Cable Sizing Guide: Which Cable Size Do You Need?

Your electrician quotes for the extension wiring and you see three cable sizes on the materials list: 1.5mm, 2.5mm, and 6mm. The numbers mean nothing to you. You search online, find a table that says "2.5mm for sockets, done," and assume cable sizing is simple. Then your electrician tells you the socket cable needs to be 4mm because the run goes through the insulated stud wall. That's an extra cost you didn't budget for, and if you'd already bought three drums of 2.5mm, you've wasted around ~£81. Cable sizing isn't complicated, but it's not a simple lookup table either. The installation method changes the answer.

The quick-reference table

Start here. For a standard kitchen extension with cables clipped to joists and studs (not buried in insulation), these are the cable sizes, MCB ratings, and circuit types your electrician will specify.

That table works for the straightforward case. But if your extension has insulated stud walls, insulated ceilings, or cables running through loft insulation, the numbers change. Read on.

Why cable sizing isn't a simple lookup

Every cable has a current-carrying capacity, measured in amps. A 2.5mm twin-and-earth cable clipped directly to a joist can safely carry 27A. That same cable buried in thermal insulation for more than 500mm can safely carry only 13.5A. Same copper. Same plastic. Half the capacity.

The reason is heat. Electric current generates heat in the conductor. Normally, that heat dissipates into the surrounding air. Wrap the cable in insulation and the heat has nowhere to go. The conductor temperature rises, the PVC sheath softens, and eventually the insulation degrades. BS 7671 (the IET Wiring Regulations, currently at the 18th Edition with Amendment 4 published April 2026) handles this through derating factors: correction values that reduce the cable's official capacity based on how it's installed.

Three factors combine to determine the actual safe current:

• Ca (ambient temperature) - if the surrounding air is hotter than 30 degrees C, the cable carries less. Rarely an issue in UK domestic work.
• Cg (grouping) - cables bundled together share heat. Five circuits through the same hole in a joist derate each other. Your electrician spaces cables apart where possible to avoid this.
• Ci (thermal insulation) - the big one for extensions. This is where cable sizing decisions change.

The formula is: safe capacity = cable rating x Ca x Cg x Ci. Your electrician runs this calculation for every circuit. You don't need to do it yourself, but you need to understand why the answer isn't always what the cable drum label suggests.

The five cable sizes explained

1.5mm twin and earth

The lighting circuit standard. Rated at 20A when clipped directly to a surface (Reference Method C), but your electrician will protect it with a 6A MCB because domestic lighting loads are low. Even with LED fittings drawing well under 1A per point, the 6A MCB provides short-circuit and overload protection.

The generous headroom between the 6A protection and the 20A capacity means derating through insulation is rarely a problem for 1.5mm lighting circuits. Even at Method 103 (fully surrounded by insulation), 1.5mm still carries 10A, well above the 6A MCB rating.

A single 100m drum at £55-£60 covers a typical extension lighting circuit with spare. Expect to use 30-50m for a single-storey rear extension, depending on the number of light points and the route back to the consumer unit.

For the full specification, see the 1.5mm twin and earth page.

2.5mm twin and earth

The socket circuit standard. Rated at 27A clipped direct (Method C). This is the cable behind every 13A socket outlet in a typical UK home, and it's the cable your electrician will run for the extension sockets unless installation conditions force an upsize.

On a 32A ring circuit, 2.5mm works because current splits both ways around the ring, so each leg carries roughly half the load. On a 20A radial, 2.5mm is comfortably within its rating. The trouble starts when the cable passes through insulation.

A 100m drum costs around ~£81. One drum typically covers the socket circuit for a 20m² extension with enough left over for a spur or two.

Full details on the 2.5mm twin and earth page.

4mm twin and earth

The size most homeowners have never heard of. 4mm doesn't appear in basic cable sizing tables because it's not the default for any standard circuit. It exists to solve a specific problem: when derating makes 2.5mm inadequate for a 32A circuit.

Rated at 37A clipped direct, 4mm carries enough current for a 32A radial circuit even after moderate derating. At Method 102 (cable touching the inner wall surface in an insulated stud wall), 4mm still delivers 27A. At Method 103 (fully surrounded), it drops to 17.5A, which is still above the minimum per-leg requirement for a ring.

