Choosing the wrong cable for a domestic circuit is not simply a code violation — undersized conductors overheat under load, damaging insulation and creating fire risk. Oversized cables are wasteful and can cause protective devices to fail to operate quickly enough under fault conditions. Getting cable selection right means understanding current-carrying capacity, installation method, and the correction factors that apply to your specific situation.
This guide covers the cable types used in UK domestic wiring, their current ratings, and typical applications, all referenced to BS 7671:2018+A2:2022 (the 18th Edition Wiring Regulations).
The Most Common Cable Types
Twin and Earth (TWE) — Flat Grey PVC
The standard cable for fixed wiring throughout a UK house. A flat PVC sheath contains two insulated conductors (live in brown, neutral in blue) and an uninsulated bare earth conductor. The earth must be sleeved green-and-yellow where exposed.
Manufacturer designation: 6242Y (twin and earth, PVC insulated, PVC sheathed).
Typical applications:
- Lighting circuits: 1 mm² or 1.5 mm²
- Socket ring mains: 2.5 mm²
- Cooker/oven circuits: 6 mm² or 10 mm²
- Shower circuits: 6 mm² or 10 mm²
- Consumer unit tails: 16 mm² or 25 mm²
Three-Core and Earth
Used where a switch-wire return is needed — commonly in two-way lighting circuits, ceiling fans with separate light/fan controls, and some appliance connections. Conductors are brown, black (must be re-identified with brown tape where used as a switch wire), grey, and bare earth.
Steel Wire Armoured (SWA)
SWA cable has individual XLPE or PVC insulated cores, a bedding layer, steel wire armour, and an outer PVC sheath. It can be buried directly in the ground, clipped to external surfaces, or run in areas where mechanical damage is a risk. The armour itself can serve as the earth conductor in many installations, though a separate earth is safer practice.
Typical domestic uses: sub-mains to outbuildings, garden power, buried runs to garages and sheds.
Flexible Cable (Flex)
Flexible cable uses finely stranded conductors to allow repeated bending without conductor fatigue — used for appliance connections, extension leads, and pendant lighting. It must not be used for fixed wiring buried in walls or chased into plaster.
Common flex types:
- 2182Y — two-core (unearth appliances, table lamps)
- 3182Y — three-core (earthed appliances)
- 3183Y — three-core, rubber-insulated (for higher-temperature applications)
MICC (Mineral Insulated Copper Clad)
A copper tube filled with magnesium oxide powder encasing copper conductors — extremely fire-resistant and mechanically robust. Used in commercial fire-alarm circuits, escape lighting, and boiler rooms. Rare in ordinary domestic work but occasionally specified in kitchens or plant rooms.
Current Ratings and Cable Sizing
Current ratings in BS 7671 depend on the installation method (clipped to surface, enclosed in insulation, in conduit, buried, etc.) and correction factors for ambient temperature, grouping with other cables, and thermal insulation proximity.
The table below gives simplified reference ratings for common twin-and-earth cables in domestic use, assuming installation method C (clipped direct to a non-thermally insulating surface) and no grouping or insulation correction required:
| Conductor Cross-section | Installation Method C (A) | Typical Circuit Use |
|---|---|---|
| 1.0 mm² | 16 A | Lighting (up to 16 A OCPD) |
| 1.5 mm² | 20 A | Lighting circuits (standard) |
| 2.5 mm² | 27 A | Socket ring main (32 A OCPD) |
| 4.0 mm² | 37 A | Cooker (small), radial socket |
| 6.0 mm² | 47 A | Shower (up to 45 A OCPD) |
| 10.0 mm² | 64 A | Cooker/range up to 45 A, EV charger |
| 16.0 mm² | 85 A | Sub-main, consumer unit tails |
| 25.0 mm² | 110 A | Service tails, large sub-mains |
Important: these figures are indicative. Cables buried in thermal insulation (e.g., within a wall cavity or under loft insulation) require a significant derating — sometimes as low as 0.5× of the clipped rating. Always apply the appropriate BS 7671 correction factors or use an approved design tool.
