SWA (steel wire armoured) cable is a multi-core mains power cable with a layer of helically wound steel wires between the core insulation and the outer PVC sheath. The steel armour provides mechanical protection against crushing, impact, and incidental damage from digging. It is the standard cable for underground electrical supplies, outdoor above-ground runs, and supplies from one building to another (e.g. house to garage or outbuilding). The armour also acts as the circuit earth conductor.

How deep does SWA cable need to be buried?

BS 7671 recommends burying SWA cable at a minimum depth of 450mm under a garden or lawn (where digging is likely to be shallow), or 600mm under a path or driveway. A layer of cable tiles and cable route markers should be placed above the cable before backfilling, to warn future excavators. Despite the armour's mechanical protection, it is not designed to withstand a direct spade or fork strike, so correct depth and warning tiles are still important.

What size SWA cable do I need for a garden shed or outbuilding?

The cable size depends on the maximum load and the cable run length. For a typical garden shed or workshop supplied via a 32A MCB at the house consumer unit, 6mm2 three-core SWA is a common choice - this handles the load at typical UK cable run lengths of 20-30m with acceptable voltage drop. For a garage with an electric vehicle charger (7kW/32A), 10mm2 SWA may be required for longer runs. A qualified electrician should calculate the correct cable size for the specific installation.

Does an SWA cable need conduit?

SWA cable can be buried directly in the ground without conduit - the armour provides its own mechanical protection. Conduit is not normally required but may be used where the cable exits the ground (to protect the cable at the vulnerable above-ground section) or where it passes through a building wall. The armour must be earthed at both ends of the cable run, typically using proprietary SWA glands that grip the armour wires and provide a connection to the earth terminal.

Electrical noun

SWA Cable

/ ɛs dʌb(ə)ljuː eɪ ˈkeɪb(ə)l /

Also known as: steel wire armoured cable, armoured cable, armoured mains cable

Definition

SWA cable (steel wire armoured cable) is a multi-core power cable constructed with a layer of helically wound steel wires sandwiched between the core insulation and the outer black PVC sheath. The armour provides mechanical protection against crushing, impact, and accidental damage during digging. SWA is the standard cable for underground electrical installations, outdoor above-ground runs, and electrical supplies between buildings.

In practice

SWA cable comes in two-core (live and neutral - for single-phase circuits where the armour is the earth) and three-core (live, neutral, and earth - the standard for UK supplies to outbuildings, where the armour is additional mechanical protection but not relied on as the sole earth conductor). Cable sizes range from 1.5mm² to 400mm² per core - for domestic outbuilding and garden socket supplies, 2.5mm², 4mm², 6mm², or 10mm² three-core is most common.

When connecting SWA cable, proprietary brass cable glands grip the armour wires at each end and provide the earth connection from the armour to the enclosure. The gland locknut must be tightened correctly to ensure good electrical contact - a poorly earthed armour creates a shock hazard. SWA cable that is buried must have route marker tiles or a draw cord installed above it, and the route should be recorded on a drawing retained with the building's electrical certificates.

Building Regulations

Relevant PartPart P - Electrical safety in dwellings

StandardBS 5467 - armoured power cables with XLPE insulation

Buried depth (garden)Minimum 450 mm with cable tiles above

Buried depth (driveway)Minimum 600 mm with cable tiles above

Installing a new circuit from a house to an outbuilding or external socket is notifiable work under Part P (as it involves a new circuit) and must be carried out by a Competent Person Scheme registered electrician, or notified to Building Control. An Electrical Installation Certificate must be issued. A supply to an outbuilding that will have plumbing or a shower also requires careful attention to RCD protection and equipotential bonding requirements in BS 7671.

Full Building Regulations guidance

SWA Cable

Just so you know...