How to wire a split load consumer unit

Helpful video reference. We use Learn Electrics' UK tutorial "Consumer units - wiring a 2 way split load board" as the video reference here. Learn Electrics is a UK electrical education channel that covers BS 7671 wiring methods, consumer unit design, and testing procedures with clear diagrams - exactly right for understanding how a split load board is assembled.

1. Understand what a split load board actually does

A standard split load consumer unit has one main switch that isolates everything, then two RCDs (residual current devices) downstream of it. Each RCD protects its own group of MCBs. If a fault causes one RCD to trip, the circuits on the other side stay live - you lose half your house, not all of it.

The board is split into two halves: left (sometimes labelled A) and right (B). Your lighting circuits might sit on one side, your socket and appliance circuits on the other. The electrician decides this during the planning stage, aiming to balance the load and put circuits used together on different sides where possible.

2. Plan which circuits go on each side

Before any cables are moved, the electrician draws up the groupings. The goal is to avoid clustering all your power-hungry circuits on one RCD (which increases nuisance tripping) and to make sure the circuit you most need in a fault scenario - typically lighting - stays live when the other side trips.

Common groupings: lights upstairs and downstairs on side A; sockets, cooker, and outdoor circuits on side B. Dedicated circuits for a shower, boiler, or EV charger are often given their own RCBO rather than sharing an RCD, so a fault there only takes out that one circuit.

3. Isolate at the meter cut-out

The electrician isolates the supply at the main fuse in the meter cabinet. This requires the DNO's permission and, in most cases, a brief supply interruption. An approved voltage tester confirms the tails are dead at the consumer unit before the old board is removed.

The meter cabinet seal is there for a reason: it marks the boundary between the distribution network and your installation. Breaking it without authority is illegal. Your electrician has the arrangements to work safely on this side of the installation.

4. Fit the new consumer unit and connect the main switch

The new unit goes on the wall in the same position or nearby, secured with the correct fixings. The incoming meter tails - typically 25 mm² brown and blue insulated cables - are terminated at the main switch. The consumer unit enclosure is bonded to the earth terminal and the main protective bonding conductors are checked and reconnected.

Metal consumer units are required in domestic premises under Amendment 1 of the 18th Edition (BS 7671:2018). Plastic units are no longer acceptable for new installations.

5. Connect the two RCDs

Each RCD is connected in series with its group of MCBs. The live bar (busbar) is split so that circuits on side A draw through RCD-A and circuits on side B draw through RCD-B. The neutral bar is usually common to both sides. Both RCDs must be Type A (detecting pulsating DC as well as AC), and 30 mA rated, for domestic use under the current regulations.

6. Wire the MCBs and route the cables

Each circuit cable is stripped back, its line conductor terminated at its MCB, its neutral at the neutral bar, and its earth at the earth bar. The cables are routed tidily through knock-outs in the unit and dressed flat. Ferruled neutrals and labelled tails make future testing easier.

The electrician works through each circuit in turn, confirming continuity before terminating, and keeps a schedule of which MCB serves which circuit for the label and certificate.

7. Test, label, and certify

Before the supply is restored, insulation resistance tests check that no cable is damaged and no conductor is touching something it should not. Once live, the RCD trip times are tested (must be 300 ms or faster at 1×I_n, 40 ms or faster at 5×I_n), and the loop impedance of each circuit is measured.

Every circuit is labelled at the board and an Electrical Installation Certificate is issued. For a replacement board in a domestic property, a Part P building notice or a certificate from a self-certifying scheme member (NICEIC, NAPIT, ELECSA) closes out the notification.

When to call us

If your split load board keeps tripping one side and you cannot identify a faulty appliance, that needs fault-finding. If you want a new board fitted, upgraded, or reconfigured for solar or battery storage, that is a full consumer unit replacement job. Richard covers Sandwich and east Kent and prices board replacements fairly - call or message for a quote.