Sizing by Circuit Type
Lighting Circuits
Most domestic lighting circuits are wired in 1.5 mm² twin-and-earth, protected by a 6 A or 10 A MCB. A single circuit should not supply more than around 1,200–1,500 W at 10 A. LED lamp loads are so low that a single circuit can serve a whole floor in most houses — the traditional practice of limiting to 8–10 outlets (assuming 100 W per lamp) is largely obsolete.
Socket Ring Mains
The standard domestic socket circuit is a ring main wired in 2.5 mm² twin-and-earth, protected by a 32 A MCB or BS 1362 fused connection. Each leg of the ring must be continuous — spurs are permissible but regulated. A ring main can serve a floor area up to 100 m² per BS 7671.
Cooker Circuits
A cooker circuit is radial and sized to the demand of the appliance. The Wiring Regulations allow diversity calculations:
- First 10 A of demand + 30 % of remainder + 5 A if a socket outlet is included in the cooker control unit.
In practice, most 10–13 kW electric ranges are wired in 6 mm² on a 32 A MCB, or 10 mm² on a 40 A MCB for induction ranges or large range-cookers.
Electric Shower Circuits
Shower circuits are radial, dedicated, and sized to the shower’s rated power:
| Shower Power | Minimum Cable Size | MCB Rating |
|---|---|---|
| Up to 7.2 kW | 6 mm² | 32 A |
| 7.2–9.5 kW | 6 mm² | 40 A |
| 9.5–12.5 kW | 10 mm² | 45 A / 50 A |
A double-pole isolating switch rated to at least 45 A must be installed adjacent to the shower in a position accessible in an emergency but outside any bathroom zone.
EV Charger Circuits
A standard 7.4 kW (32 A) home EV charger requires 6 mm² cable on a dedicated 40 A RCBO, ideally Type B (to handle DC fault currents produced by EV chargers). Runs longer than 10–12 m typically step up to 10 mm² to limit voltage drop to below 3 % of nominal (BS 7671 Regulation 525).
Voltage Drop
BS 7671 limits voltage drop to 3 % for lighting and 5 % for power circuits under normal load. For long cable runs — to an outbuilding or across a large house — voltage drop may dictate a larger conductor than the current-carrying capacity alone would require. Use the formula:
V-drop (mV) = (mV/A/m from BS 7671 Table) × current (A) × length (m) ÷ 1,000
Table values for 2.5 mm² TWE are approximately 18 mV/A/m.
Material Costs (Per Metre, Mid-2026)
| Cable Type & Size | Price per Metre (approx.) |
|---|---|
| 1.5 mm² twin & earth | £0.55–£0.85 |
| 2.5 mm² twin & earth | £0.85–£1.30 |
| 6.0 mm² twin & earth | £2.20–£3.20 |
| 10.0 mm² twin & earth | £3.80–£5.20 |
| 4.0 mm² 3-core SWA (2-core) | £3.50–£5.00 |
| 6.0 mm² SWA (2-core) | £5.50–£8.00 |
| 1.5 mm² 3-core flex | £0.60–£1.00 |
Prices vary by supplier and quantity. Builders’ merchants typically give trade discounts of 15–25 % on listed prices.
Key Rules to Remember
- All new fixed electrical work in a home is notifiable under Part P of the Building Regulations unless it is in a room other than a kitchen, bathroom, or outside, and involves only adding to an existing circuit (not creating a new one).
- Cable buried in a wall must be in a zone (within 50 mm of the top of the wall or within 150 mm of a corner), run horizontally or vertically, or protected by a mechanical barrier — unless a metal-clad wiring system is used.
- Do not mix aluminium and copper conductors without an approved mechanical connector — galvanic corrosion at the joint causes resistance heating.
- Always label all circuits clearly at the consumer unit after completing work; label direction of travel if cables are concealed.
If you are not a Part P registered electrician, have all new circuit work and consumer unit changes certified by a competent person before signing off on any building project.
