4mm Twin and Earth Cable (6242Y): The Middle Cable for Long Radial Runs

Your electrician measures the run from your consumer unit in the front hall to the new kitchen island socket at the back of the extension. 22 metres. He shakes his head and tells you 2.5mm cable won't pass the voltage drop check on a 32A radial. You'll need 4mm. The drum costs roughly twice what 2.5mm does, the cable is noticeably stiffer, and your back boxes need to be 35mm deep instead of the 25mm boxes the kitchen fitter has already chiselled into the studs. Spotting this at quote stage costs you £165 – £179 and an extra hour or two of labour. Spotting it after first fix is plastered means tearing out a wall. This page tells you when 4mm cable is the right choice, how the voltage drop calculation works, and why the cable inside has a 1.5mm earth wire rather than 4mm.

What it is and what it's for

4mm twin and earth is the middle cable in the domestic twin-and-earth family. It sits between the everyday 2.5mm cable that wires your standard sockets and the heavy 6mm cable used for cookers and showers. The "4mm" refers to the cross-sectional area of each current-carrying conductor (live and neutral), measured in square millimetres.

4mm twin and earth is specified when a socket circuit needs more current capacity or lower voltage drop than 2.5mm can deliver. The two scenarios that drive 4mm into a domestic installation are long radial runs (where the 5% voltage drop limit becomes the limiting factor) and circuits with sustained heavy loads where the derated capacity of 2.5mm in insulation falls short.

The formal designation is 6242Y: the "6" means 300/500V insulation class, "24" means flat construction with a bare earth, "2" means two current-carrying cores, and "Y" means PVC sheathed. You'll also see 6242BH (white sheath, low smoke zero halogen) used in fire escape routes, schools, and care premises. For a domestic extension, standard grey 6242Y is what your electrician will specify.

The cable is approximately 11.4mm wide and 6.25mm thick. That's noticeably bigger than 2.5mm (10mm wide), and it feels significantly stiffer in the hand because the live and neutral conductors are now stranded copper rather than solid. Bend it sharply and it stays bent.

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The earth is 1.5mm, not 4mm

Open up a length of 4mm twin and earth and the bare copper earth conductor is 1.5mm, not 4mm. Beginners assume the earth matches the live and neutral. It doesn't. The bare CPC (circuit protective conductor) is visibly thinner than the insulated cores, the same 1.5mm size used in 2.5mm twin and earth.

This is permitted under BS 7671 Regulation 543.1.4 because the calculations show the smaller earth conductor still disconnects the circuit fast enough to prevent dangerous touch voltages during a fault. Your electrician will verify this with an earth fault loop impedance test (Zs test) at commissioning. The test result must come in below the maximum value listed in BS 7671 Table 41.3 for the protective device fitted.

What this means for you in practice: the earth wire must be sleeved with green and yellow PVC sleeving at every termination point, just as with 2.5mm cable. Use 3mm green/yellow sleeving for the 1.5mm earth conductor.

When you actually need 4mm cable

4mm is not a default upgrade. It's a problem-solving cable used in specific situations. The professional consensus on electrical forums is consistent: use 2.5mm where 2.5mm works, use 4mm where the voltage drop or current rating calculation demands it, and don't upsize for the sake of it. The cost premium and the termination hassle aren't justified unless the circuit needs the headroom.

Scenario 1: kitchen island socket on a long run

This is the most common scenario in a domestic extension. The kitchen island sits 6 to 10 metres into the room from the back wall. The consumer unit is at the front of the house. The cable run, including the route through ceiling voids and down walls, totals 18 to 25 metres. On a 32A radial protected by a 32A MCB, that run length pushes the voltage drop on 2.5mm cable past the 5% limit. 4mm halves the drop and brings the circuit back into compliance.

Scenario 2: detached garage or outbuilding circuit

A 32A feed to a detached garage commonly uses 4mm twin and earth (where the run is internal or in protected containment) or 4mm SWA (where it leaves the building or is buried). The garage might have a workbench socket, lighting, and a charging point for power tools. The 32A capacity gives headroom for occasional simultaneous loads, and the 4mm size keeps the voltage drop within limits over the typical 15 to 25 metre run.

Scenario 3: garden room or home office

A garden room is a small detached outbuilding used as an office, gym, or studio. The supply is typically a 32A radial from a sub-board near the house, run as 4mm SWA underground. Inside the garden room, sockets and lighting branch off the 4mm feed. A 16A or 20A circuit on 2.5mm would do the job for many garden rooms, but 4mm gives the option to add a heat pump, an EV charger, or a workshop machine in future without rewiring the supply.

