SWA Cable: The Steel Wire Armoured Guide for Garden Rooms, Garages and Outdoor Circuits

A homeowner pays a builder £600 to dig a 25-metre trench from house to garden room, drops a 25-metre length of 4mm² armoured cable into it, backfills, lays the lawn back, and only then realises nobody fitted a banjo earth tag at either gland. The electrician returns, condemns both ends, and the lawn comes back up. SWA cable is unforgiving like that. Get the spec wrong and the trench has to come back open. This page tells you what SWA actually is, what size you need, the regulations that govern burial and earthing, and the small pieces of brass hardware that the whole circuit's safety hangs on.

What it is and what it's for

SWA stands for steel wire armoured. It's the cable used wherever electricity has to travel underground or through a hostile environment without a separate run of conduit. The construction is layered: copper conductors with cross-linked polyethylene insulation (XLPE, a thermal-set plastic that tolerates higher temperatures than PVC), wrapped in a PVC inner sheath, then a cylindrical layer of galvanised steel wires laid round the cable (the armour), then a black PVC outer sheath. Spade strikes, fork tines, and the occasional rodent meet the steel and bounce off.

You need SWA whenever a new circuit has to:

• Run underground to a garden room, garage, summerhouse or outbuilding
• Feed an outdoor hot tub on a dedicated circuit
• Supply external lighting on a long buried run
• Cross open ground between two parts of a property

The cable carries the supply itself. Inside the receiving outbuilding, it terminates at a sub-consumer unit (a small fuseboard with its own RCD and MCBs), and from that point onwards you wire normal twin-and-earth to sockets and lights. The boundary between SWA and twin-and-earth is the sub-consumer unit at the outbuilding end.

Standards: BS 5467 vs BS 6724

Two British Standards cover SWA cable, and they look identical to a homeowner.

BS 5467 is the standard used for almost all domestic and outdoor installations. The bedding and outer sheath are PVC. The conductors are insulated with XLPE. Voltage rating is 600/1000V, temperature range -15°C to +90°C, mechanical impact class 6 joules. For your garden room supply, this is the cable you want.

BS 6724 is the same cable construction but with low-smoke zero-halogen (LSZH) bedding and sheath instead of PVC. LSZH is a fire-safety material that emits less smoke and no halogen acid gas during a fire. It's required in tunnels, hospitals, schools, and other public-occupancy buildings. For a domestic garden installation it offers no benefit and costs more. Don't pay for it.

The cable designation 6943X tells you what you're holding: 600/1000V class, three-core, armoured, XLPE-insulated. 6944X is the four-core variant.

Sizes, designations and current ratings

For domestic outdoor circuits you'll choose between four sizes. Three-core is standard for single-phase UK supply (live, neutral, earth). Four-core is rare in domestic use and only needed when a separate dedicated CPC is wanted alongside three phases or where a particular earthing arrangement demands it.

The buried current ratings below come from BS 7671 Table 4D4A reference method D (cable buried direct in ground at 0.5–1m depth, 20°C ground temperature, 70°C conductor). They're the figures your electrician will use as the starting point for sizing.

The current rating is rarely what governs the cable size on a domestic run. Voltage drop does. More on that below.

Three-core or four-core?

In a single-phase UK domestic supply (which is what almost every homeowner has) you have one live conductor, one neutral, and one circuit protective conductor (CPC, the earth). That's three cores. Buy 6943X.

You'd only specify four-core (6944X) for three-phase installations (rare in domestic), or if your electrician has decided that the steel armour itself is unsuitable as the CPC and wants a dedicated copper earth conductor in addition to the three cores. That's a design decision, not a homeowner decision. If the spec says three-core, buy three-core.

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Burial depth and warning tape

UK underground cable rules come from two sources, and the difference matters when an electrician quotes you something different from what an internet article says.

The Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR), Regulation 14, is the primary legislation. It requires every underground cable to be "kept at such depth or otherwise protected" so as to avoid damage or danger, and that any non-earthed conductor cable is protected, marked or indicated to warn anyone excavating later. ESQCR doesn't specify a number. It specifies an outcome.

BS 7671:2018+A2:2022, Regulation 522.8.10 is the technical code your electrician works to. It also doesn't specify a number. It requires "sufficient depth" against "reasonably foreseeable disturbance of the ground." It's a performance standard, not a depth tape measure.

