What is the difference between a split load and an RCBO consumer unit?

A split load consumer unit contains two or more 30mA RCDs, each protecting a group of circuits. The MCBs within each group provide overcurrent protection, whilst the common RCD provides residual current (earth fault) protection for all circuits in that group. If a fault occurs on any circuit, the common RCD trips, cutting power to every circuit in the group. An RCBO consumer unit uses individual RCBOs — Residual Current Breakers with Overcurrent — for each circuit. Each RCBO provides both overcurrent protection (like an MCB) and residual current protection (like an RCD) for that individual circuit only. A fault on any circuit trips only that circuit's RCBO, leaving all other circuits unaffected.

What does BS 7671 Regulation 314.1 require about consumer unit circuit division?

BS 7671:2018+A4:2026 Regulation 314.1 states that every installation shall be divided into circuits as necessary to: (a) avoid danger and minimise inconvenience in the event of a fault; (b) facilitate safe inspection, testing, and maintenance; (c) take account of danger that may arise from the failure of a single circuit (such as a lighting circuit supplying a stairway). The RCBO approach — providing individual protection per circuit — better satisfies requirement (a): avoiding inconvenience in the event of a fault. A split load board where a fault on the kitchen circuit also kills the hallway lighting satisfies the regulation technically but not in the spirit of best practice.

What causes nuisance tripping on a split load consumer unit?

Nuisance tripping occurs when the accumulated earth leakage current from multiple healthy circuits exceeds the trip threshold of the shared RCD. Every circuit has a small inherent leakage to earth from cable capacitance, appliance EMC filters, and other sources. A single circuit may leak 1–3mA; perfectly healthy. But a group of 10 circuits sharing one RCD may have a combined leakage of 10–30mA — enough to approach or exceed the 30mA trip threshold. Any additional leakage (from an ageing appliance, a damp environment, or a long cable run) can cause the RCD to trip even though no actual fault exists. An RCBO board eliminates this problem: each circuit's individual RCBO only sees the leakage from that one circuit.

Is a split load consumer unit still acceptable under BS 7671?

Yes, a split load consumer unit remains compliant with BS 7671:2018+A4:2026. The regulations do not mandate RCBO protection for every circuit — they require appropriate RCD protection for the circuits that need it (Regulation 411.3.3 and 411.3.4). A split load board with the circuits allocated correctly across the RCD groups (critical circuits such as freezer and security alarm on separate RCD groups from general circuits) meets the requirements. However, BS 7671 Appendix 15 and the guidance in the On-Site Guide note that RCBO protection per circuit is the preferred approach for new installations due to the improved selectivity (discrimination) and fault isolation.

How many ways should circuits be split in a split load consumer unit?

There is no single prescribed maximum number of circuits per RCD in BS 7671 — the limit is practical: the combined earth leakage current from all circuits on one RCD must not approach the RCD trip threshold under normal operating conditions. As a practical limit, most installers use a maximum of 8–10 circuits per RCD in a split load board. For properties with high earth leakage (many appliances with EMC filters, long cable runs, old appliances), fewer circuits per RCD is advisable to reduce the risk of nuisance tripping. Circuits that must not lose supply unexpectedly — freezer, security alarm, medical equipment — should be on a dedicated RCD group or individual RCBO regardless of the board type.

What happens when an RCBO trips?

When an RCBO trips — either on overcurrent or residual current — the lever moves to a mid position (between on and off), indicating a trip rather than a manual off. The affected circuit loses supply whilst all other circuits remain energised. To reset: investigate and resolve the fault, then push the RCBO lever to the off position fully before switching it back on. An RCBO cannot be reset whilst the fault condition persists — if the RCBO immediately re-trips after reset, the fault is still present on the circuit and must be investigated further using insulation resistance testing (500V DC, minimum 1MΩ between all live conductors and earth).

What is the cost difference between a split load and an RCBO consumer unit?

