10mm Twin and Earth Cable (6242Y): When You Need It and Why

You order a 10.5kW induction hob for the new kitchen. The kitchen designer's quote sheet says nothing about cable size, just the model number. Your electrician runs 6mm to the cooker switch position because that's what every other cooker circuit on the job has needed. The walls get plastered. Six weeks later the hob arrives with a one-page installation manual that specifies, in plain English: minimum 10mm² supply cable, 40A or 45A protective device on a dedicated circuit. The 6mm cable already in the wall isn't compliant. Now you're cutting a chase from the consumer unit to the hob position through a finished plastered wall to swap it. This page tells you when 10mm cable is mandatory, when 6mm gets away with it, and the practical traps (back box depth, terminal connectors, conduit sizing) that catch homeowners doing first fix.

What it is and what it's for

10mm twin and earth is the heaviest-duty domestic flat cable in common use. The "10mm" refers to the cross-sectional area of each current-carrying conductor (live and neutral), measured in square millimetres. That's almost double the conductor area of 6mm cable and four times that of standard 2.5mm socket cable.

The designation is 6242Y for grey PVC versions (the standard) and 6242B for white LSOH (low smoke zero halogen) versions. Construction: two stranded copper conductors of 10mm² each (brown live, blue neutral), plus a 4mm² bare stranded copper earth wire (the CPC, or circuit protective conductor), all inside a flat grey PVC sheath manufactured to BS 6004. The overall dimensions are roughly 8.4mm thick by 16.8mm wide. That's noticeably bigger than the 6.9mm by 13.8mm of 6mm cable, and the difference matters when you try to fit it into a back box.

A 10mm cable is rated 64A clipped directly to a surface (BS 7671 Table 4D5, Reference Method C, 30°C ambient). In conduit in a wall it drops to 57A. Surrounded by thermal insulation for more than 500mm of its run, capacity falls further to roughly 32A. Compare with 6mm: 47A clipped, 41A in conduit, around 23A in insulation. The headline difference is bigger than it sounds. In an insulated kitchen ceiling void, 10mm has the headroom for a 9.5kW shower; 6mm doesn't.

When your extension actually needs 10mm

Two appliance categories drive 10mm into a kitchen extension. Get either of these wrong and you're cutting walls open later.

Large induction hobs above 7.4kW. A standard 7.4kW induction hob draws 32A at full load; with diversity applied per BS 7671 Regulation 311.1, the design current drops to about 17A and 6mm cable is comfortable on a 32A breaker. Move up to a 9, 10, or 11kW model (Bosch, Siemens, AEG, Neff all sell hobs in this range) and the picture changes. A 10.8kW hob pulls 47A at full load. Diversity calculations theoretically reduce that, but here's the catch: BS 7671 Regulation 134.1.1 requires the installation to comply with manufacturer specifications. If the hob's installation manual specifies "minimum 10mm² supply cable", that overrides any diversity calculation you might run. The datasheet wins.

Electric showers from 9.5kW upwards. Showers are simpler than hobs because diversity does not apply: you size the circuit for the full rated current. A 9.5kW shower draws 41.3A. A 10.5kW shower draws 45.7A. Both push past 6mm cable's clipped-direct rating of 47A as soon as you add any derating for cable run length, ambient temperature, or insulation. Standard practice is 10mm cable on a RCBO rated at 40A (for 9.5kW) or 45A (for 10.5kW).

There's a third reason 10mm gets specified: long cable runs where voltage drop becomes a constraint. BS 7671 limits voltage drop on power circuits to 5% of nominal supply. For a 32A circuit on 6mm copper at 17m run length, you're already approaching that limit. Upsizing to 10mm halves the voltage drop and resolves the compliance issue without re-routing the cable. This catches you out on extensions where the consumer unit is in a garage or front utility cupboard and the new kitchen is 15m+ away at the rear of the house.

The "specify before first fix" rule

This is the single most expensive mistake in extension electrics. Every working electrician will tell you the same thing: choose your hob before first fix. The rated power on the hob's installation manual determines the circuit cable size. The cable goes into the wall during first fix. The wall gets plastered. The hob arrives months later, sometimes a year later, and that's when you discover the spec.

If the hob datasheet calls for 10mm and you've installed 6mm, you have three options: cut a chase through finished plaster from the consumer unit to the hob position to pull new cable; downgrade your hob to a smaller model; or accept a non-compliant installation that will fail an EICR. None of these are cheap or pleasant.

The professional answer when the hob choice isn't finalised is to run 10mm anyway. The cost premium over 6mm is roughly £20-£30 for a typical 15m kitchen run. Against a six-figure build budget, that's noise. Against the cost of reopening walls, it's nothing.

Working with 10mm cable

You won't be doing the wiring yourself. New high-load circuits in an extension are notifiable work under Part P and must be installed by a registered competent person or notified to building control. But you'll be buying the cable, you may be pulling it through the structure, and if you're project managing first fix you need to understand what your electrician is dealing with.