The numbers come from industry guidance documents (BS 7671 Guidance Note 7, NICEIC and NAPIT publications) and from ESQCR interpretation. The defensible domestic position is:

• 500mm minimum for a normal domestic garden (lawn, beds, paths)
• 600mm minimum under any area subject to vehicle loading or deeper cultivation (driveways, vehicle access, allotment-style cultivation)
• PVC warning tape laid 150mm below finished surface level above the cable, running the full length of the trench

The 450mm figure that occasionally appears in older guides comes from highway adoption standards for cables under public footways. It doesn't apply to domestic gardens. Use 500mm.

Trench preparation

The textbook trench cross-section is straightforward and worth following:

1. Excavate to 500mm (or 600mm under driveways) with vertical or near-vertical sides
2. Lay 50mm of clean sharp sand on the trench base
3. Lie the cable on the sand without tension or kinks
4. Cover with another 50mm of sand
5. Backfill with site soil (ideally screened of sharp stones for the first 200mm above the cable)
6. Lay yellow PVC warning tape 150mm below finished surface level
7. Backfill the rest and reinstate turf or surface

Sand bedding has two purposes: it prevents sharp stones bearing directly against the sheath, and it gives a known-conductivity backfill around the cable for thermal performance. The sand isn't optional even though many installations skip it. Builders' merchants sell yellow "Caution Electric Cable Below" PVC tape on 100m or 250m drums for under under £20.

Run extra cables while the trench is open

Re-digging a 25m garden trench costs £200 – £600 in labour. Pulling extra cables alongside the SWA on day one costs almost nothing. Order CAT6 ethernet for the garden room data link. Order coax if you might want a TV aerial out there. Order a second SWA on a separate circuit if there's any chance you'll add a hot tub later. The trench is open exactly once. Use it.

Glanding: where most installations get it wrong

The cable arrives at each end of the run with bare armour and three insulated conductors. Connecting it to a metal or plastic enclosure (a junction box, a sub-consumer unit, an outdoor weatherproof socket) requires a gland, which is a brass compression fitting that grips the armour, seals against the outer sheath, and bonds the steel armour to the enclosure earth.

There are two types you'll see on data sheets, and only one of them is correct outdoors.

CW glands vs BW glands

A CW gland is a four-piece outdoor compression gland: an entry component, an internal cone, a brass armour ring, and a sealing nut that compresses an inner gasket against the cable's outer sheath. The cable enters the gland with its outer sheath intact. The seal is on the sheath. With a PVC shroud (a rubber sleeve) over the body, a CW gland is rated IP66 or better, fully weatherproof. CW20 is the size for 2.5mm² and 4mm² SWA. CW25 is for 6mm² and 10mm². This is the gland for outdoor and underground use. Always.

A BW gland is a two-piece indoor gland that grips the armour wires only, with no seal against the outer sheath. It's rated IP30 to IP54 at best, splash-resistant, not weatherproof. BW glands are for indoor termination inside dry switchgear cabinets where the cable enters the building and connects to a busbar. Never use a BW gland outdoors.

The pricing difference is trivial. A CW20 gland kit (two glands for one cable's two ends, with shrouds, locknuts, and earth tags) costs £5 – £7 from Toolstation. There's no scenario where saving a couple of pounds by buying BW makes sense.

The banjo earth tag

This is the small piece of hardware that more outdoor electrical installations get wrong than any other.

When a CW gland is fitted to a metal enclosure, the gland body sits flush against the enclosure wall and the locknut tightens from inside. Steel armour grips the brass ring inside the gland, brass body contacts the metal enclosure, metal enclosure connects to the earth bar via its dedicated earth terminal. Continuity established. That's the path that makes the armour safe to touch under fault conditions.

When the enclosure is plastic (a typical outdoor IP65 junction box, or a non-metallic sub-consumer unit), the gland body has nothing earthed to touch. The armour is electrically floating. A fault on the live conductor that breaches insulation onto the armour can raise the entire armour to mains voltage, the homeowner touches the cable later, the cable feels normal, and there's no earth path to trip the breaker. That's a fatal scenario.

The fix is a banjo earth tag: a brass washer with a flat tail and a hole punched in it, like a banjo on its side. The washer slides over the gland thread, sits between the locknut and the enclosure wall, and grips the gland body when the locknut tightens. The flat tail is then bent out from the gland and terminated with a green/yellow earth conductor (typically 6mm² or 10mm² flexible) that goes to the enclosure's earth bar. The armour is now solidly earthed regardless of whether the enclosure is metal or plastic.

