What's involved in wiring a summer house electrical supply

Helpful video reference. We use Benjamins Drive's video "How I Wired This Summer House in Conduit & SWA (UK Electrician)" as the video reference here. It covers conduit entry, SWA gland termination, the earthing decision, and the sub-board layout in a real UK installation.

1. Work out what you want to power

Write down every appliance you might ever run in the summer house: lighting, sockets, a heater, a television, a kettle. Add the wattages and divide by 230 to get the current in amps. That figure, plus a safety margin, tells you the rating you need at the sub-board.

Most summer house supplies are sized at 32 A. If you want an electric heater of 3 kW and a kettle as well, you are already at 26 A continuous, so 32 A is about right. A hot tub or workshop lathe is a different calculation entirely.

2. Measure the cable route and size the cable

Measure the actual cable path from the house consumer unit to the summer house consumer unit, including the rises through the walls at each end. Do not measure as the crow flies across the garden.

For a 32 A circuit, 6 mm² two-core-and-earth steel wire armoured (SWA) cable handles most runs up to about 30 metres before voltage drop becomes an issue (the limit is 3% for final circuits under BS 7671). Anything longer than that, or if you want 40 A protection, move up to 10 mm².

If you are unsure, use a proper voltage-drop calculator (there are free ones from the IET and cable manufacturers). The cable size must be confirmed before you order.

3. Notify building control

Installing a new supply to a separate structure, including fitting a consumer unit in the summer house, is notifiable work under Part P of the Building Regulations (England). You have two options.

The straightforward one: hire a Part P registered electrician. They can self-certify the work, notify the local authority on your behalf, and issue the electrical installation certificate that your insurer and any future buyer will want to see.

The DIY route: submit a building notice to your local authority before you start. They will want an inspection, and you will need to pay the fee (typically £150-£300). Either way, the work must be certified.

4. Dig the trench

Steel wire armoured cable is robust, but it still needs to be buried at the correct depth to protect it from garden forks and spades.

• Open garden soil: minimum 600 mm deep.
• Under a path or patio: minimum 450 mm, with cable protection tiles laid directly above the cable and a marker tape 150 mm below the surface.
• Under a driveway or roadway: 900 mm minimum, or run in heavy-duty conduit.

Keep the trench as straight as you can, and sketch a plan with measurements from fixed points (house wall, fence post) so you can find the cable again years from now. Put this plan in the property file.

5. Lay and terminate the SWA cable

SWA is heavy and stiff. Unroll it from the drum as you go, rather than coiling it off a reel by hand. Feed it into conduit where it exits the house wall and where it enters the summer house, to protect the outer sheath from abrasion.

At each end you need an SWA gland: the correct size for your cable, tightened with the right torque so the armour wires are firmly gripped. The armour connects to earth at both ends. Seal every wall entry point with outdoor-rated compound or a proper wall-seal fitting to stop moisture working its way in along the cable.

6. Install the sub-board in the summer house

You need a small consumer unit inside the summer house. In an outbuilding, every circuit must have 30 mA RCD protection, either via a dual RCD board or individual RCBOs. A two-way board with an RCD is typical for a simple summer house (one lighting circuit, one socket circuit).

Mount it on the wall close to where the SWA enters, at a height you can read easily. Label every way clearly. If the summer house is used by anyone other than yourself, a lockable enclosure is worth considering.

7. Earth the outbuilding correctly

This is the step most DIYers get wrong, and it matters a lot.

Most UK homes have a PME (TN-C-S) earth, where the neutral and earth are combined in the supply cable. You must not extend that combined earth terminal to a separate building. If there is ever a fault on the DNO's neutral, the combined earth (and everything bonded to it, including metal water pipes) can rise to a dangerous voltage relative to true earth. Outdoors, that creates a lethal situation.

Instead, drive a local earth rod into the ground outside the summer house and use a TT earthing arrangement at the sub-board. You need an RCD at the head of the summer house supply (at the house end) to provide protection because TT earth impedances are higher than TN earths.

If your house has a TN-S (separate earth) supply, confirmed by your DNO, you may be able to extend the earth cable. Check with your network operator before assuming.

8. Test and certify before switching on

Before you energise the summer house supply, carry out the following tests at minimum: insulation resistance (each core to earth, cores to cores), earth continuity from rod to sub-board earth bar, and polarity (line is line, neutral is neutral at every accessory).

The person who connects the house-end and carries out the tests must be competent to do so and must issue either a Minor Works Certificate or, more likely for a full new circuit, an Electrical Installation Certificate. Keep this with the property paperwork.

When to call us

The digging and cable-laying are often the bit homeowners are most comfortable with. The house-end connection, earthing decision, and testing are the parts where a mistake has real consequences. Richard covers Sandwich and east Kent and can handle the whole job or just the notifiable parts at the ends.