When your electrician specs 4mm for your extension sockets, this is why. It's not overengineering. It's the correct response to cables running through well-insulated walls.

A 100m drum of 4mm costs [Unknown price: twin-earth-4mm-100m-drum], compared to ~£81 for 2.5mm. The price difference is real but the safety margin it provides in an insulated extension justifies it.

6mm twin and earth

The heavy-duty cable for cookers, induction hobs, and electric showers up to about 7.5kW. Rated at 47A clipped direct. The conductors are stranded copper (unlike the solid conductors in 1.5mm and 2.5mm), which makes the cable thicker and stiffer but more flexible than a solid conductor of the same size would be.

A 25m drum costs £63-£70; a 50m drum £100-£120. For most kitchen extensions, a 25m drum is enough for the cooker circuit (typical run from consumer unit to cooker point is 10-20m).

BS 7671 diversity calculations (Regulation 311.1) mean that a 10kW cooker doesn't draw its full 43A at the socket. The diversified current for a 10kW cooker is approximately 25A, which is well within 6mm capacity on a 32A MCB. Even a 14kW cooker diversifies to about 30A.

Full specifications on the 6mm twin and earth page.

10mm twin and earth

The biggest cable you'll encounter in a typical domestic extension. Rated at 64A clipped direct. 10mm is specified for electric showers above 7.5-8kW (most modern electric showers are 8.5-10.5kW) and for large range cookers above 14kW.

10mm cable is heavy, stiff, and awkward to terminate in standard back boxes. Your electrician will use deeper back boxes (47mm minimum) and may need to use a cooker connection unit rather than a standard socket outlet to accommodate the cable.

A 50m drum costs £170-£200. You'll rarely need more than 15-20m for a single radial run to a shower or cooker position.

The derating deep-dive

This section is the reason your electrician's cable specification may differ from what a basic online table suggests. No other homeowner-facing guide explains this properly. It's the single biggest knowledge gap in domestic electrical work.

What the installation methods mean

BS 7671 defines how a cable is physically installed using reference method numbers. Each method has different current-carrying capacities because each allows different amounts of heat to escape from the cable.

Source: BS 7671:2018+A4:2026 Table 4D5, confirmed via Doncaster Cables manufacturer data and SparkyHub reference tables.

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The 50% rule

When a cable is fully surrounded by thermal insulation for 500mm or more, BS 7671 Table 52.2 applies a correction factor (Ci) of 0.5. That's a 50% reduction. A 2.5mm cable rated at 27A becomes a 13.5A cable. A 6mm cable rated at 47A becomes a 23.5A cable.

This is the rule that catches people. A 13.5A capacity on what's supposed to be a 32A ring circuit is inadequate. Even on a ring where the load splits two ways, each leg needs to carry at least 20A to satisfy the circuit design. 13.5A per leg doesn't meet that threshold.

The Ci correction factors by insulation depth

The 50% rule is the worst case (500mm+ of insulation). Shallower insulation derates less severely:

• 50mm insulation: Ci = 0.89 (11% reduction)
• 100mm insulation: Ci = 0.81 (19% reduction)
• 200mm insulation: Ci = 0.68 (32% reduction)
• 400mm insulation: Ci = 0.55 (45% reduction)
• 500mm+ insulation: Ci = 0.50 (50% reduction)

For a 2.5mm cable (27A baseline): through 100mm of insulation, it drops to 21.9A. Through 200mm, 18.4A. At 500mm, 13.5A. The deeper the insulation, the more severe the penalty.

When electricians specify 4mm instead of 2.5mm

Your electrician will spec 4mm for the extension socket circuit when any of these apply:

1. Cable routes through insulated stud walls where it's fully surrounded by insulation for more than 500mm. Method 103 drops 2.5mm to 13.5A. 4mm at Method 103 gives 17.5A, and at Method 102 (cable touching inner wall surface) gives 27A, both sufficient for a 20A radial.