A missing banjo earth tag at either end of an SWA cable, where the armour is intended as the circuit protective conductor, is a C2 (potentially dangerous) defect on an Electrical Installation Condition Report (EICR). An EICR inspector will fail the installation. C2 defects must be remedied before the installation is signed off. The fix is a £2 banjo and a £1 length of fly lead. Skipping it on the install is the most common avoidable inspection failure on outdoor circuits.

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Stripping the armour

Skipping ahead to the practical bit: getting the armour cut to length without nicking the inner cores takes patience.

Trade electricians use a CK Armourslice or similar dedicated armour cutter (a metal-bodied tool with a small rotating blade that scores the steel wires once around the cable circumference). It costs around around £25 and produces a clean cut every time. For a one-off job, a junior hacksaw works if you score lightly all the way round, then bend each armour wire back and forth until it snaps. Don't try to hacksaw straight through. A hacksaw blade ploughing through steel armour will dive through the bedding and into the conductor insulation underneath. That cable then fails an insulation resistance test and goes in the bin.

Whatever cutter you use, after the armour is bent away you trim back the bedding with a sharp knife (ringing the bedding, not pulling longitudinally), expose the three cores, and you're ready to fit the gland.

The PME vs TT decision: the part nobody explains

This is the topic the homeowner won't find in any consumer-facing guide, and it's the point where a competent electrician earns their fee.

Most modern UK homes have a PME (Protective Multiple Earth) supply. The earth at your consumer unit is provided by the distribution network operator (DNO) via the combined neutral-and-earth conductor (the PEN) in the incoming supply cable. PME is the default for new connections.

The question is what to do at the outbuilding end. There are two options.

Option A: extend PME via the SWA armour

Run three-core SWA from the house consumer unit to the outbuilding. Use the steel armour as the CPC. Connect the outbuilding's earth bar to the armour at the gland. The outbuilding inherits the PME earth.

The problem: BS 7671 requires the CPC for an extended PME earth to be cross-section-equivalent to a 10mm² copper conductor. Steel armour, because steel has roughly one-eighth the conductivity of copper, would need to be approximately 48mm² of steel to qualify as PME-grade bonding. The armour on a typical 6mm² 3-core SWA is nowhere near that. NICEIC and ELECSA technical guidance is explicit: standard SWA armour is not an adequate PME earth in most domestic configurations.

Option B: TT the outbuilding

Disconnect the outbuilding from the house PME entirely. Drive an earth electrode (a copper-clad steel rod, typically 1.2m long, with a stranded earth conductor crimped to the head) into the ground next to the outbuilding. The outbuilding now has its own local TT earth. The SWA armour still gets bonded at both ends for fault protection, but the outbuilding's protective earth comes from the rod, not from the house.

A 30mA RCD at the outbuilding's sub-consumer unit ensures fault disconnection within the time required by BS 7671 for TT systems.

This is the recommended approach in NICEIC guidance for domestic outbuildings and is what most competent electricians will do. It avoids the PME bonding problem entirely. It costs perhaps £30 in materials (the rod, the rod clamp, a length of earth conductor) and 20 minutes to install while the trench is open.

If your electrician's quote includes "earth rod at outbuilding" as a line item, that's a good sign. If they're proposing to extend the house PME via the armour without adding a separate copper CPC of the right size, ask them why. Get the answer in writing.

RCD protection: 30mA, non-negotiable

BS 7671 Regulation 411.3.3 (as restructured by Amendment 2:2022) requires 30mA RCD additional protection for socket-outlets up to 32A used by ordinary persons, and for mobile equipment up to 32A used outdoors. Regulation 522.6.204 covers buried cables and similar installation methods.

The practical consequence is that any outdoor circuit, sockets in a garden room, an external socket on the house wall, a hot tub feed, garden lighting on switched outlets, needs 30mA RCD protection. The preferred location for that RCD is at the outbuilding sub-consumer unit, not at the house end. RCDs at the house end work, but a fault on a long buried run can take time to reach the threshold, and a local RCD gives faster, more selective disconnection.

The standard arrangement looks like this:

• House consumer unit: dedicated MCB (e.g. 20A or 32A) protecting the SWA circuit
• SWA cable runs from the MCB to the outbuilding
• Outbuilding sub-consumer unit: main switch, 30mA RCD, individual MCBs (or RCBOs) for sockets and lighting
• Outbuilding earth: local TT earth electrode (rod), or extended PME via dedicated 10mm² CPC if your electrician insists on PME

If your electrician's design doesn't include a 30mA RCD somewhere on the circuit, that's a Part P certification problem and they shouldn't be signing it off.

