Steel Wire Armoured (SWA) cable is the standard choice for outdoor, underground, and exposed wiring in UK electrical installations. This guide covers cable selection, burial depth, gland types and termination, bonding the steel wire armour, testing procedures, and full BS 7671 compliance requirements.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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Steel Wire Armoured (SWA) cable is a heavy-duty power cable with an outer layer of galvanised steel wires providing mechanical protection and an earth continuity path. It is the standard cable type for underground installations, outdoor wiring, and any situation where the cable is exposed to potential mechanical damage.
SWA cable construction consists of several layers: copper (or aluminium) conductors, PVC or XLPE insulation on each conductor, an inner PVC bedding sheath, the steel wire armour layer, and an outer PVC oversheath. The steel wire armour provides substantial mechanical protection against impact, crushing, and penetration — making the cable suitable for direct burial, surface mounting on walls, and installation in exposed locations.
SWA cable is available in a wide range of sizes from 1.5 mm squared to 400 mm squared and above, in 2-core, 3-core, 4-core, and 5-core configurations. For most domestic and light commercial installations, 2-core and 3-core SWA in sizes from 2.5 mm squared to 25 mm squared covers the majority of applications — including supplies to outbuildings, garages, EV chargers, garden offices, and external lighting.
The key advantage of SWA cable over standard twin and earth (T&E) cable is its mechanical protection and the earthed metallic covering. Under BS 7671, cables with an earthed metallic covering are exempt from the requirement for additional RCD protection when installed in walls (Regulation 411.3.4), and the armour provides an inherent CPC that does not rely on a separate earth conductor.
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Selecting the correct SWA cable follows the same BS 7671 cable sizing process as any other cable type, but with specific tables for SWA current-carrying capacity and voltage drop values.
The installation method determines which current-carrying capacity table to use. SWA cable is commonly installed using Reference Method D (direct buried), Reference Method C (clipped direct to a surface), or Reference Method B (in trunking or conduit). Each method has different current ratings because the cable's ability to dissipate heat varies with the installation method.
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When installing SWA cable underground, BS 7671 and the IET On-Site Guide provide guidance on minimum burial depth. The requirements depend on the location and whether additional mechanical protection is provided.
Minimum 500 mm burial depth below finished ground level. This applies to general areas such as gardens, driveways, and fields where the cable is direct-buried with no additional mechanical protection.
Route marker tape (yellow warning tape marked "CAUTION — ELECTRIC CABLE BELOW") must be placed at least 150 mm above the cable to warn anyone digging in the area.
Where additional mechanical protection is provided — such as cable tiles, concrete troughs, or duct — the cable may be installed at a shallower depth. Cable tiles placed directly above the cable provide impact resistance and a visual warning during excavation.
Under roads and areas subject to heavy vehicular traffic, a minimum depth of 600 mm is recommended even with SWA cable, due to the increased risk of damage from construction vehicles and resurfacing work.
The cable route should be recorded on the installation drawings and provided to the client. For domestic installations, a simple sketch showing the cable route, depth, and key reference points (distance from walls, fences, and other permanent features) is essential. This prevents future damage when the client or their gardener digs in the area.
Where SWA cable crosses under a road, driveway, or patio, it should be installed in a duct (typically 63 mm or 100 mm diameter MDPE or PVC duct) to allow future replacement without excavation. The duct should extend at least 300 mm beyond the edge of the hard surface on each side.
Earthing arrangements for outdoor installations — check PME restrictions for supplies extending outside the main equipotential zone.Correct termination of SWA cable is critical for both mechanical integrity and earth continuity. The cable gland grips the steel wire armour, provides strain relief, and maintains the electrical connection between the armour and the enclosure or earth system.
There are three main types of cable gland used with SWA cable in UK installations:
The CW (Cable Wire) gland is the standard indoor gland for SWA cable. It consists of a body, a compression ring (cone), an armour lock ring, and a back nut. The compression ring bites into the steel wire armour to provide mechanical grip and earth continuity. CW glands are rated IP40 and are suitable for dry indoor environments. They are the most commonly used gland type for consumer unit and distribution board entries.
The CX gland adds a neoprene shroud (outer seal) to the CW gland design, providing weatherproof sealing to IP68 when fully assembled. CX glands are used for outdoor installations, underground cable entries, and any location where the gland may be exposed to moisture. The shroud seals around the cable oversheath, preventing water ingress at the termination point.
BW glands are designed for cable types with a continuous metallic sheath rather than wire armour — such as MICC cable or older lead-sheathed cables. They are not used with standard SWA cable but are mentioned here to avoid confusion when sourcing glands.
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Start free trialThe steel wire armour of SWA cable serves as an earth continuity path and must be bonded at both ends to ensure effective fault protection. The gland provides the primary bonding connection between the armour and the metallic enclosure (consumer unit, distribution board, junction box, or metal accessory).
For the armour to serve as the sole CPC for the circuit, its cross-sectional area must meet the requirements of BS 7671. Table 54.7 sets out the minimum CPC size based on the line conductor size, and the adiabatic equation (k squared S squared greater than or equal to I squared t) must be verified to ensure the armour can withstand the prospective fault current for the disconnection time of the protective device.
In most domestic installations using 2-core SWA cable (sizes up to 16 mm squared), the steel wire armour cross-sectional area is sufficient to act as the CPC. However, there are situations where an additional CPC conductor may be required:
In these cases, use 3-core SWA cable and connect the third core as the CPC, bonded in parallel with the armour at both ends.
At the supply end, ensure the gland makes a solid connection to the metallic consumer unit or distribution board enclosure. At the load end, if the cable terminates at a non-metallic enclosure (such as a plastic isolator or junction box), an earth tail must be connected from the gland earth tag to the circuit earth terminal.
SWA cable is the cable of choice for outdoor and underground wiring in the UK. Its mechanical armour provides protection against accidental damage during gardening, landscaping, and construction work, and the cable is rated for direct burial without additional ducting (though ducting is recommended under hard surfaces for future maintenance access).
Dig the trench to the required depth (minimum 500 mm) with a flat bottom. Remove any sharp stones, rocks, or debris that could damage the cable oversheath. Place a 50 mm layer of fine sand or sifted soil at the bottom of the trench as a bedding layer. Lay the cable on the bedding, then cover with another 50 mm layer of sand or sifted soil before backfilling.
Place yellow warning tape marked "CAUTION — ELECTRIC CABLE BELOW" at least 150 mm above the cable. This provides an early warning to anyone digging in the area. At cable route changes of direction and at each end, install permanent route markers (posts, plaques, or indelible markings on adjacent structures) so the cable route can be traced in future.
Where the cable enters a building, use a duct through the wall to prevent damage from building movement and to maintain the building's weather seal. The duct should be sealed at both ends with duct sealant to prevent moisture and vermin entry. The cable should rise vertically from ground level to the entry point — avoid running horizontally along the outside of the building at low level where it is vulnerable to damage.
For EV charger installations, SWA cable from the consumer unit to an external charge point is the standard approach. Check PME restrictions if the supply is TN-C-S — a separate earth electrode may be required for the charge point installation.
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Try it free for 7 daysTesting an SWA cable installation follows the standard BS 7671 testing sequence, but with specific attention to the armour continuity, insulation resistance between cores and armour, and the earth fault loop impedance through the armour CPC.
Record all test results on the Electrical Installation Certificate schedule of test results. For SWA circuits, note the armour resistance separately as this is a useful reference for future periodic inspection — an increase in armour resistance may indicate corrosion or deterioration of the gland connections.
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