2. Multiple cables grouped through the same route. Five circuits through the same hole in a stud introduce a grouping derating factor (Cg) on top of the insulation derating. Combined, these factors can push 2.5mm below safe limits even on a 20A radial.

3. Long cable runs with voltage drop concerns. BS 7671 limits voltage drop to 5% for power circuits (11.5V at 230V). On a 25m run at 20A, 2.5mm cable drops 9V (3.9%); 4mm drops 5.5V (2.4%). If the run is longer or the load higher, 4mm keeps you within limits where 2.5mm wouldn't.

4. A 32A radial circuit rather than a ring. BS 7671 Appendix 15 explicitly permits 4mm on a 32A MCB for radial circuits serving up to 75m² floor area.

What about cables through loft insulation?

Loft insulation creates the same problem. If your extension has a warm roof or a ceiling void packed with mineral wool, cables running across the ceiling joists and covered by insulation fall under Method 101 (insulation exceeding 100mm). For 2.5mm, that's 17A rather than 27A.

17A is below the 20A required for a radial on a 20A MCB, but it is enough for each leg of a ring circuit (where the load splits). Your electrician will either route cables below the insulation, use raised cable supports to keep the cable above the insulation, or specify 4mm.

Method 100 (insulation not exceeding 100mm) gives 21A for 2.5mm, which is adequate for most socket circuits. The distinction between "up to 100mm" and "over 100mm" matters. Modern building regulations push loft insulation well beyond 100mm, so Method 101 is increasingly the relevant scenario.

Ring vs radial: how circuit topology affects cable choice

The circuit type your electrician chooses affects which cable size is correct. This is the second most common source of confusion after derating.

Ring circuit

A ring starts at the consumer unit, loops through every socket, and returns to the consumer unit. Load splits both ways. 2.5mm cable on a 32A MCB, serving up to 100m² floor area. The ring topology effectively doubles the cable capacity because current has two paths.

Ring circuits are the traditional UK approach. They work well for wiring an entire floor of a house. But for a small extension with four to eight sockets in a single room, they're more cable and more complexity than necessary.

Radial circuit

A radial runs from the consumer unit to the first socket, then daisy-chains to each subsequent socket in sequence. No return path. Two standard configurations:

• 20A radial on 2.5mm: serves up to 50m² floor area. Perfect for a typical 20-30m² single-storey extension.
• 32A radial on 4mm: serves up to 75m² floor area. Better for larger extensions or where derating makes 2.5mm marginal.

Professional consensus has shifted towards radials for extension socket circuits. A radial is simpler to install, easier to test, and has fewer failure modes than a ring. There's no "open ring" failure where a broken connection silently overloads one leg of the cable. For a single-room extension, a 20A radial on 2.5mm (or 4mm if derating applies) is the standard recommendation.

Spurs: the one-socket rule and when a fused spur is the answer

A spur is a branch off a ring circuit feeding an additional socket. Spurs cause more wiring mistakes than any other domestic electrical detail.

Unfused spur: A single cable tapped directly from a socket on the ring. It can feed one single socket or one double socket. That's the limit. The spur cable is a single 2.5mm conductor with no return path, protected only by the ring's 32A MCB. A single socket limits the maximum draw to 13A (one plug), which is within the cable's capacity. Connect two double sockets to an unfused spur and you've created a potential overload.

Fused spur: Runs through a fused connection unit (FCU) with a 13A fuse. Behind that fuse, you can connect multiple outlets because the FCU limits total current to 13A regardless of how many sockets are downstream. Fused spurs are the right answer when you need several outlets in a location that's awkward to reach with the ring cable (island units, worktop socket strips, outdoor sockets).

For an extension, you'll generally avoid spurs entirely. A new radial or ring circuit from the consumer unit is cleaner and more future-proof than tapping spurs off an existing circuit. Spurs are a good solution for adding one socket to an existing room, not for wiring a new space.

How much cable for a typical kitchen extension

For a 4m x 5m (20m²) single-storey rear extension with the consumer unit inside the existing house, these are realistic cable quantities. Lengths include the run from the consumer unit through the existing house to the extension, plus the circuit within the extension itself.

