press the TEST button on the RCD itself — it should trip immediately. If it does not, the RCD may be faulty and must be replaced. This test should be performed every six months. Note: the test button only simulates a fault using an internal resistance; it does not verify the trip time in milliseconds. An RCD tester instrument is needed for time and current accuracy testing.

My consumer unit has no earth leakage protection. Is that still legal?

Older consumer units without RCDs are not illegal to retain in an existing installation, but they do not meet current BS 7671 requirements for new work or significant modifications. If you are adding new circuits, extending existing circuits, or selling the property, a consumer unit upgrade incorporating RCD protection is strongly recommended by the electrical safety standards body.