Summary

Every UK domestic and light commercial installer reaches for one of these two cable types on almost every job, and choosing wrong causes two very different kinds of problem. Specify T&E where SWA was needed and the cable degrades from UV, gets crushed by vehicle loading, or corrodes from ground moisture within a few years — a submains failure that's expensive and disruptive to dig up and replace. Specify SWA where T&E would have done and the customer pays for unnecessary armour, glands, and a harder first-fix, with no functional benefit indoors.

The decision comes down to one question: is the cable staying inside dry, protected building fabric, or is it going outside, underground, or somewhere it can be mechanically damaged? T&E is designed and tested for the former. SWA is designed and tested for the latter. Both are BS 7671-compliant cable types when used within their intended application, and both appear constantly on the same job — a kitchen rewire uses T&E throughout, then switches to SWA the moment the circuit leaves the building to feed a garage or garden room.

This article sets out the construction, rating, and application differences, when to specify each, and the common mistakes — bare T&E run outdoors, SWA installed indoors, unarmoured cable buried without a duct — that cause callbacks and Part P sign-off failures. For full cable sizing methodology (current capacity, voltage drop, earth fault loop impedance) see cable sizing current capacity; for SWA-specific burial depths and gland termination see armoured cable.

Key Facts

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Factor PVC Twin and Earth (T&E) SWA (Steel Wire Armoured)
Governing standard BS 6004 BS 6346 / BS 5467 / BS 6724
Typical application Internal fixed wiring — sockets, lighting, cookers External, underground, mechanically exposed runs
Conductor temperature 70°C (PVC/thermoplastic) 70°C (BS 6346) or 90°C (BS 5467/6724, XLPE)
Armour / mechanical protection None — relies on building fabric or added conduit Integral steel wire armour
Earth conductor Reduced-size bare CPC within the sheath Full-size core, or armour itself (with supplementary bond)
UV / weather resistance Poor — indoor use only Good — designed for external exposure
Direct burial Not suitable Suitable, subject to depth and bedding requirements
Bend radius Flexible, minimal restriction 8–12× cable diameter minimum
Termination method Direct into accessory/terminal, CPC sleeved Brass gland with armour clamp and earth tag
Typical sizes used domestically 1.0–10mm² 1.5–25mm²
Relative cost per metre (equivalent rating) Lower Higher (typically 2–4× for equivalent CSA)
Typical use case Ring/radial socket circuits, lighting, cooker circuits, showers Outbuilding submains, garden lighting supply, external distribution boards

Detailed Guidance

Why T&E is not suitable outdoors or underground

Standard PVC T&E sheath is not formulated for UV stability or prolonged wetting. Left exposed to sunlight, the PVC sheath becomes brittle and cracks within a few years, exposing the conductors. Buried directly, the sheath has no mechanical protection against stones, spades, or ground movement, and no defence against sustained ground moisture ingress at any nick or abrasion in the sheath. Even where an installer runs T&E through a conduit outdoors to protect it, the cable itself is still operating outside its design envelope for temperature cycling and UV where the conduit doesn't fully exclude light — SWA or an outdoor-rated equivalent is the correct specification for any permanent external run, not a workaround with conduit.

Why SWA is unnecessary — and unhelpful — for standard internal circuits

SWA's armour adds cost, weight, and a harder first-fix for no benefit inside a normal domestic wall, floor void, or joist run that already provides adequate mechanical protection once plastered, capped, or clipped correctly. The gland termination alone (stripping to the correct lengths, fitting the armour clamp, fitting an earth tag and supplementary bond) takes significantly longer than terminating T&E directly into an accessory. Specifying SWA for a standard ring final or lighting circuit inflates the job cost with no corresponding safety or performance gain — reserve it for the runs that actually leave the protected building fabric.

