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When you ask an electrician for a consumer unit replacement quote, the choice typically narrows to three configurations: a split load consumer unit, a dual RCD consumer unit, or a full RCBO board. The third (full RCBO) is the premium option and is straightforward in concept. The first two (split load and dual RCD) sound similar, are similarly priced, and are often described in interchangeable ways. They are not the same. The differences matter for resilience, fault finding, and how the installation behaves when something goes wrong. This guide explains how each type works, when each is the better choice, and what the practical implications are.
The headline difference
Both configurations divide the circuits into two banks, each protected by its own RCD. The key technical difference:
- Split load consumer unit: The circuits are divided unequally between the two banks. One bank is protected by an RCD; the other bank often has the main switch as its only upstream protection (with each circuit having its own RCBO providing earth fault detection at circuit level).
- Dual RCD consumer unit: The circuits are divided more evenly between two banks, each protected by its own RCD of equal capacity. Both banks have RCD protection.
In practice, the term “split load” has been used somewhat loosely in the industry to describe both arrangements. The specific configuration in your property depends on the electrician’s design choice and the supply characteristics. Faithful Spark explains the actual configuration at survey so you understand what is being installed.
How does each type work in practice?
Split load consumer unit
In a typical split load configuration, the consumer unit has:
- A main switch as the master supply isolator.
- One RCD covering one bank of MCBs (typically the socket and bathroom circuits where RCD protection is most often required).
- The other bank of MCBs (typically the lighting and dedicated appliance circuits) connected directly to the main switch without an upstream RCD.
If a fault occurs on a socket circuit, the RCD on that bank trips. The other bank (lighting and dedicated appliances) continues operating. If a fault occurs on a lighting circuit, only that circuit’s MCB trips; the rest of the lighting bank continues working.
This configuration meets current Wiring Regulations because RCD protection is provided on the circuits that require it. The lighting bank does not require RCD protection on most circuits unless they feed bathrooms or outdoor accessories.
Dual RCD consumer unit
In a dual RCD configuration, the consumer unit has:
- A main switch.
- Two RCDs of equal capacity, each protecting one bank of MCBs.
- Circuits divided across both banks to balance the load.
Every circuit in the unit is protected by an RCD. If a fault occurs on any circuit, the relevant RCD trips, taking out the bank of circuits sharing that RCD. The other bank continues operating.
The dual RCD approach provides more comprehensive RCD protection, which is preferred where the property has many circuits feeding accessories that benefit from RCD coverage (lighting in bathrooms, outdoor lighting, immersion heaters).
What about the full RCBO board?
The premium configuration is the full RCBO board. Each circuit has its own RCBO (a combined MCB and RCD in one device). A fault on any circuit only trips that one circuit. Every other circuit in the property continues operating normally.
The full RCBO board offers the best fault tolerance: a single tripped accessory does not take out a whole bank of circuits. The downside is cost: a full RCBO board adds £80 to £150 above the standard split load or dual RCD price.
For Scottish homes where reliability matters (rentals where tenant inconvenience drives complaints, holiday lets where guests cannot be left in a house with half the lights out, properties with home offices where productivity depends on continuous power), the full RCBO board is increasingly the preferred choice.
When is split load the right choice?
Split load consumer units are well suited to:
- Standard owner occupied homes with typical residential load patterns and where the lighting circuits are simple and predictable.
- Smaller properties with fewer circuits where the additional cost of a dual RCD or full RCBO board is harder to justify.
- Properties on a budget where the lower cost of a split load configuration aligns with a planned spending limit while still meeting current standards.
- Installations where the lighting circuits are short, simple, and unlikely to need RCD protection beyond what is provided at accessory level (bathroom downlights with their own protection, for example).
When is dual RCD the right choice?
Dual RCD consumer units are well suited to:
- Larger homes with many lighting circuits, including outdoor lighting and bathroom lighting, where RCD coverage on the lighting bank provides additional safety margin.
- Properties with extensions or alterations where the lighting circuits feed multiple types of accessory and the additional RCD protection is worth the modest extra cost.
- Scottish landlord rentals where comprehensive protection across the installation is the safer baseline.
- Properties with older wiring where RCD protection on every circuit gives an additional fault tolerance margin.
When is full RCBO the right choice?
The full RCBO board is the right choice for:
- Holiday lets and short term lets where guest inconvenience from a tripped circuit affecting multiple rooms is a serious problem.
- HMO licensed properties where multiple unrelated tenants share circuits and a tripped RCD affecting several bedrooms creates management complications.
- Properties with home offices where continuous operation of the office circuit matters and the rest of the household should not affect office availability.
