SWA Armoured Cable: Burial Depths, Gland Types, Termination and Part P Notification

Quick Answer: Steel Wire Armour (SWA) cable to BS 6346 (PVC) or BS 5467 (XLPE) is the standard UK underground and external cable. Minimum burial depth is 450mm under gardens, 600mm under driveways and roads, 750mm under cultivated land, with marker tape laid 150–300mm above the cable. Terminate with BW (indoor) or CW (outdoor weatherproof) brass glands incorporating an earth tag bonded to the armour. Installation supplying outbuildings, garages, garden lighting, or new circuits is notifiable work under Building Regulations Part P in England.

Summary

SWA armoured cable is the workhorse of UK external and underground electrical work. The steel wire armour serves three functions simultaneously: mechanical protection from spades, rodents, and ground movement; an integral protective earth conductor for the circuit; and electromagnetic shielding for clean signal performance. From garden lights to 100A submains for outbuildings, SWA is the default unless a specific reason rules it out.

The construction is straightforward — copper conductors, individually colour-insulated cores, an inner sheath (bedding), spirally-wound steel wires forming the armour, and an outer PVC or LSZH oversheath. Sizes from 1.5mm² up to 400mm² are available; the most common domestic and light commercial sizes are 1.5mm² (garden lighting), 2.5mm² (small outbuildings, 16A spurs), 4mm² and 6mm² (typical garage submains), 10mm² (workshop submains), 16mm² and 25mm² (larger outbuildings and three-phase commercial). Above 25mm² the cable becomes physically heavy and is usually reserved for industrial work.

This article covers cable selection, burial depth requirements per BS 7671 and ENA guidance, gland types and termination practice, the legal requirement for marker tape, ducting under driveways, voltage drop and earth fault loop impedance calculations, and Part P notification scope. For sizing calculations specifically see cable sizing.

Key Facts

• BS 6346 — PVC-insulated, PVC-sheathed SWA cable (general use, temperature limit 70°C conductor)
• BS 5467 — XLPE-insulated, PVC-sheathed SWA cable (higher temperature 90°C, higher current rating for same CSA)
• BS 6724 — XLPE-insulated, LSZH-sheathed SWA cable (low smoke zero halogen — for escape routes, public buildings)
• BS 8436 — SWA equivalent for fire-rated wiring in escape routes
• Steel wire armour CSA — provides the protective earth conductor; sized by cable manufacturer per BS 7671 Reg 543.1.3
• Standard sizes — 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400mm²
• Core counts — 1c (single core for high current), 2c (single phase live + neutral), 3c (single phase + earth, or 3-phase no neutral), 4c (3-phase + neutral or 3-phase + earth), 5c (3-phase + neutral + earth where armour earth is supplemented)
• Minimum burial depth under gardens / private land — 450mm (BS 7671 / ENA G81 guidance)
• Minimum burial depth under driveways / vehicular — 600mm
• Minimum burial depth under cultivated agricultural land — 750mm
• Minimum burial depth for HV / network cables — 1000mm or per DNO specification
• Warning tape — yellow polythene "ELECTRIC CABLE BELOW" laid 150–300mm above cable per BS EN 12613
• Ducting under driveways — flexible or rigid duct (orange) protecting the cable from vehicle load transfer
• Brass gland types — CW (outdoor, IP68 with seal), BW (indoor industrial), E1W (single-seal indoor), A2 (control cable indoor)
• Gland components — gland nut, claw / armour clamp, earth tag, inner seal, locknut, shroud, IP washer
• Part P notifiable in England — installation of a new circuit, replacement of a consumer unit, work in zones 1/2 of a bathroom; SWA submains to outbuildings, garages, garden buildings are new circuits → notifiable

Quick Reference Table

(Current ratings approximate, Method D direct buried, 20°C ground temperature, 70°C cable. Always refer to BS 7671 Appendix 4 Table 4D4A for definitive ratings.)

Detailed Guidance

SWA Construction and Selecting BS 6346 vs BS 5467 vs BS 6724

BS 6346 (PVC/PVC SWA) — PVC conductor insulation (70°C max), PVC outer sheath. General-purpose outdoor and buried use; most economical.

BS 5467 (XLPE/PVC SWA) — XLPE conductor insulation (90°C max), PVC outer. Approximately 15–25% higher current rating than BS 6346 of the same CSA. Preferred for derated installations, long runs, or high ambient temperatures.

BS 6724 (XLPE/LSZH SWA) — same conductor as BS 5467 but LSZH (low smoke, zero halogen) sheath. Required for escape routes, public assembly buildings, hospitals, schools, transport infrastructure.

Selection rule: default to BS 6346 for buried domestic submains; BS 5467 where higher current is needed in the same CSA; BS 6724 where the specification calls for LSZH.

Burial Depths and Marker Tape

Burial depths are based on the cable being protected from accidental damage by spades, garden digging, mole-traps, ploughs, and vehicular loads. The depths assume the cable is buried in clean, well-compacted soil with no sharp stones in contact.

