What cable size do I need for a socket ring circuit?

UK socket ring circuits use 2.5mm² twin and earth cable with a 30A or 32A MCB. This is the standard for a domestic ring final circuit serving a floor area of up to 100m².

What cable size is needed for a shower?

A shower up to 8.5kW at 240V requires 6mm² cable with a 40A MCB. A 10kW shower needs 10mm² cable with a 50A MCB. The correct size also depends on the run length and installation method.

Can I use 2.5mm² cable for a lighting circuit?

You can, and some electricians do for long lighting runs to keep voltage drop within limits, but 1mm² twin and earth is standard for most domestic lighting circuits and is correctly sized for a 6A or 10A MCB.

What happens if the cable is undersized?

An undersized cable runs hot under load. Over time this degrades the insulation, increases resistance, worsens voltage drop, and in a worst case causes a fire. The protective device should disconnect before the cable reaches a dangerous temperature — but only if the cable size and device rating are properly matched.

Do I need the full BS 7671 tables to size cable?

Not for most domestic work. The IET On Site Guide (OSG) gives simplified tables for the most common installation methods and standard load types. For unusual installations — long runs, heavy grouping, buried cables — the full BS 7671 tables give more precise figures.

How to Select the Right Cable Size for UK Domestic Wiring

Helpful video reference. The Learn Electrics channel, a UK electrical education resource covering BS 7671 and City and Guilds content, explains how to match cable size to circuit breaker using the On Site Guide in "CABLE SIZE SELECTION AND THE OSG — BS7671 — ON SITE GUIDE — MATCHING CABLE AND CIRCUIT BREAKER SIZES". The same channel's short circuit video is referenced elsewhere on this site. Their approach to the OSG tables is practical and easy to follow.

Before you start. Selecting cable size is one part of circuit design. The full design process also includes choosing the correct protective device type and rating, checking the fault current is high enough to operate the device within the required time, and confirming the earthing is adequate. For a complete new circuit, these checks belong with a qualified electrician. This guide explains the cable-sizing element so you can understand and verify the choices being made on your home.

1. Establish the design current (Ib)

The design current is the maximum sustained current the circuit will carry in normal use. For fixed appliances this is simply the rated current: a 2.4kW kettle on a 230V supply draws roughly 10.4A. For socket circuits you cannot measure a future load, so BS 7671 uses the ring circuit rules (a ring serving up to 100m² is served by 32A protection with 2.5mm² cable). For lighting, add up the wattages of all the lamps on the circuit and divide by 230.

Design current tells you the minimum MCB rating you can choose.

2. Select the protective device rating (In)

The MCB or fuse rating must be at least equal to the design current. Standard B-type MCB ratings are 6, 10, 16, 20, 25, 32, 40 and 50A. Pick the next size up from your design current if there is no exact match: a 15A load gets a 16A MCB, not a 20A.

The device rating also sets the ceiling for the cable you can use. The cable's current-carrying capacity (after all correction factors) must be at least as high as the MCB rating. If it is not, either choose a smaller MCB or go up a cable size.

3. Look up the tabulated current capacity (It)

The On Site Guide Appendix (or BS 7671 Tables 4D1 to 4D5) lists the current-carrying capacity of standard cables by cross-section and installation method. The main installation methods for domestic work are:

Clipped direct to a surface (reference method C): the cable can carry more current than when enclosed.
In conduit on a wall (reference method B): capacity is slightly lower.
In a wall, buried or in a partition (reference method B or C depending on the construction): insulation around the cable restricts heat loss.
Under thermal insulation (reference method 100 or 101): capacity drops significantly because heat cannot escape.

For most domestic socket and lighting circuits using standard 70°C PVC twin and earth, the common reference values are: 1mm² gives around 13.5A clipped direct; 2.5mm² gives around 27A; 4mm² around 37A; 6mm² around 47A; 10mm² around 65A.

4. Apply correction factors

Three main factors reduce the tabulated capacity:

Ambient temperature (Ca): if the cable runs through a boiler room or a south-facing roof void that regularly exceeds 30°C, apply the temperature correction factor from OSG Table 4C1.
Grouping (Cd or Cg): cables run together in a bundle share their heat. A group of three or more circuits touching each other in a loft typically reduces capacity to around 70% of the tabulated value.
Thermal insulation (Ci): a cable surrounded by insulation on all sides must be derated to around 50% of its clipped-direct value. This is the most significant factor for modern well-insulated homes. Where possible, route cables so they are not fully surrounded by insulation.

Multiply the tabulated capacity by all the applicable factors. The result must be greater than the MCB rating. If it is not, go up a cable size and repeat.

5. Check voltage drop

For most domestic circuits under 15 metres, voltage drop is not the limiting factor. For longer runs (a shed 40 metres from the house, or a lighting circuit serving a large open-plan ground floor), voltage drop can become significant. The OSG gives mV/A/m figures for each cable size. Multiply: mV/A/m × design current × run length in metres, then divide by 1000 to get the drop in volts. This must be below 3% of the nominal voltage for lighting (about 6.9V on a 230V supply) and 5% for power circuits.

If the drop is too high, move up a cable size. Larger cable has lower resistance and therefore lower voltage drop.

6. Common cable sizes for UK domestic circuits

In most east Kent homes, and in most UK houses built or rewired in the last 40 years, the standard sizes are:

1mm² twin and earth: lighting circuits, 6A or 10A MCB.
2.5mm² twin and earth: socket ring final circuits, 30A or 32A MCB. Radial socket circuits to a single area, 20A MCB.
4mm² twin and earth: cooker circuits for smaller ranges (up to around 30A load), 32A MCB. Also used for EV chargers where the run is short.
6mm² twin and earth: shower circuits and some cooker circuits, 40A MCB. EV chargers at 7.4kW on a moderate run.
10mm² twin and earth: large electric cookers and some high-power EV chargers, 50A MCB. Also used for long supply runs to outbuildings.

These are typical starting points, not hard rules. The actual size depends on the specific load, the run, and the installation method. An electrician will confirm the correct size once they have surveyed the job.

Stop and call an electrician if: you find existing cable that looks undersized for its purpose (check if breakers trip under normal load, or if the cable feels warm to the touch during use); you are planning a new circuit and are unsure which correction factors apply; or you need to add circuits to a consumer unit that is already heavily loaded or has no spare ways.

When to call us

Richard sizes every cable correctly for its circuit before any work starts. If you are getting quotes for rewiring, new circuits, or an EV charger and want to understand whether the cable sizes being proposed are right, feel free to ask. There is no charge for a straight answer to a straight question.

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How to select the right cable size for UK domestic wiring