- Properties with heat pumps, EV chargers, or other major loads on dedicated circuits where any nuisance trips would be particularly disruptive.
- Properties where the homeowner wants the best available installation regardless of modest extra cost.
Practical comparison: what happens when something goes wrong?
Consider a faulty kettle that develops an earth fault and trips the circuit it is plugged into. The behaviour by consumer unit type:
| Configuration | What gets disconnected | What stays on |
|---|---|---|
| Split load (1 RCD on socket bank) | All sockets in the property | All lighting and dedicated appliance circuits |
| Dual RCD | The bank of circuits sharing the affected RCD (typically half the property) | The other bank of circuits |
| Full RCBO board | Only the kitchen socket circuit | Every other circuit in the property |
The full RCBO board offers the best resilience. The trade off is the higher cost. The dual RCD strikes a middle ground between resilience and cost. The split load is the most cost effective but has the largest blast radius when a fault occurs.
Cost comparison
Indicative price differences between the three configurations for a typical 10 way Aberdeenshire residential consumer unit:
- Split load consumer unit: £550 to £700 fully installed.
- Dual RCD consumer unit: £570 to £730 fully installed (approximately £20 to £40 above split load).
- Full RCBO board: £700 to £950 fully installed (approximately £80 to £150 above split load).
The cost difference between split load and dual RCD is small. The cost difference between either of these and the full RCBO board is more meaningful, but the full RCBO board provides genuinely different fault tolerance behaviour. For more on overall consumer unit pricing, see our companion guide on fuse box replacement costs in Scotland.
Other consumer unit features to consider
The split load vs dual RCD vs full RCBO decision is the main configuration choice. Several other features may also be worth considering:
- Surge Protection Device (SPD): Protects sensitive electronics from voltage surges. Adds £80 to £150. Recommended for properties with significant computer, smart home, or AV equipment.
- Arc Fault Detection Devices (AFDDs): Required by the 18th edition Wiring Regulations on certain circuits in HMO and Higher Risk Buildings. Optional but increasingly fitted in residential installations as a fire safety upgrade.
- Spare ways: A consumer unit with extra spare ways accommodates future circuit additions without needing a further upgrade. Most modern installations specify 2 to 4 spare ways.
- Brand: The major UK consumer unit brands (Hager, Wylex, Schneider, BG, Fusebox, Lewden, Crabtree) all meet current standards. The choice often comes down to installer preference, parts availability, and the homeowner’s brand familiarity.
Frequently asked questions
Which type does Faithful Spark recommend by default?
For most Scottish residential installations, we recommend dual RCD as the standard offering and full RCBO as the upgrade option. Split load is appropriate where budget is the primary constraint. The right choice depends on the property’s specific characteristics and the homeowner’s preferences, which we discuss at the survey stage.
Can I upgrade from split load to full RCBO later?
Yes. Replacing the MCBs on the protected bank with RCBOs is technically straightforward provided the consumer unit accepts the same brand of RCBO. The cost is similar to fitting RCBOs from the start. However, replacing devices in an existing unit is sometimes more expensive overall than getting it right at the time of the original consumer unit replacement.
Does the choice of consumer unit type affect the EICR?
All three configurations satisfy current Wiring Regulations and pass an EICR. The EICR records the configuration but does not prefer one over another. The choice is about practical performance rather than compliance.
Is full RCBO a fairly recent option?
Full RCBO boards have been available for many years but have become significantly more common since the 17th edition of the Wiring Regulations introduced more comprehensive RCD requirements. Falling RCBO prices have also made the full RCBO option increasingly cost effective. As of 2026, full RCBO boards are the most common new installation in Aberdeenshire residential consumer unit replacements.
What about three phase consumer units for larger properties?
Three phase consumer units are different again, with separate considerations for the three phase distribution. They are most often used in larger commercial premises. For three phase residential properties (rare in the UK but occasionally seen in larger detached homes with significant load), the design is bespoke. Faithful Spark advises on three phase consumer unit specification at survey.
Book a consumer unit survey
Faithful Spark provides NICEIC certified consumer unit installations across Aberdeen and Aberdeenshire. We discuss the split load vs dual RCD vs full RCBO choice at the survey stage and provide a fixed price quote for the configuration that best suits your property. See our pillar guide on consumer unit upgrades in Aberdeen for the full overview.
Faithful Spark Electricians. NICEIC approved. Local Aberdeen team. Consumer unit upgrades, fuse box replacements, and Electrical Installation Certificates for Aberdeen, Peterhead, Ellon, Fraserburgh and across Aberdeenshire.
