Running a permanent electricity supply to a garden shed is one of the most useful outdoor electrical projects — it enables lighting, power tools, a freezer, EV charging, or a home office setup. The right approach is steel wire armoured (SWA) cable buried in the ground, protected at the house consumer unit and at the shed by an RCD.
This is notifiable work under Part P of the Building Regulations in England, which means either a qualified electrician completes and self-certifies the job, or you notify your local authority building control (LABC) before starting if you’re doing it yourself.
What You’ll Need
Cable:
- 2.5 mm² 3-core SWA (steel wire armoured) cable for runs up to approximately 25 m supplying lighting and light sockets
- 4 mm² 3-core SWA for runs over 25 m or where you want to run workshop tools or EV chargers simultaneously
- 6 mm² if you need a 32 A supply (e.g. dedicated EV charge point or large workshop)
At the house end:
- 32 A MCB (for 6 mm² cable) or 20 A MCB (for 2.5–4 mm²) in the main consumer unit
- RCD protection at the consumer unit (most modern consumer units have this; check with your electrician)
- SWA gland and back box to terminate the cable where it exits the house
At the shed end:
- Small consumer unit or RCD switch-fuse unit (with 30 mA RCD protection, as required by BS 7671 Regulation 411.3.3 for any socket outlets in the shed)
- SWA gland and back box at the cable entry
- Sockets and light fittings as required
For the ground run:
- Cable route marker tape or cover tiles
- Sand bedding (optional but recommended in stony ground)
- Cable route markers at each end
Burial Depth Requirements
Per BS 7671 (18th Edition, Amendment 3) and the IET Wiring Regulations:
| Installation Method | Minimum Depth |
|---|---|
| Under a garden (lawn, flower bed) | 500 mm minimum |
| Under a driveway, patio, or area subject to vehicles | 600 mm minimum |
| Under a road (unlikely for domestic) | 750 mm minimum |
SWA cable’s steel wire armour provides mechanical protection, so cover tiles are not mandatory — but they are good practice and will prevent a future gardener from cutting through the cable with a spade. Yellow cable route marker tape laid 100–150 mm above the cable is standard.
Route the cable to avoid areas that are regularly dug (vegetable beds, flower borders) where possible.
Step-by-Step Process
1. Plan the route
Mark out the shortest safe path from the house to the shed, avoiding tree roots, existing drainage runs, and areas that flood or hold water. Measure the total run including vertical rises at each end.
2. Notify Building Control (if DIY)
In England, electrical work in outbuildings is notifiable under Part P. If you’re a competent DIYer and not using a registered electrician, submit a Building Notice to your LABC before starting. Fee is typically £90–£180. The inspector will want to see the completed work tested to BS 7671 before issuing a completion certificate.
Most homeowners use a registered electrician (Part P registered, under a scheme such as NICEIC, NAPIT, or ELECSA) who can self-certify the work.
3. Isolate and work on the consumer unit
This step must be done with the mains supply isolated — or by a qualified electrician. Add the new MCB to the consumer unit. Run a short length of SWA from the consumer unit to an exit point at the house wall, using a suitable SWA gland to terminate at the wall.
4. Dig the trench
Depth: 500 mm for most garden runs. Width: 100–150 mm (wide enough for the cable and to allow backfill without sharp stones against the insulation). A trenching spade is the right tool for straight runs; a mattock helps on compacted ground. For long runs, a hired electric trenching tool (about £100–£150/day from a hire centre) saves significant effort.
5. Lay the cable
Lay the cable in the bottom of the trench. If the ground has sharp stones, lay a thin bed of sand first. Uncoil the cable as you go — don’t drag it. Allow extra at each end for the connections (minimum 500 mm).
6. Backfill and mark
Backfill with fine soil or sand, then lay cable route marker tape 150 mm above the cable, then continue backfilling. Tamp firm and reinstate the ground surface.
7. Terminate at the shed
Bring the cable up through the shed floor or wall. Use an SWA gland to clamp the armour and provide strain relief. Connect the three conductors (brown = line, grey = neutral, green/yellow = earth) to the shed consumer unit. Earth the armour at both ends — this is important for fault protection.
8. Test and commission
Test with an insulation resistance tester and continuity tester before switching on. A registered electrician will carry out a full initial verification test to BS 7671, record results on an Electrical Installation Certificate (EIC), and submit the Part P notification.
Cable Sizing Guide
| Run Length | Shed Load | Recommended Cable |
|---|---|---|
| Up to 25 m | Lights + 2–3 sockets | 2.5 mm² 3-core SWA |
| 25–40 m | Lights + sockets | 4 mm² 3-core SWA |
| Up to 40 m | Workshop (power tools) | 6 mm² 3-core SWA |
| Any | EV charge point (7 kW) | 6 mm² 3-core SWA min., dedicated circuit |
Note: cable sizing must account for the run length (voltage drop must not exceed 5% for the final circuit — check using the voltage drop tables in BS 7671). An electrician will calculate this properly.
Cost Summary
| Item | Typical Cost |
|---|---|
| 2.5 mm² SWA cable (per metre) | £3.50–£6.50/m |
| 4 mm² SWA cable (per metre) | £5–£9/m |
| SWA glands (pair) | £15–£35 |
| Shed consumer unit / RCD unit | £60–£150 |
| Cable route marker tape | £5–£15 |
| Electrician labour (full job, 30 m run) | £300–£600 |
| Part P notification (electrician self-certifies) | Included in above |
| LABC Building Notice (DIY route) | £90–£180 |
For a typical 25 m run with a small shed consumer unit and 3 sockets fitted:
- Materials: approximately £200–£350
- Electrician labour and certification: £300–£600
- Total professionally installed: £500–£950
Common Mistakes to Avoid
- Not earthing the armour at both ends: this defeats the fault protection the armour provides.
- Burying too shallow: a 300 mm trench might be quicker, but a single spade strike in future will cut through the cable.
- Using non-armoured cable outside: ordinary T&E (twin and earth) flex or even conduit without armoured cable is not suitable for direct burial.
- Forgetting to notify: an un-notified installation can cause problems when you sell the house — a buyer’s solicitor will ask for the Part P certificate.
- Undersizing the cable: using 1.5 mm² or 2.5 mm² on a long run powering a large load will result in voltage drop and potentially overheating.
