How to Wire a Consumer
Unit in a Garage
A practical homeowner guide to wiring a sub-board in a detached or attached garage. Cable sizing, earthing, weather protection plus why every UK install needs a registered electrician under Part P. No DIY shortcuts.
A garage consumer unit is a small sub-main board fed from the main consumer unit in the house, normally over SWA armoured cable sized at 6mm² or 10mm² depending on the load plus run length. The garage board carries its own RCBOs for sockets, lights plus any EV charge point. Earthing follows the supply system used at the property (TN-S, TN-C-S or TT). Like all consumer unit work, this is notifiable Part P work plus must be installed by a registered electrician.
Four figures that shape
any garage sub-board
These figures change with the load plus the cable run. Every garage install is sized to the actual circuits being supplied not a one-size-fits-all rule.
SWA Sub-main
Typical armoured cable size feeding a domestic garage. Sizing depends on load plus cable run length to avoid voltage drop.
Feed MCB
The breaker in the main board protecting the sub-main feed. Sized to the diversified load of the garage circuits.
Enclosure Rating
Recommended ingress rating for garage boards plus all sockets in damp, unheated or external locations.
Notifiable Work
A garage sub-board is always notifiable. It must be done by a Part P registered electrician then certified.
How a garage board
actually goes in
Whether the garage is attached, detached or 30 metres down the drive, the install follows four stages. Skipping any one breaks compliance plus risks a failed EICR at sign-off.
Survey plus Size
Total load is calculated (sockets, lights, EV charger). Run length is measured. SWA cable plus feed MCB are sized to BS 7671 Appendix 4 voltage drop limits.
Run the SWA
Armoured sub-main is run from the main board. Buried direct (with marker tape) plus capped at 600mm or run on cable tray. Glands fitted at both ends.
Mount Garage Board
IP-rated enclosure mounted at sensible height. SWA terminated, armour earthed via brass gland plus banjo, RCBOs fitted for each garage circuit.
Test plus Certify
Full BS 7671 test on the new sub-main plus all garage circuits. Earth fault loop measured at the garage board. EIC issued plus Building Control notified.
Wiring a garage consumer unit follows the same rules as the main board with a few extra considerations
A garage in a UK home almost always runs off a sub-main from the main consumer unit. That sub-main feeds a smaller dedicated consumer unit in the garage, which then distributes power to lighting, sockets plus any vehicle charger. This guide explains how that wiring is done so you know what to expect when an electrician quotes the job. It is not a how-to for DIY. Garage consumer unit work is notifiable under Part P of the Building Regulations like any other consumer unit work.
Sizing the sub-main
The starting question is always the same: what is the garage going to run? A typical garage with lights, a few sockets plus an outdoor light might pull 16 to 20 amps in real terms. Add a 7kW EV charger plus that jumps to 32 amps minimum on its own dedicated circuit. The sub-main has to carry the diversified total of all those loads.
For most domestic UK garages the sub-main is SWA (Steel Wire Armoured) cable. The two common sizes are:
- 6mm² SWA for shorter runs (under about 25 metres) feeding standard garage circuits without an EV charger.
- 10mm² SWA for longer runs, higher loads or where an EV charger is included on the garage board.
Cable size is calculated to two limits: the current carrying capacity of the cable plus the BS 7671 voltage drop allowance. For lighting plus mixed loads the limit is normally 5% of nominal supply voltage between the origin plus the furthest point. On a 230V supply that is roughly 11.5V total drop. The cable is sized to stay under that figure across the whole run.
The route plus the protection
SWA can be buried directly in the ground at a minimum depth of 600mm, marked with yellow warning tape laid above. It can also run inside ducting or on cable tray for surface installs. Where it enters the house plus the garage it terminates through a brass gland which connects to the armour, plus an earthing banjo links the armour to the main earth at both ends. The armour itself is what gives SWA its name plus its protection. It also doubles up as a circuit protective conductor in many installs.
At the house end the sub-main lands under a dedicated MCB or RCBO in the main consumer unit. This is the feed protection, normally a 40A or 63A device sized to match the SWA. At the garage end the sub-main lands on the main switch of the garage board.
