Armoured Cable Installation UK: SWA Cable Guide

Steel Wire Armoured (SWA) cable is a multi-core power cable for fixed wiring where mechanical protection is required. It is the default choice for underground runs, external sub-mains between buildings, and industrial and commercial wiring exposed to accidental damage.

The construction is a copper or aluminium conductor, insulation on each core, a bedding layer, a layer of galvanised steel wires laid helically around the cable (the armour), and an outer sheath. The armour does two jobs: it takes the mechanical punishment, and when correctly terminated it acts as the circuit protective conductor for the circuit.

The regulation that puts SWA underground

Regulation 522.8.10 is the one that matters and it is worth reading in full. Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor. The location of buried cables shall be marked by cable covers or a suitable marker tape. Buried cables, conduits and ducts shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground.

That is why plain twin-and-earth in a garden trench fails an inspection and SWA does not. SWA already carries an earthed armour suitable for use as a protective conductor, so it needs no additional duct — only depth and marking.

Which British Standard is the cable made to?

The standard printed on the sheath decides which Appendix 4 rating table applies. BS 7671 Table 4A3 sets out the mapping.

BS 6346 is retained in BS 7671 Table 4A3 for historical purposes only — the standard is withdrawn. New SWA bought in the UK today is normally BS 5467, or BS 6724 where a low smoke and fume cable is specified for escape routes and public buildings. Both are 90 °C thermosetting cables rated from Tables 4E3 and 4E4, not the 70 °C 4D tables.

Regulation 522.6.204(a) also names BS 5467 and BS 6724 among the cables accepted as incorporating an earthed metallic covering complying with the requirements for a protective conductor of the circuit concerned.

Install in accordance with BS 7671:2018+A4:2026 and the associated IET Guidance Notes.

The core count decides where the earth comes from. Get this wrong and either the CPC is undersized or you have paid for a core you never connect.

Whichever configuration you choose, the CPC has to satisfy Regulation 543.1.1 — either calculated with the adiabatic equation of Regulation 543.1.3, or selected in accordance with Regulation 543.1.4. Where the armour alone will not carry the prospective earth fault current for the disconnection time, move up to a cable with a dedicated earth core.

Conductor sizes run from 1.5 mm² up to 400 mm² and beyond. Aluminium conductor SWA is used for large distribution cables where copper would be prohibitively heavy and expensive; it is rated from the 4H and 4J series of tables rather than the 4D and 4E series.

There is no such thing as “the rating of 10 mm² SWA”. The tabulated value depends on the cable standard, the reference method, and then the correction factors for your site. Work through it in that order.

Step 1 — pick the right reference method

Appendix 4 defines the installation conditions each column of a rating table assumes. Getting D1 and D2 the wrong way round is a common and expensive error: burying direct dissipates heat better than a duct, so D2 carries the higher rating.

Appendix 4 notes that D1 and D2 values are based on conservative installation parameters. Where the actual ground thermal resistance, ground ambient temperature and cable depth are known, the cable manufacturer or the ERA 69-30 series may support a smaller cable.

Step 2 — read Iz from the table that matches the cable

Do not carry a number across from another table. For 90 °C thermosetting armoured cable to BS 5467 or BS 6724, multicore ratings are in Table 4E4A and voltage drop in Table 4E4B. For 70 °C thermoplastic armoured cable, the equivalents are Tables 4D4A and 4D4B. Aluminium conductor cables use the 4J series.

Step 3 — apply the correction factors

Each factor has its own symbol and its own table. Applying the wrong one — or an invented “installation method multiplier” on top of a tabulated column — is how cables end up undersized.

Two further factors catch people out on buried SWA. Under Regulation 433.1.203, for direct buried cables and cables in buried ducts, condition (c) of Regulation 433.1.1 is satisfied where the rated current of the protective device does not exceed 0.9 times the current-carrying capacity of the lowest rated conductor — an effective 0.9 factor on Iz. And under Regulation 433.1.202, where the protective device is a semi-enclosed fuse to BS 3036, the factor is 0.725.

Step 4 — check voltage drop

Appendix 4, Section 6.4 and Table 4Ab give the voltage drop between the origin of an installation and any load point, expressed against the nominal voltage.

On a private LV supply the voltage drop within each final circuit should still not exceed the 3% and 5% figures. SWA runs are often long, and Table 4Ab allows for that: where the wiring systems of the installation are longer than 100 m, the values above may be increased by 0.005% per metre beyond 100 m, without that increase being greater than 0.5%. The calculated voltage drop should include any effects due to harmonic currents.