The safe-zone and RCD rules for chased-in T&E

BS 7671 permits T&E to be chased into a wall without further mechanical protection only within the defined safe zones — horizontally within 150mm of the ceiling or floor junction, or vertically/horizontally in a zone extending from a socket, switch, or other accessory. Cable run outside these zones, or at a depth of less than 50mm from either face of a wall, must have additional protection: earthed metal conduit/capping, mechanical protection sufficient to prevent penetration by nails and screws, or 30mA RCD protection on the final circuit. In practice on modern consumer units with RCBOs or RCD-protected ways as standard, this condition is usually already satisfied — but always confirm and record it, since it's a common point of query at EICR inspection.

Choosing between BS 6346, BS 5467, and BS 6724 SWA once you've decided armour is needed

Once the decision to use SWA is made — typically for a submain to an outbuilding, garden lighting supply, or any run leaving the building envelope — the choice between the three SWA standards follows the same logic as for any SWA specification: BS 6346 (PVC/PVC) for standard buried domestic submains, BS 5467 (XLPE/PVC) where a higher current rating is needed from the same cable size, and BS 6724 (XLPE/LSZH) where low-smoke performance is specified. See armoured cable for burial depth, gland type, and CSA selection detail.

The CPC sizing difference and why it matters at the design stage

T&E's bare CPC is manufactured at a reduced cross-section relative to the line conductors — a cost and flexibility optimisation that is compliant provided the adiabatic check (BS 7671 Table 54.7 / Regulation 543.1.3) confirms the reduced CPC can carry prospective fault current for the disconnection time of the protective device. SWA, by contrast, either carries a full-size dedicated earth core or uses the armour itself, sized by the manufacturer to satisfy the same regulation by a different route. When specifying either cable for a circuit close to its rated limit, always verify the CPC or armour CSA against the actual fault current and disconnection time rather than assuming the manufactured default is automatically adequate — this is particularly relevant on long SWA submains and high-fault-current circuits close to the origin of supply.

Frequently Asked Questions

Can I run T&E through a garden in conduit instead of using SWA?

Not for anything beyond a very short, well-protected run, and not as standard practice. Standard PVC T&E is not rated for the temperature cycling, UV exposure at any imperfect conduit joint, and prolonged dampness that a buried or garden-exposed conduit run experiences over years. SWA is the correct product for external and underground circuits; using T&E in conduit as a cost-saving substitute is a common cause of premature cable failure and is not what BS 7671 anticipates for this application.

Does SWA need an RCD in the same way T&E does?

The RCD requirement in BS 7671 Regulation 522.6.202 (with the alternative protective measures listed in 522.6.204) specifically addresses cables at risk of penetration (nails, screws) in walls and partitions — a risk that mainly applies to T&E and other unarmoured cable buried in masonry or stud partitions. SWA's armour itself resists penetration, so this specific regulation is not the driver for RCD protection on SWA circuits. However, general RCD protection requirements elsewhere in BS 7671 (additional protection for socket outlets, mobile equipment, outdoor circuits, and so on) still apply independently of cable type.

Is there a low-smoke version of T&E for escape routes, the way BS 6724 exists for SWA?

Yes — BS 7211 is the thermosetting, low-smoke-and-fume (LSZH) equivalent for non-armoured internal wiring, used in escape routes, public buildings, and anywhere a specification calls for reduced smoke and toxic gas emission in a fire. It sits alongside BS 6004 T&E in the same way BS 6724 sits alongside BS 6346/5467 for SWA.

Can I join T&E to SWA directly, or does the circuit need a junction box?

A circuit commonly transitions from T&E indoors to SWA for the external run — this needs a proper enclosure (weatherproof junction box or a sub-consumer unit/isolator) rather than a direct twist-and-tape joint, both for mechanical protection of the joint and to correctly terminate the SWA armour to earth via a gland at that point. Never leave an unenclosed joint between the two cable types, indoors or out.

Which cable type is cheaper to install for a garden room supply?

SWA, despite its higher material cost per metre, is the only compliant option once the circuit leaves the building — so the comparison isn't really T&E vs SWA on cost, it's whether the garden room's supply cable can be routed entirely within safe zones inside existing structure (rare) or must cross open ground (almost always). Budget for SWA, correct burial depth, and proper gland termination at both ends as the realistic cost of any new outbuilding circuit.

Regulations & Standards