Which Cable for Which Job? The Complete UK Electrician's Cable Guide

Picking the wrong cable is one of the most costly mistakes on an electrical job. Too small a conductor and the cable overheats under load — at best a nuisance trip, at worst a fire. Too large and you're wasting money and struggling with stiff cable in tight conduit. This guide covers every cable type a UK electrician encounters on domestic and light-commercial jobs: Twin and Earth (T&E), three-core and earth, SWA armoured, flexible (flex), and data cable — what each is for, what size to use, and the BS 7671 18th Edition rules that govern the choice.

The Most-Used Cables at a Glance

Twin and Earth (6242Y) — The UK's Standard Domestic Cable

Twin and Earth is the cable used for virtually every fixed domestic circuit in the UK. It contains two insulated conductors (live and neutral) plus a bare earth wire, all sheathed in grey PVC. The designation "6242Y" is the British Standard reference. It complies with BS 6004.

1.0mm² T&E — Lighting Circuits

The standard cable for domestic lighting circuits. A 1.0mm² cable is rated at 13.5A in free air (installation method B, clipped direct), giving comfortable headroom above the 6A MCB that typically protects a lighting circuit. On a standard lighting circuit with 10–15 pendants or downlights, a 6A MCB and 1.0mm² T&E is the correct combination.

When to upsize: If the lighting circuit also feeds a long cable run (over 25m one-way), or if the circuit supplies high-wattage luminaires or emergency lighting packs, check the voltage drop calculation — you may need 1.5mm². Long garden lighting runs almost always require 1.5mm² to stay within the 3% voltage drop allowance.

1.5mm² T&E — Lighting, Showers (old installs), Some Spurs

1.5mm² is used where 1.0mm² would be marginal on voltage drop (longer lighting runs), and on older shower circuits where 15A showers were standard. In modern installs, 1.5mm² is also used for fan circuits, smoke alarm circuits, and doorbell circuits where the load is light but the run is long.

2.5mm² T&E — Ring Mains and Socket Circuits

The most-bought cable size in any electrical wholesaler. 2.5mm² T&E is the cable for ring final circuits (the standard BS 7671 configuration for socket outlets in domestic premises, protected by a 32A MCB). It is also used for radial socket circuits up to 20A.

On a ring main, the cable leaves the consumer unit, visits each socket in turn, and returns to the same MCB — giving each socket two cable paths to the source and halving the effective impedance. The ring must use 2.5mm² throughout; you cannot spur from a ring using 1.0mm².

Cut-to-length at our Acton counter: We sell 2.5mm² T&E from the drum — no coils, no minimum order. Pay for exactly what the job needs. In-store only at 24 Western Avenue, Acton W3 7TZ.

4mm² T&E — Cookers, High-Draw Appliances

4mm² is rated at 25A clipped direct (method B) and is used for electric cooker circuits (typically 32A MCB — derate applies as per Table 4B1 grouping factors), electric shower circuits where the shower is 7.5–8.5kW, and double ovens. For showers above 8.5kW (10kW units require approximately 45A at 230V), you will need 6mm².

6mm² T&E — High-Power Showers, Larger Cookers

6mm² T&E (rated 32A clipped direct) covers 10kW electric showers (45A MCB — check the derating tables for your grouping/insulation situation), large range cookers, and some heat pump connections. It is also used for outbuilding feeds where the load does not justify SWA armoured cable and the run is short and surface-mounted.

10mm² T&E — Consumer Unit Tails, Main Feeds

10mm² is rarely used for final circuits. Its main domestic use is as incoming tails from the meter to the consumer unit (the 25mm² service cable from the street terminates at a 100A cut-out fuse; 10mm² tails connect the cut-out to the meter and the meter to the consumer unit). Also used for electric vehicle charger feeds on dedicated circuits, and for larger outbuilding sub-mains where the run is short.

Three-Core and Earth (6243Y)

Three-core and earth contains three insulated conductors (brown/live, blue/neutral, black/switched live or second live) plus a bare earth. Despite the blue conductor's standard colour in T&E, the third core in 6243Y is black — used to carry the switched live in a two-way switching arrangement.

When to Use Three-Core and Earth

• Two-way switching (staircase, corridor): The strapper cable between two 2-way switches uses 3-core and earth. Wire colours: brown = common, black and grey (or black and blue in older cable) = strappers.
• Intermediate switching (3+ positions): The cable between the two 2-way switches and the intermediate switch(es) uses 3-core.
• Some motor and timer circuits where three active conductors are required.

3-core and earth is not used for socket circuits or cooker circuits in standard domestic installations.