Minimum depths:

Bedding and backfill: excavate to depth plus 50mm, lay 50mm clean sand bedding, lay the cable on the bedding with no tension and a gentle curve at any bend (min 8× cable diameter), cover with 100mm of the same bedding material, lay yellow electric cable marker tape (BS EN 12613) across the trench at 150–300mm depth, then backfill with excavated material — well-compacted, no large stones against the bedding.

Bend radius: SWA must not be sharply bent. Manufacturer specifications typically require 8–12× cable diameter; sharp bends crush the armour and can damage core insulation.

Ducting Under Driveways and Roads

Buried SWA under a driveway is at risk from point loading from car tyres (as soil settles), future excavation by the homeowner, and damage during resurfacing. Best practice is to install the SWA inside a duct under any vehicular surface.

• Duct material: orange "electrical" twinwall corrugated duct or rigid PVC pipe
• Duct size: internal diameter ≥1.5× cable outer diameter for pulling without damage
• Duct depth: top of the duct at the minimum cable depth (600mm under driveway); cable inside is deeper
• Draw rope: polypropylene rope inside the duct for future cable replacement
• Sealing: seal duct ends with foam or compound against water ingress and rodents
• Marker tape: still required above the duct

Gland Termination — Brass Glands and Bonding

The brass gland serves three functions: clamps the cable mechanically, provides ingress protection at the enclosure entry, and establishes electrical continuity between the steel wire armour and the enclosure / earth bar.

Common gland types:

For outdoor terminations — exterior of building, garage exterior, weatherproof junction box — always use CW glands with the outer rubber seal that grips the cable outer sheath plus the inner seal that grips the inner bedding. The IP washer between the gland body and the enclosure provides the final water seal.

Gland kit components (typical CW gland): gland nut (compresses outer seal), outer compression seal (against outer sheath), armour clamp/claw ring (grips steel wires), cone (splits armour wires evenly), inner seal (against inner bedding), gland body (threads into enclosure), earth tag (brass washer with terminal for supplementary earth conductor), locknut (secures body from inside enclosure), IP washer (between gland face and enclosure exterior), and shroud (PVC sleeve over the finished gland).

Termination procedure:

SWA gland termination — sequence of operations
┌─────────────────────────────────────────────┐
│ 1. Slide shroud onto cable                  │
│ 2. Slide gland nut onto cable               │
│ 3. Slide outer seal (CW glands)             │
│ 4. Strip outer sheath to expose armour      │
│    (length = gland armour-clamp depth)      │
│ 5. Fan steel wires around cone              │
│ 6. Slide armour clamp / claw into position  │
│ 7. Trim wires to length of clamp            │
│ 8. Strip inner sheath / bedding to expose   │
│    cores (length = enclosure depth + 50mm)  │
│ 9. Pass cores into enclosure                │
│ 10. Tighten gland body with locknut         │
│ 11. Tighten gland nut to compress seals     │
│ 12. Install earth tag with bonding wire     │
│ 13. Slide shroud over completed gland       │
│ 14. Test continuity of armour to earth bar  │
└─────────────────────────────────────────────┘

Earth bonding via gland: the armour is the CPC. The armour clamp bonds the armour to the gland body, and the gland body bonds to the enclosure via the locknut. However, BS 7671 Reg 543.3.2 requires a supplementary earth conductor (the "banjo" lead at the earth tag) so the earth path does not rely solely on the mechanical joint. Run a green/yellow lead from the earth tag terminal to the main earth bar.

For 2-core SWA where the armour is the earth, this banjo is essential. For 3-core or 4-core SWA with a dedicated G/Y core, the banjo provides redundant earthing — best practice and recommended.

CSA Selection — Current, Voltage Drop, Earth Fault Loop Impedance

Sizing SWA submains involves three parallel checks:

1. Current carrying capacity (Iz) — look up the rating in BS 7671 Appendix 4 Table 4D4A for SWA, Method D (buried) or Method C (in duct). Apply grouping factor (Table 4C5) and ground-temperature correction (Table 4B2). Iz must exceed both design current Ib and protective device rating In.
2. Voltage drop — limits per Appendix 12 are 3% for lighting and 5% for other circuits, from origin of installation. Vd = mV/A/m × Ib × length / 1000. Example: 40A on 10mm² over 30m → 4.4 × 40 × 30 / 1000 = 5.28V → 2.3%, acceptable.
3. Earth fault loop impedance (Zs) — low enough for the device to operate within disconnection time (0.4s for socket circuits ≤63A, 5s for distribution). Zs = Ze + (R1+R2). Verify against BS 7671 Table 41.3.

See cable sizing for worked examples.

Part P Notification — When Is SWA Installation Notifiable?

In England, Building Regulations Approved Document P (Electrical safety in dwellings) applies to electrical work in or attached to a dwelling, its garden, or outbuildings serving the dwelling.

