6mm Twin and Earth Cable (6242Y): Cooker and Shower Circuit Wiring

You install a new induction hob in your kitchen extension, wired with 6mm cable on a 32A breaker. It works perfectly for three years. Then the loft insulation goes in above the ceiling, surrounding the cable run for two metres. Nobody recalculates. The cable that was safely carrying 47A is now derated to roughly 23A, and your 7.4kW hob pulls 32A at full load. The breaker doesn't trip because the sustained current sits below 32A most of the time. But the cable runs hot. Insulation degrades slowly. You won't know until something fails. This is why understanding 6mm cable matters. The size alone tells you nothing without knowing how installation conditions change its capacity.

What it is and what it's for

6mm twin and earth is the heavy-duty cable used for high-power domestic circuits. The "6mm" refers to the cross-sectional area of each current-carrying conductor (live and neutral), measured in square millimetres. Compared to the 2.5mm cable that wires your sockets or the 1.5mm cable for lighting, 6mm is physically larger, noticeably stiffer, and carries far more current.

The cable designation is 6242Y for standard PVC-sheathed versions (grey) and 6242B for LSOH (low smoke zero halogen) versions (white). The construction is two 6.0mm stranded copper conductors (brown live, blue neutral) plus a 2.5mm solid copper earth wire, all inside a flat grey PVC sheath. The overall dimensions are approximately 6.9mm x 13.8mm. That's noticeably wider and thicker than 2.5mm T&E, which matters when you're routing it through walls and into back boxes.

An important difference from smaller cable sizes: 6mm conductors are stranded copper, not solid. Below 4mm, twin and earth uses solid single-strand conductors. At 4mm and above, the conductors are made of multiple fine strands twisted together. Stranded conductors are more flexible than solid ones of the same size, but 6mm cable is still substantially stiffer than anything else you'll handle in domestic wiring. The earth conductor, however, remains a single solid 2.5mm wire regardless.

When your extension needs 6mm cable

Three appliance categories drive 6mm cable into a kitchen extension:

• Electric cookers and ovens (standalone or built-in, typically 3kW to 14kW)
• Induction hobs (typically 6kW to 11kW)
• Electric showers (up to approximately 7.5kW on 6mm; above this, you need 10mm)

Each of these appliances gets its own dedicated radial circuit from the consumer unit, protected by a 32A MCB or RCBO. You don't share a 6mm circuit between two high-power appliances. A cooker and a hob that are separate units need separate circuits, each on its own cable run from the consumer unit. Trying to daisy-chain two heavy appliances from one 6mm cable is a common planning mistake that an electrician will flag immediately.

Current capacity and the derating problem

This is the single most misunderstood concept around 6mm cable, and it's the one that actually matters for your installation.

The headline figure for 6mm twin and earth is 47A when clipped directly to a surface (BS 7671 Reference Method C, at 30 degrees ambient temperature). That number appears on every product listing and cable specification sheet. It gives you a comfortable margin above a 32A MCB. Seems straightforward.

It isn't. That 47A figure only applies when the cable is clipped to a wall or joist surface in free air, able to shed heat in all directions. Change the installation method and the number drops:

Look at that last row. Fully surrounded by insulation, your 6mm cable drops to roughly 23A. That's below the 32A MCB protecting the circuit. The MCB won't trip at 23A, so the cable overheats before the protection device activates. This is not a theoretical risk. It's the exact scenario that causes thermal damage to cable insulation over months and years of use.

Click to enlarge

The practical consequence: your electrician chooses cable size based on the worst-case installation condition along the entire cable route. If even one section passes through insulation for more than half a metre, the derated capacity applies to the sizing calculation. This is why some electricians default to 10mm for cooker circuits in new builds, even when the diversity calculation says 6mm is adequate. The margin of safety matters.

The 6mm vs 10mm decision

This is the question that fills forum threads. The answer depends on what you're connecting and how the cable is installed.

Cookers and ovens

BS 7671 includes a diversity formula (Regulation 311.1) for cooking appliances. Diversity recognises that a cooker rated at 10kW doesn't draw 10kW continuously because not every element runs at maximum simultaneously. The formula reduces the design current, which is why 6mm cable on a 32A MCB handles cookers with much higher headline ratings than you'd expect:

These figures assume cable clipped directly to a surface at 30 degrees ambient, with a 5A allowance for a cooker control unit socket. Under ideal installation conditions, 6mm handles domestic cookers up to about 14kW with diversity applied.

But "ideal installation conditions" is doing a lot of work in that sentence. If any section of the cable run passes through insulation, if the ambient temperature exceeds 30 degrees (a loft space in summer easily hits 40-50 degrees), or if the run is long enough for voltage drop to matter, the calculation shifts.

Range cookers (Rangemaster, Britannia, Lacanche) are the edge case. These are often rated at 15kW or above. The diversity calculation technically puts a 15kW range cooker within 6mm territory at 31.6A diversified. But the professional consensus on forums and among experienced electricians is clear: use 10mm for range cookers. The margin is too thin, and the cost difference for a typical 15m run is roughly £20 – £30. That's not worth the risk.