The gland is not just a cable entry. On a 2-core or 4-core cable it is the joint in the circuit protective conductor, so a poor termination is an earthing fault, not a cosmetic one.

1. 1 — Measure and mark

   Measure the depth of the gland entry plus the length of armour the cone has to grip. Mark the outer sheath. Allow extra length inside the enclosure for dressing and connecting the cores.

2. 2 — Remove the outer sheath

   Score around the sheath with a sharp knife without cutting into the armour, bend the cable to crack it, and pull the section off. An armoured cable stripping tool is safer and gives a cleaner cut.

3. 3 — Dress the armour

   Fan out and straighten the steel wires, then cut them square to length with sharp side cutters. Uneven armour gives poor cone grip and a high-resistance earth path.

4. 4 — Fit the shroud first

   Slide the shroud onto the cable before anything else. Once the gland is tightened onto the enclosure the shroud cannot be passed over it, and it cannot be retrofitted without stripping the termination back. This is the single most common SWA termination error on site.

5. 5 — Fit the gland

   Slide the back-nut then the cone over the armour, taper facing the gland body. Feed the cable through the entry, then tighten the back-nut to draw the cone under the armour and clamp the wires. Firm, not crushed — over-tightening cuts the wires you are relying on.

6. 6 — Make the earth connection

   Fit an earth tag between the gland body and the enclosure lock-nut and run a green/yellow conductor from the tag to the earth bar. Regulation 543.2.7: where the protective conductor is formed by the metal sheath or armour of a cable, the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure.

Verifying the termination

Prove continuity of the armour with a low-resistance ohmmeter from end to end, then measure earth fault loop impedance and compare it with the maximum for the protective device: Table 41.2 for fuses on a 0.4 s disconnection time, Table 41.3 for circuit-breakers, and Table 41.4 for fuses on a 5 s disconnection time.

Those tabulated values assume the conductor is at its normal operating temperature, and on site you are usually testing a cold cable. Appendix 3 gives the correction: the requirement is considered met where the measured loop impedance Zs(m) does not exceed 0.8 × (U₀ × Cmin / Ia) — that is, 0.8 times the tabulated limit, or equivalently multiply your measured Zs by 1.25 before comparing. For a low voltage supply given in accordance with the ESQCR, Cmin is 0.95. Appendix 3 also notes this is one method of correcting for temperature and others are not precluded.

Regulation 522.8.10 sets a performance requirement, not a number: buried cables, conduits and ducts shall be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground. There is no general depth figure in BS 7671 for an ordinary domestic or commercial run — the depth has to be justified against the risk on that site.

Where BS 7671 does put a figure on it, it does so for particular locations. These are the depths the standard itself uses, and they are widely adopted as practical minima elsewhere.

In each case the depth is only excused where additional mechanical protection is provided. In practice that means deeper under anything carrying vehicles than under a footpath or border, and deeper again where groundworks or machinery are foreseeable.

Remember the rating consequence as well as the safety one: Appendix 4 tabulates buried ratings at a depth of laying of 0.7 m. Bury shallower or deeper and Cd from Table 4B4 applies.

Marking and bedding

Regulation 522.8.10 requires the location of buried cables to be marked by cable covers or a suitable marker tape, and buried conduits and ducts to be suitably identified. That marking is the regulatory requirement — surface route marker posts and as-fitted drawings are sound practice that make the route traceable later, but they are not what 522.8.10 asks for and they do not replace tape or covers.

Lay the cable on fine sand or selected fill free from sharp stones, glass and debris, cover with further selected fill, then lay the warning tape or covers above it for the full run before backfilling.

Where minimum depth cannot be achieved — a shallow area of bedrock, for example — provide supplementary mechanical protection such as duct, tiles or concrete encasement, and record the route on as-fitted drawings retained for the life of the installation.

The gland has to suit the environment it sits in. A gland with too low an IP rating lets moisture and dust into the termination, and the first thing to degrade is the earth path through the armour.

The assembly is only as good as its weakest part: an IP68 gland in an IP66 enclosure gives you IP66. Match the gland to the enclosure rating or better, and choose the seal material — neoprene, EPDM or silicone — for the temperature and chemical exposure at that entry.

These are the SWA defects that turn up as observations on EICRs.

Certify every SWA installation with an Electrical Installation Certificate (EIC), or a Minor Electrical Installation Works Certificate where the work does not extend to a new circuit. Keep the cable sizing calculation and the as-fitted route drawing with the certificate — both are what a future inspector, and your own insurer, will ask for.

Reviewed by Andrew Moore, founder of Elec-Mate. Content is grounded in BS 7671:2018+A4:2026 and the IET Guidance Notes series.