Scenario 4: kitchen radial when the spark prefers it

Some electricians specify a 4mm 32A radial in preference to a 2.5mm 32A ring for kitchens with several high-draw appliances clustered around the worktop. The radial is simpler to test, easier to fault-find, and provides headroom. This is a design preference, not a regulatory requirement. If your electrician proposes a 4mm radial for the kitchen sockets, the question to ask is "Why this rather than a ring?" If the answer is about voltage drop, derating, or future-proofing, it's a sound choice.

The voltage drop calculation

This is the technical reason 4mm exists in the first place, and it's worth understanding even if your electrician handles the calculation.

BS 7671 Regulation 525.1 sets the maximum voltage drop on a power circuit (sockets, cookers, immersion, showers) at 5% of nominal supply voltage from the origin of the installation to the furthest point of utilisation. On a 230V supply, that's 11.5 volts. Lighting circuits are held to a tighter 3% limit (6.9 volts).

The formula in BS 7671 Appendix 4 is:

Voltage drop (V) = (mV/A/m × Ib × L) / 1000

Where:

• mV/A/m is the cable's voltage drop coefficient from Table 4D5
• Ib is the design current of the circuit in amps
• L is the cable length in metres

The relevant coefficients for 70°C PVC twin and earth:

• 2.5mm: 18 mV/A/m
• 4mm: 11 mV/A/m

That 39% reduction is why 4mm solves voltage drop problems. For the same load over the same distance, 4mm cable drops 39% less voltage than 2.5mm.

Worked example: kitchen island socket

A homeowner wants a double socket at the kitchen island, 22 metres of cable run from the consumer unit. The circuit is a 32A radial. Design current under typical kitchen loads is 26A (the IET On-Site Guide assumption for a kitchen socket circuit).

Check on 2.5mm cable:

VD = (18 × 26 × 22) / 1000 = 10.30 V (4.48% of 230V)

That's just inside the 5% limit. But add the upstream submain voltage drop (typically 1 to 2 volts in a domestic installation), and the cumulative drop pushes past 5%. The circuit fails the calculation.

Check on 4mm cable:

VD = (11 × 26 × 22) / 1000 = 6.29 V (2.74% of 230V)

Comfortable margin. The circuit passes with room for upstream drop and future load growth.

The IET On-Site Guide gives a practical headline figure for this scenario: a 4mm/1.5mm radial on a 32A MCB can run up to 43 metres at the assumed 26A design current. At the full 32A load, the practical maximum drops to roughly 30 metres. Your electrician's calculation will use the actual design current for your circuit, which depends on the appliances connected.

Reference methods and the derating problem

Cable current ratings depend on installation method. The Method C clipped-direct figure on the product datasheet is the headline, but it only applies when the cable runs in free air across the surface of a wall or joist. Push it through conduit, sandwich it in insulation, or group it with other cables, and the rating drops.

This is the area where 4mm cable installations most commonly fall foul of compliance. Many homeowners and even some installers see the 37A figure and assume it applies in all situations. It doesn't.

Look at Method 103. The 4mm cable rating drops to roughly half its clipped-direct value when fully buried in insulation for a continuous run of 500 mm or more. 18.5A is below the 32A MCB protecting the circuit. That means the MCB will not trip before the cable overheats. The installation is not compliant.

The practical scenario that catches installers out: cable runs through a modern, well-insulated stud wall on the way to a kitchen island. The wall has 100mm of mineral wool between studs, with PIR insulation on the warm side. Drop a 4mm cable down inside that insulation cavity for two metres, and you've put the cable into Method 103. The 32A MCB is no longer adequate protection for that section of cable.

The fixes (in order of preference):

1. Route the cable in a different way that keeps it clipped direct or in protected zones rather than buried in insulation
2. Use steel oval conduit that creates an air gap around the cable
3. Downsize the MCB to 25A or 20A to match the derated cable rating (which reduces what the circuit can supply)
4. Upsize the cable to 6mm if Method 103 is unavoidable on the route

How to work with it

You won't be installing the cable yourself. New socket circuits in an extension are notifiable work under Part P of the Building Regulations, meaning a registered competent person (NICEIC, NAPIT, ELECSA) must certify the installation. But you'll often be buying the cable, and you may be helping pull it through joists and studs under your electrician's direction. Here's what matters.

Stiffness and termination

4mm cable is significantly stiffer than 2.5mm. The stranded conductors give it some flexibility, but the overall profile is thicker and the bend radius is larger. You can't dress 4mm into a back box the way you can 2.5mm. It resists folding into the corners.