Electric showers

Showers are different from cookers. There's no diversity to apply because an electric shower draws its full rated current whenever it's running. The cable must carry the actual load, not a reduced calculated load.

The practical boundaries:

• Up to 7.5kW: 6mm cable is adequate (draws approximately 32.6A, within the 47A clipped-direct rating and comfortably below a 40A MCB)
• 8kW and above: 10mm cable is required (an 8.5kW shower draws 37A; a 10.5kW shower draws 45.7A)

Triton, one of the UK's major shower manufacturers, states 6mm may be adequate for showers "up to 8kW in some cases," but the professional consensus is more conservative. Most electricians will spec 10mm for anything above 7.5kW, and that's the right approach. The cable run to a shower often passes through floor voids, ceiling spaces, and potentially insulated areas, all of which derate the capacity.

If you're installing an electric shower in a kitchen extension (common when adding a utility room), the electrician needs to know the exact model and kW rating before ordering cable. A 7.5kW shower on 6mm is fine. A 9.5kW shower on 6mm is not. The difference between those two products is about ~£30 at the retailer. The difference in cable specification is the cost of ripping out a cable run and starting again.

Voltage drop on long runs

BS 7671 limits voltage drop to 5% for power circuits. At 20m of cable run for a 9.5kW shower (drawing approximately 41A), the voltage drop through 6mm cable exceeds this limit. 10mm cable solves it. For cooker circuits with diversity applied (typically 25-32A design current), voltage drop on 6mm is rarely the limiting factor unless the consumer unit is at the opposite end of the house from the kitchen, creating runs of 25m or more.

Your electrician calculates this. But if your consumer unit is in a hallway at the front of the house and the new kitchen extension is at the rear, mention the distance when getting quotes. It affects material specification.

How to work with it

You won't be wiring the circuit yourself. New high-power circuits are notifiable work under Part P of the Building Regulations, meaning a registered electrician must design, install, test, and certify the work. But you'll likely be buying the cable, and understanding how it handles helps you plan the job and spot problems.

Physical handling

6mm cable is a different beast from 1.5mm or 2.5mm. The stranded conductors give it some flexibility, but the overall stiffness means you can't route it around tight corners the way you do with lighter cable. The minimum inside bend radius is approximately 41mm (three times the overall cable diameter of 13.8mm, per BS 7671 guidance). Force it tighter and you risk cracking the PVC sheath or deforming the conductors.

In practice, this means planning cable routes with gentle curves rather than sharp right-angle turns. Where cable enters a back box (the metal or plastic enclosure behind a cooker connection unit or switch), you need 35mm deep back boxes, not the standard 25mm ones. The cable is too stiff to dress neatly into a shallow box, and forcing it causes stress on the insulation at the terminal connections.

Clipping and routing

Fix 6mm T&E to surfaces using flat cable clips at 300mm intervals on horizontal runs and 400mm on vertical runs. The cable runs in BS 7671 safe zones: vertically above or below accessories, and horizontally within 150mm of ceiling or floor level. These zones tell anyone working on the wall in the future where cables are likely to be.

When running through joists, drill through the centre (the neutral axis where stress is lowest). Holes for 6mm cable need to be at least 16mm diameter (the cable is 13.8mm at its widest), but 20mm gives you enough clearance to pull the cable through without scraping the sheath. Holes should be no larger than one-quarter of the joist depth.

Termination

6mm cable typically terminates at a cooker connection unit (CCU) for cooker circuits. The CCU is a faceplate with a 13A socket and a 45A switch, mounted on the wall near the cooker position. The 6mm twin and earth runs from the consumer unit to this CCU. A short length of H07RN-F flexible rubber cable then runs from the CCU to the actual appliance for the final connection.

Stripping 6mm cable requires a sharp knife and a steady hand. Slit the grey outer sheath lengthways, taking care not to nick the brown or blue conductor insulation underneath. The stranded conductors need to be neatly trimmed and inserted fully into the terminal without stray strands escaping. A single stray strand touching the wrong terminal or the back box metalwork is a short circuit waiting to happen.

The bare 2.5mm earth conductor must be covered with green/yellow PVC sleeving (5mm diameter for 6mm cable) at every termination point, just as with smaller cable sizes.

How much do you need

A typical single-storey kitchen extension needs one or two dedicated 6mm circuits:

• Circuit 1: Consumer unit to cooker connection unit (oven/hob position). Typical cable run: 10-15m depending on consumer unit location.
• Circuit 2 (if applicable): Consumer unit to electric shower position, if installing one. Typical run: 8-12m.

Each circuit needs its own unbroken cable run from the consumer unit. You can't branch or spur off a 6mm radial circuit the way you can with a ring main.

