Glossary · Wiring, circuits and faults
Key points
- Sizes such as 2.5mm mean 2.5 mm² of copper per conductor, not the diameter or the width of the cable.
- Twin and earth is made in sizes from 1.0 to 16 mm². The smallest three are solid copper and the larger four are stranded.
- A ring final circuit must use copper line and neutral of at least 2.5 mm² and a cable rated at not less than 20 A, according to NICEIC.
- The size must be matched to the protective device and the installation method, so a bigger breaker never makes a cable safe.
- Choosing or changing a cable size is work for a registered electrician.
How cable sizes work
When an electrician says 2.5mm cable, they mean each copper conductor has a cross-sectional area of 2.5 mm². Manufacturers list the sizes in that unit. Prysmian’s twin and earth range, for instance, is 1.0, 1.5, 2.5, 4, 6, 10 and 16 mm².[1]Prysmian, Twin and Earth Cable The number is area, not width, so a 6 mm² conductor has 2.4 times the copper area of a 2.5 mm² one.
A larger area has less resistance and can carry more current without overheating. That is why a socket circuit, a lighting circuit and a cooker circuit use different sizes. Our guide to twin and earth cable shows how the cable is built.
What decides the right size
The size is not picked from the appliance alone. NICEIC explains that a circuit design must coordinate three things: the design current of the circuit, the rating of the protective device and the lowest current-carrying capacity of the conductors, written Iz.[2]NICEIC, Overcurrent protection - overload In words, the load must not exceed the protective device rating, and the device rating must not exceed what the cable can carry. When the load is more than the cable can carry, the result is an overload, and the cable heats up.[2]NICEIC, Overcurrent protection - overload
Current-carrying capacity also depends on how the cable is installed. NICEIC refers to the capacity corrected for the particular installation conditions.[2]NICEIC, Overcurrent protection - overload The same cable carries less when buried in insulation than when clipped in free air, so the size is chosen after the route is known.
The size rules that are easy to quote
Few sizing rules fit in one sentence, but NICEIC gives these for a ring final circuit supplying standard 13 A sockets: the circuit must be protected by a 30 A or 32 A device, wired in copper with line and neutral conductors of at least 2.5 mm², and the corrected current-carrying capacity must be not less than 20 A.[2]NICEIC, Overcurrent protection - overload Our guide to the ring final circuit explains the layout.
Earth conductors are sized separately. A 2.5 mm² flat cable typically has a 1.5 mm² earth conductor.[3]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable? Prysmian lists the full pairing: 1.0 mm² earth for the 1.0 and 1.5 sizes, 1.5 mm² for the 2.5 and 4 sizes and 2.5, 4 and 6 mm² for the 6, 10 and 16 sizes.[1]Prysmian, Twin and Earth Cable The 1.0, 1.5 and 2.5 mm² sizes are solid copper and the larger sizes are stranded.[1]Prysmian, Twin and Earth Cable
Sizes in older homes
Cable made before metric sizing used imperial sizes. The IET says metric sizing replaced imperial from about 1969 to 1972, and that 2.5 mm² flat cable was made with a 1.0 mm² earth conductor until 1981, after which the earth was enlarged to 1.5 mm².[4]IET, Estimating the age of an electrical installation If your home has cable of either type, the age is a prompt for an inspection, not a verdict.
Example
Marcus in Pudsey is adding an electric shower. He does not pick a cable from the shop. His electrician checks the shower’s rating, the existing consumer unit, the route the cable will take and the protective device, then chooses the size to match and tests the circuit. Marcus receives a certificate listing what was installed.
What people get wrong
Many people think that a thicker cable is always safer, or that a bigger breaker fixes a breaker that keeps tripping. Fitting a bigger breaker without changing the cable leaves the cable unprotected against overload.
NICEIC is clear that the device rating must not exceed what the cable can carry.[2]NICEIC, Overcurrent protection - overload The other mistake is reading the number as a width. A 2.5mm label on a drum means 2.5 mm² of copper.
The bottom line
A cable size is the copper area in mm². The right size depends on the load, the protective device and how the cable is installed, so it is chosen by a registered electrician, not by a rule of thumb. For standard sockets on a ring, the minimum is 2.5 mm² on a 30 A or 32 A device. Never swap a breaker or cable to cure a fault.
Common questions
Does 2.5mm mean the cable is 2.5 millimetres wide?
No. It means each conductor has a cross-sectional area of 2.5 square millimetres. Manufacturers such as Prysmian list the sizes as mm², for example 1.0, 1.5, 2.5, 4, 6, 10 and 16 mm².
Can I use thicker cable to be safe?
A thicker cable carries more current, but it must still suit the circuit design, the protective device and the accessories it connects to. Choosing the size is a design decision for a registered electrician.
What size is a ring final circuit cable?
NICEIC says a ring final circuit supplying standard sockets must be wired in copper with line and neutral conductors of at least 2.5 mm². The protective device is 30 A or 32 A and the cable must be rated at not less than 20 A after corrections.
Why does the earth wire look thinner?
In twin and earth the earth conductor is made smaller than the live cores. A 2.5 mm² cable typically contains a 1.5 mm² earth conductor, according to NICEIC, and Prysmian lists the same pairing.
Sources
We cite the original source for every fact: legislation, GOV.UK, HSE, the IET, registration bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Prysmian. Twin and Earth Cable. Checked 11 November 2026. ↩
- NICEIC. Overcurrent protection - overload. Checked 11 November 2026. ↩
- NICEIC. Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?. Checked 11 November 2026. ↩
- IET. Estimating the age of an electrical installation. Checked 11 November 2026. ↩