The practical consequence is back box depth. A standard 25mm metal back box is too shallow for a 4mm twin and earth termination. Forum electricians describe single-gang 25mm boxes as "a nightmare" or "impractical" for 4mm cable. The conductors don't bend tightly enough to sit cleanly behind the faceplate, and the cable physically pushes the socket forward off the wall.

Specify 35mm deep back boxes for any socket on a 4mm cable. Both single and double gang. The extra 10mm of depth gives you the room to dress the cable properly. The price difference between 25mm and 35mm boxes is roughly £1 per box at retail. On a kitchen with a few sockets on the 4mm radial, that's pennies.

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A second consideration: you cannot easily fit a spur off a 4mm radial into the same back box as the through-cable. A double socket on a ring or radial typically has two cables landing in the box (incoming and outgoing). With 2.5mm, this is comfortable. With 4mm, two cables plus the socket terminals fills the available volume in even a 35mm double-gang box. If you need spurs, plan them at first fix as separate junctions, not as in-socket spurs.

Stripping and dressing

Stripping the grey outer sheath on 4mm cable requires a sharp knife and a steady hand. The PVC is thicker than on 2.5mm, and the stranded conductors below are easier to nick if you push the blade too deep. Score the sheath lengthways, peel it back, and trim it cleanly.

Cut the cable about 100mm longer than you think you need at each termination. The stiffness means you need extra length to form gentle curves into the terminals. Cable pulled tight into a back box creates stress fractures in the insulation over time.

The earth conductor must be sleeved at every termination. Cut a length of 3mm green/yellow sleeving for each connection point and slide it over the bare 1.5mm copper before connecting to the earth terminal of the socket, the consumer unit, or any junction.

Clipping and routing

Fix 4mm twin and earth with cable clips at 300 mm on horizontal runs and closer at bends and near accessories. The correct clip size for 4mm cable is 13mm by 6mm (clip manufacturers like Linian list this size). Using too small a clip crushes the sheath; too large and the cable rattles loose.

Cable runs follow BS 7671 prescribed safe zones: vertically above or below electrical accessories, or horizontally within 150mm of the ceiling or floor. Cables outside these zones, buried less than 50mm from the wall surface, must be in earthed steel conduit or behind earthed metal capping.

When drilling joists for 4mm cable, drill at the centre of the joist depth (the neutral axis). Holes must be no larger than one-quarter of the joist depth, and a minimum 14mm hole gives clean clearance for an 11.4mm-wide cable.

How much do you need

A typical extension with a long-run socket circuit needs one of these patterns:

• One 4mm radial circuit for a kitchen island or far-end socket cluster: 25 to 35 metres of cable for a 4m by 6m extension with the consumer unit at the front of the house
• One 4mm circuit to an outbuilding (garage, garden room, home office): 15 to 25 metres of cable depending on the outbuilding distance
• Both in projects that include a kitchen extension and a garden room

Worked example for a kitchen island radial in a 4m by 6m rear extension:

• Consumer unit (front of house) to ceiling void: 4m
• Across ceiling void to back wall of extension: 8m
• Down wall to floor level: 2.5m
• Under-floor or in floor void to island position: 6m
• Up into island base unit, dressed to socket positions: 1.5m
• Subtotal: 22m
• Plus 10% contingency: 24m
• Order: 1 x 50m drum (saves money over a 100m drum if you only need this one circuit) or 1 x 100m drum if you have multiple 4mm circuits

A 50m drum is the right purchase for a single 4mm radial. A 100m drum makes sense if your project has two or more 4mm circuits, or if you have other 4mm work planned in the medium term.

Cost and where to buy

Current retail prices for a standard 100m drum of 4mm 6242Y twin and earth (April 2026):

The standard grey PVC 100m drum range across mainstream retailers sits at £165 – £179. Smaller quantities are available but the per-metre cost is much worse. A 50m grey drum from Screwfix is £94.99 (£1.90 per metre) versus roughly £2 – £2 per metre for the 100m drum. If you need more than 30 metres, buy the full 100m drum.

How 4mm compares to 2.5mm at retail

4mm twin and earth costs roughly 70 to 100% more per drum than 2.5mm. A 100m drum of 2.5mm sits at £79 – £84 at the same retailers; the equivalent 4mm drum is £165 – £179. On a single circuit upgrade from 2.5mm to 4mm, the material cost difference is around £75.

That premium is why 4mm is not used as a default. On a typical extension with one or two socket circuits and one cooker circuit, switching all socket cable to 4mm adds roughly £150 to material cost compared to 2.5mm. Worth doing only when the calculation requires it.