SWA Armoured Cable

Steel Wire Armoured cable (SWA) is used wherever the cable is exposed to mechanical damage, laid underground, or run outdoors. The steel wire armouring protects the cable from physical damage and, if properly terminated and earthed, also provides a low-impedance fault path.

When SWA Is Required

• Underground runs — from house to garage, outbuilding, garden lighting supply, EV charger in a detached garage. SWA buried direct (without conduit) complies with BS 7671 if at the correct depth (typically 500mm under garden areas, 600mm under driveways and areas subject to mechanical disturbance).
• Exposed outdoor runs — surface-mounted on an outside wall from the meter cupboard to an outbuilding.
• Sub-main cables — feeding a distribution board in a commercial or industrial unit from the main incomer.
• Plant room connections — anywhere the cable is routed through an area where it could be struck, crushed, or subject to rodent damage.

SWA Termination: SY Glands

SWA cable must be terminated with SY (steel wire armour) cable glands that grip the armour wires. The gland provides mechanical retention and an earth continuity path through the armour. A common error is to terminate SWA in a standard PVC gland — this secures the outer sheath only, leaves the armour unearthed, and is non-compliant.

2-Core vs 3-Core SWA

For a single-phase feed to an outbuilding or sub-board where the earth is provided by the armour (TN-S or TN-C-S system), 2-core SWA is correct. The armour acts as the circuit protective conductor (CPC/earth). Install an earth continuity test on the armour and connect both glands to earth at each end.

For 3-phase circuits, 3-core or 4-core SWA is required. Always use 4-core if a separate neutral is needed for a 3-phase 4-wire system.

SWA Sizing for Outbuilding Feeds

These figures assume direct burial or free-air installation without grouping with other circuits. Always apply the derating factors in BS 7671 Table 4B1 for your specific installation method and grouping.

Flexible Cord

Flexible cord (flex) is used for connections where relative movement between the fixed wiring and the appliance is expected — pendant luminaires, appliance leads, and extension leads. Common types:

• 3182Y (2-core): For Class II (double-insulated) appliances and luminaires with no earth connection required
• 3183Y (3-core): The standard 3-core flex for all Class I appliances with an exposed metal body — electric kettles, toasters, washing machines, luminaires with metal fittings
• 0.75mm²: Lighting pendants, small appliances (up to 6A)
• 1.0mm²: General appliances up to 10A
• 1.5mm²: High-draw appliances: electric kettles, irons, fan heaters, washing machines
• 2.5mm²: Electric lawnmowers, high-load extension leads, some heating appliances

Key BS 7671 Rules: Sizing, Derating, and Voltage Drop

Voltage Drop

Under BS 7671 Regulation 525, the voltage drop between the origin of the installation (meter/cut-out) and any load must not exceed 3% for lighting circuits or 5% for power circuits (using 230V as the base voltage). On long runs, voltage drop is the binding constraint — you may need to upsize from what the current rating alone would suggest. For any run over 20m on a lighting circuit, or over 30m on a socket circuit, check the voltage drop table in Appendix 4 of BS 7671.

Grouping and Derating

When multiple cables are grouped together in a bundle, duct, or trunking, the maximum current-carrying capacity of each cable is reduced. The derating factors are in Table 4B1 of BS 7671. For most domestic work with cables clipped direct to the surface and not bunched, the tabulated values apply directly. In trunking with many cables, apply the factor — two cables grouped together derate to 0.80 of the tabulated current.

Thermal Insulation

Cable buried in or in contact with thermal insulation loses its ability to dissipate heat and must be derated significantly. A cable fully surrounded by insulation (e.g. run through a loft floor in blown mineral wool) should be derated by a factor of 0.5. In practice, this means a 2.5mm² T&E that would normally carry 20A clipped direct may only safely carry 10A in insulation — always check the route before specifying cable size.

Buy Cable at APM Electricals, Acton

We stock a full range of cable for same-day collection from our trade counter in Acton. Grey T&E in 1.0mm², 1.5mm², 2.5mm², 4mm², 6mm², and 10mm² is available by the metre from the drum — no coils, no minimum, no overbuying. SWA armoured cable in popular sizes. Flexible cord in 0.75mm², 1.0mm², 1.5mm², and 2.5mm².

For large orders on specific drum sizes, call ahead to confirm stock: 020 8702 8080. Visit us at 24 Western Avenue, Acton, London W3 7TZ. Trade accounts available in store — 30-day terms for regular trade customers.

Related guides: T&E cable sizes explained | APM trade blog