Worked example for a kitchen extension with a single cooker circuit:

• Cable route from consumer unit (hallway) through ceiling void to kitchen extension: 12m
• Allowance for drops through walls and routing around obstacles: 3m
• Slack at each end for termination: 0.5m
• Subtotal: 15.5m
• Plus 10% contingency: 17m
• Order: 1 x 25m drum (gives you 8m spare for re-routes)

If you need two circuits (cooker plus shower), a 50m drum works out cheaper per metre than two 25m drums. A 50m drum at around £100 – £120 versus two 25m drums at £63 – £70 each saves you roughly ~£25.

Cost and where to buy

Current retail prices for 6mm 6242Y twin and earth cable in the UK (March 2026):

Small coils carry a substantial premium. The 10m coil at £3.10/m per metre is 50% more expensive per metre than the 50m drum at £2 – £2. If you need more than 15m of 6mm cable, a 25m drum is always better value. If you need more than 30m, go for the 50m drum.

Trade wholesalers (CEF, Edmundson Electrical, Rexel) sell cheaper on account, typically 15-20% below retail. If your electrician buys materials separately, they'll source through these channels. If you're supplying materials yourself, Screwfix and Toolstation with click-and-collect are your best options.

Which brand?

Any cable carrying the BASEC approval mark meets BS 6004 and is suitable for installation. Beyond that mark, Prysmian (UK-manufactured) and Doncaster Cables (also UK-manufactured) are the standard professional choices. Pitacs carries BASEC approval and is consistently cheaper, but some electricians report the PVC sheath being harder to strip cleanly. The price difference between Pitacs and Prysmian on a 50m drum is roughly ~£4. Not worth agonising over.

PVC vs LSOH

Standard 6242Y cable has a grey PVC sheath. The LSOH variant (6242B) has a white sheath and produces less toxic smoke in a fire. For a standard domestic kitchen extension, grey PVC is the default and what your electrician will expect. LSOH is required in certain high-occupancy buildings and fire escape routes. Unless building control specifically requires it, buy standard PVC.

Alternatives

10mm twin and earth is the next size up, used on a 40A or 45A MCB. It's required for electric showers above 7.5kW, range cookers, and any 6mm application where derating through insulation pushes the capacity below the circuit protection level. It's heavier, stiffer, and more expensive (roughly £170 – £200 per 50m drum), but the per-circuit cost difference on a typical 15m run is about £20 – £30. If there's any doubt about whether 6mm is adequate, 10mm removes it.

4mm twin and earth is occasionally used for long radial runs to individual sockets or smaller appliances where 2.5mm would suffer from voltage drop. It sits between 2.5mm and 6mm in both current capacity and physical size. Not used for cooker or shower circuits.

2.5mm twin and earth is the standard for socket ring mains and radial circuits. It handles up to 20A clipped direct. Don't confuse it with 6mm. If a circuit needs a 32A MCB, 2.5mm cable is not adequate regardless of the installation method.

Where you'll need this

• First fix electrics - high-power circuit cables run to cooker and shower positions during first fix, before walls are closed up
• Electrical layout planning - the decision about appliance types and positions determines how many 6mm (or 10mm) circuits you need and where they route

Cable ordering should happen during pre-construction, once the electrical layout is agreed and your electrician has provided a cable schedule. Don't leave it until first fix week. Cable prices track copper commodity markets and stock availability fluctuates at merchants.

Common mistakes

Assuming the headline 47A capacity applies everywhere. The 47A figure is for cable clipped directly to a surface in open air. The moment cable passes through insulation, conduit, or a confined space, the effective capacity drops. This is the number one misunderstanding in every forum discussion about 6mm cable. Your electrician calculates the actual capacity for your specific installation method.

Feeding two appliances from one circuit. A cooker and a separate hob are two appliances, not one. Each needs its own dedicated radial circuit from the consumer unit, each with its own 6mm (or 10mm) cable and its own 32A MCB. Running a single 6mm cable to a junction box and splitting it to two appliances is dangerous and non-compliant.

Using 25mm deep back boxes for cooker connection units. 6mm cable won't dress neatly into a shallow back box. Use 35mm deep boxes for any accessory terminating 6mm cable. This is a £1 difference at the merchant and saves a frustrating half-hour on site trying to force stiff conductors into a cramped space.

Specifying 6mm for a high-power shower without checking the kW rating. A 7.5kW shower on 6mm is fine. A 9.5kW shower on 6mm is not. The shower model must be confirmed before the cable is ordered. Upgrading from 6mm to 10mm after the cable is already in the wall means ripping it out and starting again.

Trusting online cable calculators without understanding their assumptions. Several free calculators produce results based on ideal conditions (clipped direct, 30 degrees ambient, no insulation). Real installations rarely match these assumptions. The calculator says 6mm is fine; the actual installation through a loft void surrounded by mineral wool says otherwise. Let a qualified electrician run the calculation for your specific route.

Installing new circuits without Part P notification. New cooker and shower circuits in an extension are notifiable work under Part P (England and Wales). This means either a Part P registered electrician carries out the work, or you notify building control before starting. Connecting a new appliance to an existing circuit with an existing connection unit may not require notification, but running a new cable from the consumer unit always does.