Voltage drop: the calculation that actually sizes domestic SWA

Here's the part that catches most homeowners. They look at the current rating tables, see that 2.5mm² SWA is rated 36A buried, calculate that their garden room only needs 16A, and conclude 2.5mm² is plenty. Then the electrician spec's 6mm². The homeowner thinks they're being upsold.

They're not. Voltage drop governs the sizing of long domestic runs before current rating does.

BS 7671 Appendix 4 gives a 4% maximum voltage drop budget on distribution circuits from the supply origin to the point of use. On a 230V supply that's 9.2V end-to-end. On a long buried run, the cable's own resistance eats most of that budget before you've even powered anything.

The voltage drop per metre per amp (mV/A/m) for 3-core SWA, single-phase, from BS 7671 Table 4D4B:

• 2.5mm² SWA: ~18 mV/A/m
• 4mm² SWA: 11 mV/A/m
• 6mm² SWA: 7.3 mV/A/m
• 10mm² SWA: 4.4 mV/A/m

Worked example: 25m garden room, 20A design current

A typical garden office with a kitchenette, two or three sockets, lights and a small electric heater might draw 20A under load. The supply runs 25m from the house consumer unit to the garden room sub-consumer unit.

Voltage drop with 4mm² SWA: 11 mV/A/m × 20A × 25m = 5,500 mV = 5.5V, which is 2.4% of 230V. Within the 4% budget. Compliant.

Voltage drop with 2.5mm² SWA: 18 mV/A/m × 20A × 25m = 9,000 mV = 9.0V, which is 3.9% of 230V. Just inside the 4% budget on calculation, but with no margin for measurement tolerance, conductor temperature derating, or future load increase. A competent electrician won't sign that off.

Voltage drop with 6mm² SWA: 7.3 mV/A/m × 20A × 25m = 3,650 mV = 3.65V, which is 1.6% of 230V. Comfortable headroom.

The 4mm² is electrically compliant. The 6mm² is what most electricians will fit because the marginal cost is roughly £30 – £50 over 25m and it leaves room for a future hot tub, EV charge point, or workshop upgrade.

The general rule: on any run over 20m carrying more than 16A, expect 6mm² as the minimum specification. On runs over 30m or for 32A circuits, expect 10mm². If your electrician spec's smaller, ask them to show the voltage drop calculation.

Cost and where to buy

SWA cable costs vary substantially between merchants and between purchase formats. The cheapest way to buy is a full drum (50m or 100m) from Screwfix or TLC-Direct. The most expensive way is cut-to-length from Toolstation, which charges a premium of 30–50% over drum pricing.

Cable per metre (3-core, drum pricing, 2026)

• 2.5mm² 6943X: £2.15 per metre. Toolstation 25m coil from £1.80/m, Screwfix 50m drum £2.40/m, TLC-Direct £2.48/m
• 4mm² 6943X: £3.25 per metre. Screwfix 50m drum £3.10/m, TLC-Direct £3.36/m
• 6mm² 6943X: £4.10 per metre. Medlocks £3.55/m cut, Screwfix £4.20/m drum, TLC-Direct £4.68/m
• 10mm² 6943X: £6.90 per metre. Screwfix £6.00/m drum, TLC-Direct £7.75/m

For a 25m garden room run in 6mm², that's roughly £100 in cable. A 30m run in 10mm² for a hot tub circuit is closer to £200.

Glands and accessories

A CW20 gland kit (two glands, two shrouds, two locknuts, two earth tags) is £6 from Toolstation. One kit covers both ends of one cable run. Order one kit per circuit.

PVC warning tape: a 100m roll is roughly £8 – £15 from any builders' merchant. Order a length equal to your trench length, not the cable length (the tape only goes in once).

Earth rod for a TT installation: 1.2m copper-clad steel rod with rod clamp is £15 – £25 from CEF, Edmundson, or Toolstation. Add a 6mm² green/yellow earth conductor to connect it.

Full installed circuit cost

For a typical 25m garden room supply (cable, glands, sub-consumer unit, RCD, MCBs, earth rod, labour, Part P certification, electrical installation certificate), expect £1,500 as a mid-range cost. Circuit Works' 2025 figures put a standard 25m garden office at £1,000 – £1,800; a higher-spec installation with a 32A consumer unit, multiple circuits and underfloor heating push closer to £2,000 – £3,000. WhatPrice's submissions cluster in the £1,200 – £2,400 range.