Notifiable work involving SWA:

• Installation of a new circuit supplying an outbuilding (garage, garden room, workshop)
• Installation of garden lighting on a new circuit
• Installation of a sub-consumer unit in an outbuilding fed from the main consumer unit
• Replacing the main consumer unit (where the SWA submain terminates)
• Any work in special locations (bathroom zones 1 and 2, swimming pools)

Not normally notifiable:

• Replacing an existing SWA cable like-for-like on an existing circuit (no new circuit)
• Repairs and maintenance to existing installations
• Adding a socket or fused spur to an existing circuit (not in special locations) — BUT the work must still comply with BS 7671 and an EIC or Minor Works Certificate issued

Compliance routes:

1. Registered competent person — NICEIC / NAPIT / ELECSA / Stroma registered electrician self-certifies and notifies via the scheme's portal. Building control informed automatically.
2. Building control notification — non-registered worker submits a Building Notice or Full Plans application to local building control, who inspect and certify.
3. Third-party verification — independent registered electrician inspects and certifies completed work.

Equivalent regulations apply in Wales (Approved Document P), Scotland (Building (Scotland) Regulations Section 4.5), and Northern Ireland (Building Regulations Part F). Always verify the local requirement.

AC vs DC SWA Runs

SWA is rated primarily for AC distribution but is widely used for DC applications (solar PV strings, battery DC submains). Inductive heating is minimal for DC, so single-core SWA can carry higher current on DC than AC — consult manufacturer DC ratings. Multi-core SWA is standard for PV string interconnection between roof and inverter. Armour bonding to the PV earth bar is required at both ends per BS 7671 Section 712. For combined AC and DC paths in solar installations, see battery storage.

Frequently Asked Questions

Can I run SWA in the same trench as a water or gas pipe?

Best practice is to maintain 300mm horizontal separation between an electrical cable and a service pipe. Where this is impossible, install in separate ducts and verify the local water / gas authority's requirements. Cables running parallel to gas pipes at less than 150mm separation can cause induction risk — keep separation or place a fire-resistant divider.

Do I need to use a separate green/yellow earth core, or is the armour enough?

The armour is sized by the manufacturer to satisfy BS 7671 Reg 543.1.3 as a CPC, so for most installations the armour alone is adequate. However, where the armour CSA is marginal (very long runs, high fault currents), or where the regulation requires a CPC of specific size (e.g., domestic ring final circuits per BS 7671 Reg 543.2.4), a supplementary green/yellow conductor is required. Most 3-core SWA includes earth-equivalent armour by design.

How do I make a joint in buried SWA cable?

In-ground joints in SWA are possible using resin-filled joint boxes (e.g., Raychem MJI series) that re-establish full IP68 sealing and continuous armour bonding. However, in-ground joints are a permanent point of weakness — moisture ingress over decades is common. Best practice: avoid joints by running a single continuous length, and where a joint is unavoidable bring the cable above ground into a sealed junction box for the joint, then re-bury.

What's the difference between 2-core and 3-core SWA for a 230V circuit?

A 2-core SWA carries live and neutral; the armour acts as the protective earth. A 3-core SWA carries live, neutral, and a dedicated earth conductor; the armour provides additional redundant earthing. For modest single-phase loads where the armour CSA is adequate, 2-core is sufficient and cheaper. For high-current circuits or where local guidance requires a dedicated earth core, use 3-core.

Is there a problem using SWA at very low currents (e.g., a 12V garden light)?

No — SWA is overspec for very low current applications but not harmful. The mechanical and weather protection it provides justifies its use even where the conductor CSA is far larger than needed for current. For 12V garden lighting, consider whether the supply transformer feeds directly into the cable or whether SWA is needed only for the mains side; the secondary low-voltage side can use external-rated low-voltage cable.

Regulations & Standards

• BS 6346 — Electric cables — PVC insulated, armoured cables for voltages of 600/1000V and 1900/3300V

• BS 5467 — Electric cables — Thermosetting insulated, armoured cables of rated voltages of 600/1000V and 1900/3300V for fixed installations

• BS 6724 — Electric cables — Thermosetting insulated, armoured cables for voltages of 600/1000V having low emission of smoke and corrosive gases when affected by fire

• BS 7671:2018+A2:2022 — IET Wiring Regulations; particularly Section 522 (selection and erection — environmental influences), Section 543 (protective conductors), Appendix 4 (current carrying capacity)

• BS EN 12613:2009+A1:2014 — Warning tapes for buried services

• ENA G81 — Energy Networks Association guidance on framework for design and planning of underground network cables (DNO reference)

• Building Regulations Approved Document P — Electrical safety in dwellings (England)

• Building Regulations Approved Document P (Wales) — equivalent in Wales

• Building (Scotland) Regulations Section 4.5 — Scottish equivalent

• IET Guidance Note 1: Selection and Erection — practical SWA guidance

• BSI Standards Catalogue — BS 6346, BS 5467, BS 6724

• Electrical Safety First — Outdoor and Underground Cables

• NICEIC Technical Information — gland termination and bonding

• NAPIT Technical Guidance — SWA submains to outbuildings

• Energy Networks Association — ENA G81 cable burial framework

• UK Government — Approved Document P

• cable sizing — full CSA selection methodology with worked examples

• workshop supply — SWA submains to workshops and outbuildings

• consumer units — sub-consumer unit at the supplied building

• safe isolation procedure — safe isolation before working on SWA terminations

• battery storage — SWA in PV and battery storage DC circuits