How to Run Armoured Cable Shed

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 registered competent person completes and self-certifies the job, or you notify your local authority building control (LABC) before starting if you’re doing it yourself.¹

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)²
• 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

BS 7671 does not set a burial depth: it asks for buried cables to be deep enough to avoid damage from any reasonably foreseeable disturbance of the ground, and for the route to be marked with cable covers or marker tape.³ IET guidance treats 0.5 m as the general minimum depth, rising to 0.6 m under a road carriageway.³ These depths are common practice:

SWA cable’s steel wire armour provides mechanical protection, so cover tiles are not mandatory as long as the route is marked with tape, but they are good practice and will stop a future gardener’s spade.³ Yellow cable route marker tape laid about 150 mm below the finished ground level, well above the cable, is the usual guide.⁴

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, running a new circuit is notifiable under Part P, and a supply to a shed is a new circuit.¹ If you’re a competent DIYer and not using a registered electrician, submit a Building Notice to your LABC before starting. Council fees for this vary: Haringey, for example, charges £523 for 2026/27 for electrical work by someone who is not a registered competent person.⁵ The inspector will want to see the completed work tested to BS 7671 before issuing a completion certificate.¹

Most homeowners use a registered competent person (an electrician registered with NICEIC, NAPIT or another scheme authorised by government) who can self-certify the work.⁶

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 powered trencher saves significant effort: one Surrey hire firm lists a 20 hp tracked trencher at £120 for one day’s hire.⁷

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 to within about 150 mm of the surface, lay cable route marker tape, then finish 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 to the shed consumer unit. Standard 3-core SWA has brown, black and grey cores.⁸ Brown is used as line and the other two as neutral and protective conductor, sleeved blue and green/yellow respectively at each end. Earth the armour at the supply end; it is normally connected at the shed end too, unless your electrician makes the shed a separate TT installation.

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.¹

Cable Sizing Guide

Note: cable sizing must account for the run length (BS 7671 guidance limits voltage drop to 3% for lighting and 5% for other uses; check using its voltage drop tables).⁹ An electrician will calculate this properly.

Cost Summary

Screwfix lists Prysmian 3-core 4 mm² SWA at £156.59 for a 50 m drum, about £3.13 a metre.¹⁰ Builder Depot sells a pair of 20 mm outdoor (CW) armoured cable glands, supplied with shrouds and earth tags, for £4.86 including VAT.¹¹ Toolstation lists a BG metal IP65 garage consumer unit, supplied with a 40 A 30 mA RCD and two MCBs, at £39.99 including VAT.¹²

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: it must be connected to earth at the supply end, and usually at both ends, or it gives no fault protection.³
• 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) cable is not suitable for direct burial; BS 7671 only permits unarmoured cable underground inside a conduit or duct that gives equivalent mechanical protection.³
• Forgetting to notify: an un-notified installation can cause problems when you sell the house, a buyer’s solicitor will ask for the Building Regulations compliance 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.