Trench digging adds £35 per metre if a contractor does it, or roughly £200 – £600 for a typical 25m run. Many homeowners dig the trench themselves over a weekend to save that cost, especially if a mini-digger is available. The cable can't go in the trench until an electrician is on site to terminate both ends, but the digging is fair game for the homeowner.

Where to buy

For drum quantities, Screwfix and TLC-Direct are the best retail prices for a non-trade buyer. Both deliver. CEF and Edmundson Electrical sell at 10–20% below retail but require a trade account.

For cut-to-length under 30m, Toolstation cut-to-length or Medlocks (cut to the metre, mail order) are convenient if you don't want a full drum.

Alternatives

Twin-and-earth in conduit. Some installations run twin-and-earth cable inside steel or heavy-duty plastic conduit underground instead of SWA. It works, but it's rarely cheaper. The conduit cost (steel galvanised conduit at £4 – £8, plus couplers and inspection fittings) plus the cable cost typically exceeds SWA at the same size, and the conduit must be properly sealed and drained against water ingress. SWA is the simpler choice for buried runs.

Direct-burial flexible cable (HO7RN-F or similar). Rubber-insulated flexible cable rated for direct burial exists, but it has lower mechanical protection than SWA and isn't the standard for UK installations. Not recommended for new domestic work.

Hyperclad / armoured flexible cable. Used in industrial and commercial settings. Not relevant to domestic outbuilding supplies.

For a domestic buried circuit, BS 5467 SWA is the right material. The "alternative" decisions are about size, cores, and earthing arrangement, not the material itself.

Where you'll need this

• First fix electrics - if the build includes a garden room, garage conversion, or external lighting, the SWA goes in during first fix or earlier
• External works - the trench for SWA is often dug at the same time as drainage and soakaway works, while the ground is already disturbed and the digger is on site

The trench-opportunism principle matters here. If a drainage trench is already open along the route to the outbuilding, lay the SWA in the same trench (in a separate channel, with its own warning tape and at the correct depth). The cost of doing it then is a fraction of doing it later. SWA appears in any extension or renovation project where a new buildable structure is added at the back of the garden, where outdoor lighting needs upgrading, or where a hot tub or EV charge point is being installed.

Common mistakes

Sizing for current rating, not voltage drop. The 2.5mm² SWA is rated 36A buried. The 25m garden room circuit drawing 20A "fits." It doesn't. Voltage drop on the buried run takes the supply at the garden room consumer unit close to or below the 4% budget, and any future load increase pushes it over. Always size for voltage drop on runs over 20m. 4mm² minimum for short runs, 6mm² for typical garden rooms.

No banjo earth tag at one or both ends. The single most common defect on EICR inspections of older outdoor circuits. The armour is connected at one end via the gland body to a metal enclosure, and at the other end into a plastic outdoor junction box without a banjo. The armour at the plastic-end is electrically floating. C2 defect, must be remedied. Specify banjos at both ends for any new install, the cost is a £2 brass washer.

Hacksawing through the armour. A blade through bedding nicks the conductor insulation. The cable fails an insulation resistance test. Use an armour cutter (CK Armourslice, around around £25) or score lightly all round and bend each wire back and forth until it snaps.

Skipping the warning tape. The yellow tape costs perhaps £10 for 100m. Without it, every future garden project (new patio, replacing a fence post, planting a tree, hedge maintenance with a power tool) is a cable strike risk. With it, the next person digging gets a clear visual warning 350mm before they reach the cable.

Using a BW gland outdoors. Splash-rated, not weatherproof. Water tracks up the armour into the cable bedding and corrodes the run from the inside out. Always specify CW20 (for 2.5mm² and 4mm²) or CW25 (for 6mm² and 10mm²) for outdoor and underground installations.

Extending PME to the outbuilding via the armour. The standard SWA armour cross-section doesn't meet the 10mm²-copper-equivalent requirement for PME bonding. Either run a separate dedicated 10mm² CPC alongside the SWA, or, far simpler, disconnect the outbuilding from PME and use a TT arrangement with a local earth electrode. A 1.2m earth rod and a 30mA RCD at the outbuilding sub-consumer unit costs perhaps £40 and avoids the design problem entirely.

Forgetting the second cable. The trench is open exactly once. Pulling CAT6 ethernet alongside the SWA on day one costs almost nothing. Pulling it later costs another £200 – £600 in groundwork. If there's any chance the outbuilding will want network, aerial, or a second power circuit in the next decade, lay the cables now.