The garage board itself
A garage consumer unit is no different in principle to the one in the house. It contains a main switch, a busbar plus protective devices. The differences are size plus environment. Garage boards are usually 4 to 8 way enclosures. They should carry an IP rating of at least IP3X internally and IP4X is preferable in damp or unheated garages. Sockets in the garage should be IP44 minimum if they could be exposed to water splash.
Each garage circuit gets its own RCBO. Lights on one. Sockets on another. The EV charger gets a dedicated 32A circuit. AFDD protection is now recommended on circuits feeding sleeping accommodation plus is increasingly being added on EV circuits where regional schemes require it.
Typical garage consumer
unit install costs
Pricing depends on cable run length, whether trenching is needed plus how many circuits are on the garage board. These are typical Milton Keynes plus Bedfordshire ranges.
Garage Sub-board Install Cost Bands
Prices include parts, labour, EIC certification plus Building Control notification. Trenching, ducting or making good driveways is normally a separate quote.
From survey to certificate
across one or two days
A garage sub-board with a buried SWA run normally splits across two visits. A short surface install often completes in a single day.
Survey plus Trench
Load is calculated. Cable route surveyed. If buried, trench dug to 600mm with warning tape laid. Existing supply earthing checked.
SWA Pulled
Armoured cable pulled through the trench or surface route. Glands fitted at both ends. Trench backfilled with cable safely placed.
Boards Terminated
Main board terminated under a dedicated MCB. Garage board mounted, fed in, RCBOs fitted plus garage circuits wired off.
Test plus Certify
Full BS 7671 test sequence. Earth fault loop measured at the garage. EIC issued plus the work notified to Building Control.
Four checks for any
garage board install
SWA size matched to load
A 6mm² cable will not cope with a long run plus an EV charger. Insist on a sized cable calculation including voltage drop before any digging starts.
IP-rated garage enclosure
Garages are damp, dusty environments. The board enclosure plus all sockets need an appropriate IP rating, IP44 minimum on garage sockets.
Dedicated EV circuit
A 7kW EV charger needs its own 32A radial circuit on the garage board, with the right RCD type plus earthing strategy. It cannot share with sockets.
EIC plus building control notice
Get the Electrical Installation Certificate plus Building Control notification in writing. Both are needed for property sale, EICR plus insurance.
Consumer Unit Upgrades in Milton Keynes
C-Lec Electrical fits garage sub-boards, EV charger feeds plus full consumer unit upgrades to BS 7671 across Milton Keynes, Bedford plus surrounding Bedfordshire. Free quote, certified install, paperwork done in full.
Surface SWA vs
buried SWA run
For a detached garage the sub-main run is the biggest variable. Both surface plus buried installs are compliant. Each suits a different garden setup.
Quicker to install
- ●Cable runs along walls, fences or cable tray. Clipped at regular intervals to BS 7671.
- ●No trenching needed. Install completes in a single day for short runs.
- ●Lower labour cost. Easier to inspect plus replace later if needed.
- ●Visible cable run can affect garden aesthetics. UV resistant SWA holds up well outside.
- ●Best where the garage is close to the house plus a tidy fixing route exists.
Cleaner finish
- ●Cable buried at minimum 600mm depth with yellow warning tape laid 150mm above.
- ●Trenching required across lawn, drive or paving. Reinstating the surface adds cost.
- ●Cable hidden plus protected from physical damage. Long service life expectation.
- ●Higher initial labour cost. Best done before landscaping or new driveways are laid.
- ●Best on longer runs across open garden where surface fixing is impractical.
For the wider context on consumer unit types, RCDs, RCBOs plus when an upgrade is genuinely needed, head back to our full guide to consumer units where every common homeowner question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers garage sub-boards, RCBO boards, AFDDs, landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit wiring
If you have not seen a consumer unit broken down to its parts before, our short explainer on what is a consumer unit covers every component in plain language. To understand the wiring sequence on the main board itself, our walkthrough on how to wire a consumer unit is the right next read. Once you are ready to look at numbers for the main board, our pricing breakdown for how much to change a consumer unit sets out the current 2026 ranges. If you need a board fitted in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.