Stiffness and routing

10mm 6242Y is genuinely stiff. The combination of larger copper conductors, thicker insulation, and the 4mm² CPC inside the sheath makes the cable significantly harder to bend than 6mm. Pulling a 50m drum off the reel solo is impractical: you need one person feeding cable while another routes it. Tight bends (less than six times the cable diameter) damage the insulation. Plan radius generously around corners.

Through stud walls, drill 32mm holes for clean pulls; 25mm is too tight when you account for cable lubricant and the natural twist. Through joists, drill at the centre (neutral axis), holes no larger than one quarter of joist depth. A 200mm joist takes a 50mm hole comfortably.

The conduit problem

If your build includes a screeded floor over insulation (typical for an underfloor-heated kitchen extension), and the cable run goes under that floor, your electrician will want conduit. Standard 20mm conduit is borderline for 10mm T&E (the cable jams when pulling around bends). Specify 25mm or 32mm conduit if there are any direction changes underfloor.

A practical insurance trick from professionals: lay 32mm waste pipe as conduit during groundwork, even if you're certain you only need 6mm cable. The pipe costs around £8 for a 3m length at any builders merchant. If you upgrade your hob in five years and need to swap to 10mm, you pull the new cable through the conduit without touching the screed. A homeowner who skips this and later wants to run a different cable to the hob position has to break up an underfloor heated screeded floor. Don't be that homeowner.

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The back box trap

This is the single most common practical complaint about 10mm cable: standard back boxes are too shallow.

A typical 13A socket back box is 25mm deep. A standard cooker switch box is often 35mm. Neither is adequate for 10mm cable. The cable's stiffness, combined with the bulk of the 4mm² CPC needing to be sleeved in green and yellow at termination, means there is simply no room to dress the conductors into the terminals on a 25mm or 35mm box. Electricians who have tried describe the brass terminals as "virtually impossible to tighten down" because the conductor and the cable jacket fight you for space.

Specify 47mm minimum depth for any single-gang cooker switch terminating 10mm cable. 56mm is more comfortable. Better still, use a double-gang format box and switch (the extra width gives the cable room to bend properly). If you're surface-mounting (sometimes the practical choice in a utility room or behind kitchen units), a surface-mounted enclosure removes the depth constraint entirely.

The DIY shed shelves do not stock 47mm deep boxes by default. You will need to order them in advance from an electrical wholesaler, Screwfix Trade, or TLC Direct. Don't leave this to the day of second fix.

Sleeving the bare CPC

The 4mm² bare copper earth conductor inside 10mm T&E must be covered with green and yellow PVC sleeving at every termination point. Same as on smaller cables, but the bigger CPC needs 6mm sleeving rather than the 3mm sleeving that fits inside 2.5mm cable. Buy a roll of 6mm green/yellow sleeving alongside the cable drum or your electrician will have nothing to use at termination.

How much do you need

10mm cable runs are typically dedicated single-circuit feeds, not networked like socket rings. For a typical kitchen extension you need a single run from the consumer unit to the cooker connection unit position, plus separately for any electric shower circuit if applicable. Worked example for a 4m × 6m kitchen extension with the consumer unit in a garage 12m from the kitchen:

• Consumer unit to ceiling void: 1m
• Through ceiling void to kitchen: 12m
• Down the wall to cooker switch height: 1.5m
• Tail allowance for terminations at both ends: 1m
• Subtotal: 15.5m
• Plus 15% contingency (10mm has tighter routing than smaller cable, so wastage is higher): 18m
• Order: 1 × 25m drum or buy cut-to-length from a wholesaler

For longer runs (kitchen extensions on the opposite side of the house from the consumer unit) you'll comfortably use 25-30m. A 50m drum is the standard purchase size and gives you plenty of margin plus a useful offcut for any future second circuit.

100m drums of 10mm exist but are heavy (around 45kg) and rarely needed for a single domestic job. Stick with 50m drums for almost any extension scenario.

Cost and where to buy

A standard 50m drum of 10mm 6242Y from a UK retailer runs £170-£200 as of April 2026. The market is consistent across the major outlets:

Per-metre cost from a 50m drum works out at roughly £3.80 – £4.00. Cut-to-length retail (under 10m) carries a steep premium: £4.84 – £6.50 per metre. If you need more than 10m, always buy the drum.

The cost premium over 6mm cable is the figure most relevant to the "should I run 10mm anyway?" decision. A Screwfix 50m drum of 6mm Prysmian is £119.99; the 10mm equivalent is £199.99. That's a premium of around £80 per drum, or roughly 58% more per metre. The brief number "2-3x" sometimes quoted for the cost difference reflects cut-to-length pricing or older market data; for drum purchases the real premium is closer to 1.6x. On a typical 15m run for a hob circuit, the cable upgrade cost is approximately £20-£30.

Which brand?

BASEC approval is the non-negotiable buying criterion. The BASEC diamond mark on the drum label and printed along the cable sheath confirms compliance with BS 6004. Without it, your electrician's Part P certificate is at risk.

