Helpful video reference. The video above explains how to extend consumer units and add a second consumer unit in a UK domestic installation, covering the supply connection options and the earthing considerations. It gives a clear overview of what the job involves before you speak to an electrician.
1. Why add a second consumer unit?
The most common reasons are: the main board is full with no spare ways; a house extension or converted loft needs several new circuits; a detached garage or outbuilding needs its own board; or the existing installation is TT (earth electrode) and a new part of the house has different earthing requirements.
Adding a second board inside the same building is straightforward. Running supply to a separate structure (garage, shed, summer house) involves additional considerations around earthing and cable burial depth - see the greenhouse supply guide and the outbuilding supply guide for those cases.
2. Assess the existing installation
Before anything else, your electrician will check the main consumer unit: how many spare ways, the rating of the main switch, and whether the earthing arrangement can support additional circuits. On a TN-C-S (PME) supply - the most common in the UK - the second board shares the main earthing terminal. On a TT supply, the earth electrode resistance needs measuring to confirm it is adequate for the increased load.
The DNO's meter and cutout fuse also limit the total load. If the main fuse is 60 A and the house already uses close to that, adding a second board for heavy circuits like an EV charger or workshop machines may require a DNO upgrade first.
3. Choose how to supply the second board
There are three common supply methods:
- Spare way in the main board. If a suitable MCB position is free, a cable runs from it to the second board's main switch. Clean, simple, and the MCB provides fault protection on the supply cable.
- Henley block on the meter tails. A Henley block (a parallel connector) tees off the meter tails after the meter and before the main switch. The tapped-off supply feeds a switch-fuse isolator, then on to the second board. This is used when the main board is full or when an independent main switch for the second board is needed.
- Switch-fuse isolator. When a Henley block is used, a switch-fuse unit (typically 60 A or 100 A) acts as the main isolator and provides fault protection for the supply cable to the second board.
4. Size the supply cable correctly
The cable between the supply point and the second board is sized for the maximum demand of the new circuits under BS 7671 Appendix 4. The calculation takes into account grouping (if cables share containment), thermal insulation (if cables run through insulated cavities), and ambient temperature.
For a typical domestic sub-board serving sockets and lighting, 10 mm2 twin and earth is common. A board serving an EV charger, electric shower, or workshop equipment will need a larger cable. Getting this wrong is one of the most consequential errors in domestic installations - undersized cable overheats.
5. Choose the right type of second consumer unit
The 18th Edition of BS 7671 requires all circuits to have 30 mA RCD protection. A second consumer unit must comply, which means choosing between:
- Dual-RCD split-load board. Two RCDs cover groups of circuits. If one RCD trips, half the board stays on. Less expensive, but a single fault can still take out several circuits at once.
- All-RCBO board. Each circuit has its own RCBO (combined MCB and RCD). A fault on one circuit trips only that circuit. More expensive but the most resilient option.
6. Earthing and bonding
The earthing conductor from the second board runs back to the main earthing terminal (MET) at the first board. It must be sized correctly: typically 10 mm2 for a board supplied by 10 mm2 or 16 mm2 cable. A separate earth electrode is not permitted for a sub-distribution board on a PME supply.
If there are any extraneous conductive parts in the area served by the second board (gas or water pipes, structural steelwork), supplementary bonding may be required.
7. Testing and certification
Once the installation is complete, full initial verification tests are carried out: insulation resistance on every circuit, earth fault loop impedance, RCD trip times, and polarity. The results are recorded on an Electrical Installation Certificate (EIC), which is the document that registers the notifiable work with your local authority through the electrician's scheme provider.
You should receive the EIC, keep it with your property deeds, and hand it to the next owner when you sell. Buyers' solicitors increasingly ask for it.
When to call us
Richard installs second consumer units across east Kent: in house extensions, garage conversions, home workshops, and annexes. He will assess the existing installation, calculate what is needed, carry out the work, test it fully, and issue the certificate. Call or message to discuss what you are planning.
Need a second consumer unit in east Kent?
Richard handles second board installations, from assessing the existing supply through to full certification. Covering Sandwich, Deal, Dover, and surrounding areas.
Contact Richard