An RCBO consumer unit costs more in materials than a split load equivalent because individual RCBOs are more expensive than MCBs. A 10-way RCBO consumer unit from a reputable manufacturer (Hager, Schneider, Legrand) costs approximately £180–£350 in materials. The equivalent split load (2-way RCD + MCBs) costs approximately £80–£150. The material cost difference is typically £100–£200 for a standard domestic consumer unit. However, the RCBO option reduces call-back costs when nuisance tripping occurs, simplifies fault finding, and is more likely to satisfy a periodic inspection. Most electricians now install RCBO boards as standard for all new domestic consumer units — the marginal material cost is justified by the better performance and reduced nuisance tripping complaints.

CONSUMER UNIT GUIDE

Split Load vs RCBO Consumer Unit: Which to Install and Why

RCBO boards provide individual circuit protection — a fault on one circuit trips only that circuit. Split load boards protect circuits in groups — one fault can kill multiple circuits. This guide explains the technical differences, BS 7671 requirements, nuisance tripping, and cost comparison.

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12 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1A split load consumer unit has two or more RCDs protecting groups of circuits. All circuits in each group share a single RCD — if a fault occurs on any circuit in the group, the RCD trips and all circuits in the group lose power.
2An RCBO (Residual Current Breaker with Overcurrent) consumer unit uses individual RCBOs for every circuit. Each circuit has its own combined overcurrent and residual current protection — a fault on one circuit trips only that circuit.
3BS 7671:2018+A4:2026 Regulation 314.1 requires that the consumer unit is divided into circuits to avoid danger and inconvenience in the event of a fault. The RCBO approach provides superior circuit independence and fault discrimination.
4Nuisance tripping — where an RCD trips due to accumulated earth leakage current from multiple healthy circuits — is a significant problem with split load consumer units. RCBO boards eliminate this problem by isolating fault current to a single circuit.
5RCBO consumer units cost more in materials but provide better protection, easier fault finding, and greater resilience. They are the preferred choice for new domestic installations and replacements under BS 7671:2018+A4:2026.

01 · Consumer Unit Guide

Split Load vs RCBO Consumer Unit: Which Should You Install?

Consumer unit specification is one of the most common design decisions for UK domestic electricians. The choice between a split load consumer unit (with grouped RCDs) and a full RCBO consumer unit (with individual RCBOs per circuit) affects fault protection, nuisance tripping, cost, and how the installation performs for the customer over its lifetime.

Both types meet the requirements of BS 7671:2018+A4:2026, but the industry trend is firmly towards RCBO boards for all new domestic consumer unit installations. This guide explains why — and when a split load board may still be the appropriate choice.

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02 · Consumer Unit Guide

Split Load Consumer Units: How They Work

A split load consumer unit divides circuits into two or more groups, each protected by a shared 30mA RCD. Within each group, individual MCBs provide overcurrent protection. The RCD provides residual current (earth fault) protection for all circuits in that group.

Typical Split Load Layout

• Main switch: 100A double pole isolator
• RCD 1: 30mA 80A, protecting circuits 1–6 (e.g. ring final, cooker, immersion)
• RCD 2: 30mA 80A, protecting circuits 7–12 (e.g. lighting, garage, boiler)
• MCBs: Individual overcurrent protection within each group
• Fault on any circuit in RCD 1 group → all RCD 1 circuits lose supply

The split load approach was the standard for UK consumer units from the introduction of the 17th Edition (2008) until RCBO boards became more cost-competitive. It remains a valid and compliant approach, but nuisance tripping and poor fault discrimination are well-recognised limitations.

03 · Consumer Unit Guide

RCBO Consumer Units: Individual Protection Per Circuit

An RCBO (Residual Current Breaker with Overcurrent) combines the functions of an MCB and an RCD in a single device. Each circuit in the consumer unit has its own RCBO, providing both overcurrent protection (MCB function) and 30mA residual current protection (RCD function) independently.