Beyond BASEC, brand matters in practice. Prysmian is the default professional recommendation: UK-manufactured, consistent sheath quality, holds shape well in clips, and strips cleanly. Doncaster Cables is the other major UK manufacturer; some electricians prefer their slightly softer insulation for tight terminations. Both carry a pre-sleeved earth conductor variant (Doncaster's Earthsure range) which eliminates field sleeving at every termination. Pitacs and Time (Screwfix's budget line) are imports from Turkey and Eastern Europe. They carry BASEC approval and are functionally compliant, but persistent trade complaints describe Pitacs sheath as "dark grey, not very flexible, tears easily" when stripping. The price gap between Prysmian and budget imports is rarely more than £10 per drum. Buy Prysmian or Doncaster.

For trade buyers with accounts, CEF and Edmundson Electrical price 10-20% below retail. If your electrician sources materials, they'll go through a wholesaler. If you're buying yourself, Screwfix or B&Q Trade Point is the practical option.

Alternatives

6mm twin and earth (covered in detail here) is the right choice for cooker and shower circuits up to 7.4kW hobs and 8kW showers, where derating doesn't reduce headroom too aggressively. Save the 10mm premium when 6mm is genuinely sufficient and you know your appliance specs in advance.

16mm twin and earth is the next size up. You'll never need it in a domestic kitchen extension. It exists for very long runs, for industrial circuits, or as a tail from a meter to a consumer unit (the meter tails specifically, not branch circuits). If anyone suggests 16mm for a hob or shower, get a second opinion.

SWA (steel wire armoured) cable is sometimes confused with high-current T&E by homeowners reading forum threads. SWA is a different cable entirely: it has steel wire armour around the conductors and is rated for direct burial or external runs. You'd use SWA from your consumer unit to a garden room or detached garage, not for an indoor kitchen circuit. Don't buy 4-core SWA when single-phase 10mm T&E is what your electrician needs. This mistake comes up regularly on forums when homeowners try to source materials independently.

Single core cables in conduit are an alternative to flat T&E for some commercial installations, but no domestic UK electrician will use them in your extension. Stick with flat T&E in 6242Y.

Where you'll need this

• First fix electrics - high-load cooker and shower circuits run during first fix, before walls close up
• Electrical layout planning - the cable size decision is triggered by the hob and shower kW ratings, which must be locked down before first fix begins

The buying decision is upstream of first fix. Cable schedules should be on your shopping list during pre-construction. Order at least a week before the electrician's start date; deep back boxes and 6mm green/yellow sleeving are not always in stock at the local shed.

Common mistakes

Not checking the hob datasheet. The single biggest cause of remediation work in kitchen extensions. Kitchen designers, builders, and even electricians sometimes assume "cooker cable is 6mm" and run cable before the appliance is finalised. A real case from BuildHub: kitchen designer told the homeowner the hob was "4kW" when the actual rating was 7kW. Electrician ran 4mm cable. Five hours of remediation work before the finishes went in. Always download the manufacturer's installation manual for your specific hob model and read the cable specification page yourself. Don't take anyone's word for it.

Using 6mm "to future-proof" a hob upgrade. This logic is backwards. 6mm cable does not future-proof anything. It locks you into hobs of 7.4kW or smaller for the lifetime of the cable. If you might upgrade in future, run 10mm now. The £20 – £30 cost difference at first fix is meaningless against the cost of cutting a chase through finished plaster later.

Buying 4-core SWA when single-phase 10mm T&E is the spec. A genuine forum mistake. Domestic UK kitchen circuits run on single-phase 230V supply. They need single-phase cable: live, neutral, earth. SWA cable comes in 2, 3, 4, or 5-core variants for various industrial and outdoor applications. None of them are what your kitchen needs. If you're buying 10mm cable for a domestic hob or shower, the only correct product is 10mm 6242Y flat twin and earth (grey PVC sheath, two cores plus bare earth).

Forgetting that 10mm doesn't connect to the hob terminal. The hob's terminal block typically accepts up to 6mm² flexible cable. Running 10mm T&E directly into a hob is mechanically impossible. The cable terminates at a cooker connection unit; a short length of 6mm heat-resistant butyl flex bridges the CCU to the hob.

Specifying a 40A breaker for a 10.5kW shower. 10.5kW at 230V is 45.7A. A 40A device is undersized. Specify 45A RCBO. Some installations get away with 40A because thermostatic shower controls limit sustained max draw, but EICR inspectors will flag it as C2 and you'll need to replace the breaker eventually.

Buying standard 25mm back boxes. The DIY shed default. Useless for 10mm cable. Order 47mm minimum depth boxes from an electrical wholesaler before second fix.

Skipping the conduit-under-screed insurance. A 32mm waste pipe laid under the screed costs around £8 and takes ten minutes during groundwork. It's a one-time investment that pays back the moment you change your mind about a hob or shower five years from now. Skip it and any future cable change requires breaking up the floor.

Forgetting that induction hobs need a Type A or Type F RCBO. Induction hobs leak around 10mA to earth from their RF suppression filters. Standard Type AC RCDs aren't designed for this leakage and will nuisance trip. The dedicated cooker circuit needs a Type A (or Type F for the most modern hobs) RCBO at the consumer unit. Mention this to your electrician at the planning stage; if they specify Type AC, that's a red flag.