Video reference. We use the Green Hawk Electrical UK tutorial "How to install Armoured Cable" as the video reference here. Green Hawk Electrical is a UK-based electrical contractor filming real installations across the country. The video covers correct trench depth, SWA gland termination and the armour-as-CPC connection that BS 7671 requires.
1. Plan the route and choose the cable size
Walk the proposed cable route and sketch it out. The cable should follow the shortest practical path, avoiding tree roots that could damage the sheath over time, and avoiding areas where you might later need to dig for drainage or landscaping.
Cable size depends on the design current and the length of the run. For a small garden shed with lights and one socket circuit, 2.5 mm² two-core-and-earth SWA covers most jobs up to 30 metres. A garage or home office with a consumer unit, multiple socket circuits and a workshop tool load will need 6 mm² or 10 mm². If in doubt, go one size up — the difference in material cost is small compared to the cost of re-digging a trench.
2. Notify building control
Do this before you lift a spade. In England and Wales, a new circuit to an outbuilding is Part P notifiable. You can notify your local building control authority and have the work inspected on completion, or you can use a registered Part P electrician who self-certifies the work. Either way, you need a completion certificate at the end. Skipping this step creates problems when you come to sell the property.
3. Dig the trench to the correct depth
BS 7671 and the IET On Site Guide specify burial depth based on the ground conditions. In a garden used only by pedestrians, 500 mm deep is the minimum. Under a driveway, path or any area where vehicles could park or drive: 600 mm minimum.
Where the cable rises from the ground to enter a building, it needs mechanical protection from ground level to at least 300 mm above ground — usually achieved with a length of rigid conduit, painted yellow to indicate electrical services.
4. Lay the cable and cover tiles
Lower the cable carefully into the trench with a gentle S-shape rather than pulling it tight between entry and exit points — this gives it room to move with ground settlement. Before backfilling with soil, lay bright orange plastic cable cover tiles (available at any electrical wholesaler or builders merchant) directly above the cable. These protect against an accidental spade strike and serve as a warning to anyone digging later.
Backfill with fine soil or sharp sand first — avoid rubble or stones that could damage the sheath — then replace the turf or paving on top.
5. Terminate with SWA glands at each end
SWA glands are the component that makes armoured cable work properly as an earthed system. At each end of the cable:
- Strip back the outer PVC sheath by 50 to 60 mm to expose the armour wires.
- Slide the gland body, then the cone ring, over the cable sheath.
- Comb out the armour wires evenly over the cone ring.
- Slide the gland body forward to clamp the armour wires between the cone ring and the gland back-nut.
- Tighten the back-nut firmly to grip the armour wires. They should not be able to pull free by hand.
- Thread the gland into the enclosure knockout and tighten the locknut inside.
The gland provides both mechanical retention and the electrical connection between the armour wires and the enclosure. A green/yellow earth tail from the gland earth lug to the earth terminal inside the enclosure completes the connection.
6. Connect the conductors inside each enclosure
Inside the house, the supply end of the cable connects to the consumer unit (or a new sub-board in the garage). Grey outer conductors in a two-core SWA are brown (line) and black (neutral) in modern cable, or red and black in older cable. The bare armour wires — once connected via the gland — serve as the circuit protective conductor.
At the far end, the conductors connect to the outbuilding's consumer unit or distribution board. The armour earth connects to the outbuilding's main earth terminal.
7. Test before energising
Before restoring power, carry out a continuity test on the armour (CPC), an insulation resistance test at 500 V DC between each conductor pair, and a polarity check. These are the tests required by BS 7671 before putting any new circuit into service. Record the results — they form part of the electrical installation certificate.
When to call us
Digging the trench is straightforward with the right tools. The electrical work — terminating the glands correctly, selecting the consumer unit equipment for the outbuilding, connecting to the house board and testing — is where most problems happen. Richard quotes garden circuit jobs as a fixed price, often including the outbuilding consumer unit if needed.
Garden power in Sandwich or east Kent?
Richard installs underground circuits for garages, garden offices and outbuildings, with all the required Part P certification included in the price.
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