MCBs, RCBOs, and Consumer Units — Circuit Breaker Types, Sizing, and Replacement for UK Electricians

Every UK domestic rewire, consumer unit replacement, or new circuit installation requires the right combination of MCBs, RCBOs, and a compliant consumer unit. Understanding Type B vs Type C breakers, knowing when an RCBO replaces a breaker and RCD, and selecting an Amendment 3-compliant board are the core decisions every UK electrician faces. This guide covers the practical selection criteria, sizing rules, and product options available at APM Electricals.

MCB vs RCBO — What Is the Difference?

An MCB (Miniature Circuit Breaker) protects a circuit against overcurrent (overload and short circuit). It trips when the current exceeds the rated value for a sustained period, or instantly on a severe fault. MCBs come in Type B, Type C, and Type D trip characteristics — the difference is the level of instantaneous trip current relative to the rated current:

• Type B: trips instantly at 3–5× the rated current — correct for resistive loads such as lighting, sockets, and immersion heaters in domestic properties
• Type C: trips instantly at 5–10× the rated current — used for inductive loads such as motors, air handling units, and fluorescent lighting with magnetic ballasts
• Type D: trips instantly at 10–20× the rated current — reserved for high-inrush loads such as large transformers and welding equipment

In most domestic consumer units in the UK, Type B is standard. Using Type C where Type B is correct does not improve protection — it reduces it, because the instantaneous trip threshold is higher.

An RCBO (Residual Current Breaker with Overcurrent) combines the overcurrent protection of an MCB with the earth leakage protection of an RCD in a single 1-module device. Under BS 7671:2018+A2:2022 (18th Edition Amendment 2) and the associated Amendment 3, RCBOs on individual circuits are preferred over split-load consumer units because each circuit has independent RCD protection. If one circuit's RCD trips, all other circuits remain live.

Consumer Unit Standards — Amendment 3 and All-Metal Requirements

Since January 2016, all new consumer units installed in domestic properties must be housed in a non-combustible enclosure — in practice, an all-metal consumer unit — under BS 7671:2018 Regulation 421.1.201. Plastic consumer units may not be installed in new domestic work. When replacing a consumer unit, the new unit must comply.

Amendment 3 (published 2020, mandatory from September 2022) introduced requirements for:

• Surge Protective Devices (SPDs) — required where the cost of damage from a surge is greater than the cost of the SPD, which in practice means residential installations should have SPD fitted unless a risk assessment determines otherwise
• Arc Fault Detection Devices (AFDDs) — required for specific circuit types including socket circuits in high-risk premises (student accommodation, HMOs, care homes) under 18th Edition Regulation 421.1.7
• Type A RCD protection for circuits that may supply IT or mixed-frequency equipment — standard RCDs are Type AC (detecting sinusoidal AC fault currents only); Type A RCDs detect both AC and pulsating DC fault currents and are required for circuits serving EV chargers, solar inverters, and variable-speed drives

Sizing MCBs — Current Ratings and Cable Size

The MCB rating must be selected so that the breaker protects the cable in the circuit, not just the load. The maximum current-carrying capacity of the cable (Iz) must equal or exceed the rated current of the breaker (In), which in turn must be greater than or equal to the design current of the circuit (Ib). This is Regulation 433.1.1 of BS 7671.

Common domestic MCB sizing by circuit:

• 6A Type B: dedicated lighting circuit (1.0mm² or 1.5mm² T&E)
• 16A Type B: ring final circuit serving 13A sockets (2.5mm² T&E), or shower up to 3.6kW
• 20A Type B: dedicated circuit for dishwasher or washing machine (2.5mm² T&E)
• 32A Type B: ring final circuit (2.5mm² T&E) or electric shower up to 7.2kW (6mm² T&E)
• 40A Type B: cooker circuit for ranges up to 9.2kW (6mm² T&E)
• 50A Type B: larger cooker circuit or EV charger (10mm² T&E)

Always verify cable current-carrying capacity against BS 7671 Appendix 4 tables, accounting for grouping derating factors and thermal insulation where applicable.

When to Use an RCBO Instead of an MCB

An RCBO is the preferred solution when you need to provide 30mA RCD protection to an individual circuit without tying it to a shared RCD in a split-load board. Use an RCBO when:

• Adding a new circuit to an existing consumer unit that has no spare protected ways
• Upgrading from a split-load board where nuisance tripping affects multiple circuits at once
• Circuits require Type A RCD protection (EV chargers, solar PV, inverters) but other circuits on the same board are Type AC
• BS 7671 Regulation 314.1 demands that circuits be arranged so that a fault on one circuit does not interrupt supply to others

RCBOs typically occupy one module space in a consumer unit — the same space as a standard MCB. This means an RCBO-only board gives maximum flexibility, as each circuit is individually protected.

Products at APM Electricals

APM Electricals stocks MK and Axiom MCBs, RCBOs, and consumer units for domestic and light commercial installations. All products are available for same-day collection from the Acton W3 trade counter. Browse the full range at our Consumer Units collection.

MK MCB Single Pole Type B 16Amp 230V

MK Type B MCB rated at 16A, suitable for ring final socket circuits and general purpose domestic circuits wired in 2.5mm² T&E. Fits standard DIN rail consumer units. Single-pole, 230V, 6kA breaking capacity. Compliant with BS EN 60898-1.

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MK MCB Single Pole Type B 32Amp 230V

MK Type B 32A MCB for electric shower circuits up to 7.2kW (with 6mm² T&E) or as an additional ring final circuit protection device. Also used in EV charger sub-boards where a 32A circuit is required. 6kA breaking capacity, BS EN 60898-1 compliant. Same snap-in DIN rail fit as the 16A MK MCB range.

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AXIOM RCBO 16Amp S.P. 1 Module Type A Mini

Single-pole RCBO combining 16A overcurrent protection with 30mA Type A RCD protection in a single 18mm module. Type A detection is essential for circuits supplying EV chargers, variable-speed drives, and any load with a DC component. Fits into standard consumer unit ways. Ideal for upgrading circuits to individual RCD protection without changing the consumer unit. Compliant with BS EN 61009-1.

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Axiom Consumer Unit 12 Way 100A Main Switch

All-metal 12-way consumer unit with 100A double-pole main switch, complying with the BS 7671 requirement for non-combustible enclosures in domestic installations. 12 ways provides sufficient capacity for modern homes with EV charger, solar PV, heat pump, and dedicated appliance circuits alongside standard lighting and socket rings. Compatible with MK and Axiom MCBs and RCBOs. Supplied with neutral bar, earth bar, and mounting hardware.

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Consumer Unit Replacement — Notifiable Work Under Part P

Consumer unit replacement is notifiable work under Part P of the Building Regulations in England. The installation must be carried out by a competent electrician who is either:

• Registered with a Part P competent person scheme such as NICEIC, NAPIT, or Stroma, who can self-certify the work, or
• Working under a building notice submitted to the local authority, with inspection by a building control officer

An Electrical Installation Certificate (EIC) or Minor Works Certificate must be issued on completion. Where a consumer unit is being replaced, a full inspection and test of the existing installation is typically required before the new board is energised.

Browse our full range of consumer units, MCBs, and RCBOs online, or visit APM Electricals at the Acton W3 trade counter for same-day collection.

APM Electricals — 24 Western Avenue, Acton, London W3 7TZ
Call us: 020 8702 8080 | www.apmi.uk
Trade counter open for same-day collection. West London's electrical and plumbing trade supplier.