RCBO Consumer Unit Advantages

• Fault on one circuit trips only that circuit — all others remain live
• Eliminates nuisance tripping from accumulated leakage across multiple circuits
• Easier fault finding — the tripped RCBO identifies the faulty circuit immediately
• Better compliance with BS 7671 Regulation 314.1 (circuit independence)
• Preferred approach recommended in BS 7671 and the On-Site Guide

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04 · Consumer Unit Guide

BS 7671 Requirements for Consumer Units

The key BS 7671 regulations governing consumer unit design are:

Regulation 314.1 — every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. RCBO boards better satisfy this requirement than split load boards.
Regulation 411.3.4 — in domestic (household) premises, all socket outlet circuits not exceeding 32A and all circuits in bathrooms must be protected by a 30mA RCD. RCBO boards comply automatically; split load boards must ensure these circuits are in an RCD group.
Regulation 531.3 — RCDs shall be selected and installed to minimise the risk of unwanted tripping and to ensure that only the protective device associated with the fault operates. This is the discrimination requirement — RCBO boards provide inherently better discrimination than split load boards.
Regulation 421.1.201 — consumer units in domestic premises must have a non-combustible enclosure (metal or thermoplastic with a metal insert). Applies to both split load and RCBO board types.

05 · Consumer Unit Guide

Nuisance Tripping and Discrimination

Nuisance tripping is the most common complaint about split load consumer units. It occurs when healthy circuits have combined earth leakage current approaching the 30mA trip threshold of the shared RCD:

Common Causes of Nuisance Tripping

• Long cable runs — capacitive leakage increases with cable length
• Appliances with EMC filter capacitors (washing machines, dishwashers, computers)
• Older appliances with degraded insulation — increased leakage without actual fault
• Damp environments — higher leakage through cable insulation
• Solar PV inverters and EV chargers — may have significant leakage by design

An RCBO board eliminates nuisance tripping from accumulated leakage because each RCBO only measures the leakage from its own single circuit. The leakage from a healthy washing machine (2–3mA) is far below the 30mA trip threshold of its individual RCBO, even if 20 other circuits have similar leakage.

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06 · Consumer Unit Guide

Cost and Installation Comparison

Factor	Split Load	RCBO Board
Material cost (10-way)	£80–£150	£180–£350
Installation time	Similar	Similar
Nuisance tripping risk	Higher	Minimal
Fault discrimination	Group level only	Individual circuit
Preferred by BS 7671	Compliant	Preferred

07 · Consumer Unit Guide

Which Consumer Unit to Choose?

Consider Split Load When:

• Budget is very tight and client declines RCBO upgrade
• Extending an existing split load board (matching protection type)
• Simple low-circuit-count installation with low leakage risk

Choose RCBO Board For:

• All new domestic consumer unit installations
• Properties with EV chargers, solar PV, heat pumps (high leakage)
• Older properties where nuisance tripping is likely
• Clients who work from home (cannot afford power loss)
• All rewires — best practice and standard specification

08 · Consumer Unit Guide

For Electricians: Consumer Unit Replacement Best Practice

Consumer unit replacement is one of the most common domestic electrical jobs. Always carry out an EICR condition check on the existing installation before fitting a new consumer unit — defects in the existing circuits (particularly high Zs values or failed insulation resistance tests) must be rectified before the new board is energised. The EIC for the new consumer unit is your certification of the new work, not the existing wiring.

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Frequently Asked Questions: Split Load vs RCBO Consumer Unit

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Split Load vs RCBO Consumer Unit: Which to Install and Why

Key Takeaways

Split Load vs RCBO Consumer Unit: Which Should You Install?

Get the BS 7671 A4:2026 Cheat Sheet — free

Split Load Consumer Units: How They Work

Typical Split Load Layout

RCBO Consumer Units: Individual Protection Per Circuit

RCBO Consumer Unit Advantages

Complete EIC certificates for consumer unit replacements

BS 7671 Requirements for Consumer Units

Nuisance Tripping and Discrimination

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Cost and Installation Comparison

Which Consumer Unit to Choose?

Consider Split Load When:

Choose RCBO Board For:

For Electricians: Consumer Unit Replacement Best Practice

Complete EIC certificates for consumer unit replacements

Frequently Asked Questions: Split Load vs RCBO Consumer Unit

What is the difference between a split load and an RCBO consumer unit?

What does BS 7671 Regulation 314.1 require about consumer unit circuit division?

What causes nuisance tripping on a split load consumer unit?

Is a split load consumer unit still acceptable under BS 7671?

How many ways should circuits be split in a split load consumer unit?

What happens when an RCBO trips?

What is the cost difference between a split load and an RCBO consumer unit?

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