How to wire a full garage electrical installation

Helpful video reference. Artisan Electrics covers a complete garage wiring job in "How to wire a garage. Garage electrics installation. Sockets, lighting, conduit, consumer unit!". The channel is one of the most-watched UK electrical trade channels, with over 93 million views, and the work shown reflects current BS 7671 practice. Worth watching in full before you start planning your own installation.

1. Plan the circuits you need

Start with a written list of everything you intend to run: general-purpose double sockets, dedicated high-load outlets, lighting, and any fixed equipment such as a compressor, lathe, or EV charger. Each fixed appliance drawing over about 3kW should have its own radial circuit. General sockets can share a ring final or radial depending on the floor area of the garage.

Work out the total connected load and check whether your house main board has capacity. A 100A supply at the house can typically support a 60-80A sub-board at the garage after you account for diversity, but run the numbers properly rather than guessing.

2. Choose a sub-distribution board

Fit a small consumer unit or garage sub-board at the garage rather than running individual circuits all the way from the house. This keeps overcurrent protection local, reduces fault current at the remote end, and makes it far easier to add circuits in future.

Choose a unit with individual RCBO protection on each way rather than a single RCD at the front. That way a tripping circuit takes down only itself, not everything in the garage. Size the enclosure for the number of circuits you need now, plus two spare ways.

3. Size and route the supply cable from the house

The supply cable from the house to the garage sub-board carries the full garage load, so it needs to be sized correctly for both current-carrying capacity and volt drop over the run length. Volt drop must stay within 3% for lighting circuits and 5% for power circuits over the total run from the origin.

• Standard garage with sockets and lighting: 6mm2 two-core steel wire armoured cable is a common starting point.
• Workshop with heavy machinery or EV sub-board: 10mm2 SWA is often needed.
• Underground route: Bury at a minimum depth of 500mm under a garden, 600mm under a path, and 1000mm under a road. Use cable marking tape above the cable.
• Overhead route: Minimum height of 3.5m if there is any vehicle access under the span, 3m if foot traffic only. A catenary wire is required for spans over about 3m.

The supply cable must be protected by an overcurrent device at the house end. Size the protective device to the cable rating, not the expected load.

4. Install cable containment inside the garage

Cables inside the garage that are exposed to mechanical damage must be protected. In a working garage that means steel conduit or heavy-gauge steel trunking, not plastic.

Fix conduit at the correct spacing for the conduit diameter and run it level and plumb. Use steel inspection fittings at every change of direction so you can pull in cables and deal with any future issues without cutting conduit. Where conduit enters the back of a socket or switch box, fit a bush to protect the cable insulation from the cut conduit edge.

5. Fit back boxes for sockets and switches

Use metal back boxes in a workshop environment. Plastic boxes crack under impact and are unsuitable where tools or materials could strike them. Fit boxes at consistent heights: sockets at 450mm from the floor in a workshop are lower than in a house but keep cables out of reach of vehicles. Switches at 1400mm.

Allow at least 35mm depth for socket boxes where the cables make tight bends. In masonry walls, chop the correct size recess rather than letting the box sit proud, which puts strain on the faceplate fixings.

6. Wire the lighting circuit

A garage lighting circuit is typically a 6A or 10A radial in 1.5mm2 twin and earth, run in conduit. LED batten fittings rated IP44 or above are suitable for a damp garage environment and use very little current, which means you can run a reasonable number of luminaires on a single circuit.

Position luminaires to light the work areas evenly. A single central light often leaves the workbench in shadow when you stand at it. Two rows of battens, or battens positioned over each work area, give much better coverage. Put the switch at the entry point so you can light the space before you walk in.

7. Wire the socket circuits

A 20A radial in 2.5mm2 twin and earth, protected by a 20A RCBO, covers up to 50m2 of floor area. A ring final in 2.5mm2 covers up to 100m2. For a standard domestic garage one radial is usually sufficient for general sockets. Any circuit with a socket outlet must have 30mA RCD or RCBO protection under BS 7671.

If you are installing a 32A Commando socket for an EV or workshop equipment, that needs its own dedicated radial circuit sized for the load. Do not connect a Commando socket to the general ring final.

8. Earth and bond all metalwork

Connect the sub-board earth terminal back to the main earthing terminal at the house using a correctly sized protective conductor. If the supply cable is SWA, the armour can serve as the circuit protective conductor provided it has the correct cross-sectional area, or run a separate earth core alongside.

Bond any gas or water services entering the garage to the earthing system. Connect all exposed metalwork: conduit, the sub-board enclosure, metal socket faceplates and any structural steelwork that could become live if a fault occurred.

When to call us

A full garage installation is notifiable work and must be certified. Richard carries out garage supply installations, designs sub-boards for workshop and EV use, and provides the Part P certificate. Covers Sandwich CT13 and surrounding east Kent.