Lighting circuit: 30-50m of 1.5mm twin and earth. One 100m drum (£55-£60) covers this comfortably with spare for a second lighting circuit or a future modification. Buy the 100m drum even if you only need 40m. The per-metre price on small coils is roughly double the drum rate, and leftover cable stores indefinitely.

Socket circuit: 50-80m of 2.5mm twin and earth for a radial or ring. One 100m drum (around ~£81) is the right purchase. If your electrician specifies 4mm due to insulation derating, the drum costs [Unknown price: twin-earth-4mm-100m-drum]. See the 4mm section above for why this happens.

Cooker/hob circuit: 10-20m of 6mm twin and earth. A 25m drum (£63-£70) is normally enough. If the consumer unit is at the opposite end of the house from the kitchen, measure the actual route and buy a 50m drum (£100-£120) to be safe.

Shower circuit (if applicable): 10-20m of 6mm (up to 7.5kW) or 10mm (8kW+). Most kitchen extensions don't include a shower, but if yours does (en suite utility room, for example), budget for a separate radial. A 10mm 50m drum is £170-£200.

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Total cable cost estimate

For a standard 20m² kitchen extension (lighting + sockets + cooker circuit, no shower), the cable materials are a small fraction of the total first-fix electrics cost, which runs £2,500–£4,000 including labour.

Part P: who installs it and what's notifiable

Every new circuit in a domestic extension is notifiable electrical work under Part P of the Building Regulations (England and Wales). This isn't optional. It means either:

1. A Part P registered electrician (NICEIC, NAPIT, or equivalent registered competent person scheme) designs, installs, tests, and certifies the work. They self-certify to building control. This is the standard approach for extensions.

2. You or an unregistered electrician do the work, and you notify your local building control before starting. A building control inspector then arranges inspection and testing. This costs more in building control fees and creates more administrative friction.

In practice, option 1 is the only sensible choice for a kitchen extension. The wiring involves multiple new circuits, which means the electrician must issue an Electrical Installation Certificate (EIC) with full test results. An unregistered installer can't issue this certificate, and your building control completion certificate depends on it.

Scotland has separate building standards (Section 4, Safety) with an equivalent requirement for competent-person certification of new circuits. Northern Ireland's Building Regulations also require equivalent sign-off. The cable sizing tables (BS 7671) are UK-wide, and the practical outcome is the same everywhere: use a registered electrician for new extension circuits.

Where you'll need this

• Electrical layout planning - deciding which circuits you need and where sockets, lights, and appliance points go determines which cable sizes to order
• First-fix electrics - your electrician runs all cable during first fix, before plasterboard goes up. Cable sizing decisions must be finalised before this stage

Cable sizing decisions feed into every electrical aspect of an extension or renovation project. The principles here apply to loft conversions, garage conversions, and garden rooms in exactly the same way. Modern insulation standards mean the derating question comes up on virtually every project where cables pass through insulated structures.

Common mistakes

Buying cable before the electrician specifies. You find a deal on 2.5mm cable and stock up. Your electrician arrives, surveys the insulated stud walls, and tells you the socket circuit needs 4mm. Those drums of 2.5mm have no use on this job.

Assuming the drum label is the actual capacity. The 27A printed on a 2.5mm cable drum assumes Method C (clipped direct). In your insulated extension, that same cable may only be rated at 13.5A. The label isn't wrong, but it's incomplete.

Running 6mm cable to a cooker point through insulation without recalculating. 6mm at Method 103 drops to 23.5A. A 10kW cooker diversifies to about 25A. That's a margin of less than 2A. If the cable route is long or the insulation deep, your electrician should upsize to 10mm or reroute to avoid the insulation.

Confusing "cable size for my shower" with a single right answer. A 7kW shower and a 10.5kW shower need different cable sizes, different MCB ratings, and potentially different circuit arrangements. The question isn't "what cable for a shower?" It's "what cable for this specific shower, on this specific route, through these specific conditions?"

Ignoring grouping derating. Your electrician bundles five circuits through the same hole drilled in a joist. Each cable derates the others because they share heat. A 4mm cable that comfortably carries 37A alone might only manage 28A when grouped with four other circuits. Your electrician accounts for this. Online calculators sometimes don't.