Trade vs retail pricing

Trade wholesalers (CEF, Edmundson Electrical) sell at roughly 10 to 20% below retail with a trade account. Your electrician sourcing materials through a wholesaler will pay £125 to £140 for the same 100m drum. If you're buying materials yourself, Toolstation, TLC Direct, and Internet Electrical are the practical mainstream options.

Which brand?

Any cable with a BASEC approval mark meets BS 6004 and is fit for purpose. Beyond that, brand matters for sheath quality and consistency. Prysmian is UK-manufactured and the default recommendation; consistent stripping, holds shape in clips, predictable behaviour at terminations. Doncaster Cables is the other major UK manufacturer, slightly softer insulation. Pitacs is the budget Turkish import most commonly seen at Toolstation. It carries BASEC approval and works fine, but trade forum complaints about Pitacs sheath quality on 2.5mm carry over to 4mm.

The price gap between Prysmian and the cheapest BASEC-approved generic on a 100m 4mm drum is roughly £20. On a single drum that's not material. Buy Prysmian or Doncaster.

Alternatives

2.5mm twin and earth is the standard for socket circuits on shorter runs. A 32A ring circuit on 2.5mm covers most domestic installations comfortably. Use 2.5mm where the run length and load allow, and reserve 4mm for the cases where the calculation demands it.

6mm twin and earth is the next size up, used for dedicated cooker, oven, hob, and shower circuits where the load is higher. 6mm is rated 47A clipped direct, which gives capacity for circuits running 32A or 40A protective devices. The cable is significantly stiffer than 4mm and noticeably more expensive. Don't use 6mm where 4mm would do; the termination problems multiply with cable size.

Steel wire armoured cable (SWA) is the right choice when the cable run leaves the building, runs underground, or needs mechanical protection. A garden room feed buried 600mm in a trench uses SWA, not twin and earth. The steel wire armour can serve as the circuit protective conductor when correctly terminated through SWA glands. SWA is more expensive per metre than twin and earth and requires gland kits at each end.

Larger cable sizes (10mm, 16mm, 25mm) come into play for sub-mains feeds to consumer units and for very high-load circuits. These are outside the scope of typical extension socket and lighting work; your electrician specifies them where required.

Where you'll need this

• First fix electrics - 4mm cable for long radial socket circuits is run during first fix, before walls are closed up and plastered
• Electrical layout planning - the cable schedule produced at this stage determines whether 4mm is needed and how much to order

The decision about 4mm versus 2.5mm happens at electrical layout planning, not on the day of first fix. Once your electrician has the kitchen layout and the consumer unit position, they can run the voltage drop calculation and tell you exactly which circuits need 4mm. Do this before you order cable.

Common mistakes

Specifying 25mm back boxes for 4mm sockets. Single-gang and double-gang 25mm boxes are too shallow for clean 4mm termination. The cable physically pushes the faceplate forward, and the bent conductors stress the insulation over time. Use 35mm boxes for any socket on a 4mm radial, including spurs. The price difference is pennies per box.

Mixing 4mm and 2.5mm on the same circuit. A 4mm radial protected by a 32A MCB cannot have 2.5mm spur cable tapped off it. The 2.5mm spur is rated 27A clipped direct (less in containment), below the 32A protective device, which is non-compliant. If you need to add sockets to a 4mm radial, run them in 4mm or use a fused connection unit to limit the spur load.

Assuming 32A always works on 4mm regardless of installation method. The 32A MCB only matches the 4mm cable rating in Method C (clipped direct, 37A). In conduit (Method B, 30A), in insulated walls (Method A, 26A), or in insulation (Method 103, 18.5A), the 32A breaker is too large. Plan the cable route at first fix to keep the cable in clipped-direct or near-equivalent installations along its full length, or downsize the protective device to match the worst section.

Forgetting that the earth is 1.5mm. When testing a 4mm circuit, the CPC end-to-end resistance will be measurably higher than the live or neutral resistance because the earth conductor is smaller. That's expected, not a fault. A reading lower than calculated means there's a parallel earth path somewhere (bonded metalwork, adjacent circuits), worth investigating but usually benign.

Buying without a BASEC mark. Cheap cable from online marketplaces may look identical to approved products but may not meet BS 6004 for insulation thickness, conductor purity, or voltage rating. Your electrician will refuse to install non-BASEC cable, and rightly so. Check the drum label and the repeating print on the sheath itself before you accept delivery.

Ordering before the cable schedule is produced. Cable runs depend on consumer unit position, kitchen layout, and the sequence of fixed appliances. Ordering 4mm cable before your electrician has produced a written cable schedule risks ordering the wrong quantity, the wrong size, or both. Wait for the schedule. Buy from the schedule.