Glossary · Wiring, circuits and faults
Key points
- The cable has three conductors: a brown live, a blue neutral and a bare copper earth, all inside one flat sheath.
- Colours changed in 2004 to 2006: older cable has red and black cores, newer cable has brown and blue, according to the IET.
- Cable sizes run from 1.0 to 16 mm². The earth wire is smaller than the live cores in most sizes.
- Old cable is not automatically unsafe, but certain 1968 to 1975 PVC cable is known to degrade, so age is a reason to have it tested.
- Only a registered electrician should cut, join or add to it.
How twin and earth cable works
Prysmian, a UK manufacturer, describes twin and earth as flat PVC-insulated and sheathed general wiring cable, also called 6242Y. It has two insulated cores and an uninsulated bare copper circuit protective conductor, all inside a flat outer sheath.[1]Prysmian, Twin and Earth Cable The live core is brown, the neutral core is blue and the protective conductor is the earth.[2]Prysmian, Live, neutral, earth - is it that simple?
Each conductor has a job. The live carries current to the load. The neutral is the return path that completes the circuit. The earth carries current only if a fault occurs, and connects to the protective devices at the consumer unit.[2]Prysmian, Live, neutral, earth - is it that simple? Our guide to the earth fault shows what that earth path does.
The cable is rated at 300/500 V and designed for indoor use. Prysmian lists fixed installation in domestic premises, in walls, clipped direct, in trunking or embedded in plaster, and says it suits ring and radial circuits, lighting and switching circuits, and electric cookers and showers.[1]Prysmian, Twin and Earth Cable The PVC allows a maximum working temperature of 70 °C.[2]Prysmian, Live, neutral, earth - is it that simple?
Sizes and construction
Prysmian’s range runs from 1.0 to 16 mm². The 1.0, 1.5 and 2.5 mm² sizes use solid copper and the 4, 6, 10 and 16 mm² sizes use stranded copper. The earth conductor is smaller than the live cores: 1.0 mm² 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 Our guide to cable sizes covers what the sizes mean.
NICEIC makes the same point about the common 2.5 mm² flat cable: it typically contains a 1.5 mm² earth conductor.[4]NICEIC, Why can an unfused spur be taken off a ring final circuit wired in 2.5 mm² cable?
Reading the age of old cable
The IET gives dates that help an electrician judge how old an installation is.[3]IET, Estimating the age of an electrical installation
| Feature | What it indicates |
|---|---|
| Red and black live and neutral | 1950s to 2004 to 2006 |
| Brown and blue live and neutral | 2004 to 2006 onwards |
| Grey sheath | Now almost always used on PVC cable |
| White sheath | Used to signal low smoke halogen-free cable |
| 2.5 mm² flat cable with 1.0 mm² earth | About 1969 to 1981 |
| 2.5 mm² flat cable with 1.5 mm² earth | From 1981 onwards |
| No built-in earth conductor in some cables | 1950s to early 1970s, although an earth was required at all points in all circuits from 1966 |
Those details all come from the IET article.[3]IET, Estimating the age of an electrical installation It also notes that certain PVC cables from 1968 to 1975, with untinned conductors, were later found susceptible to slow chemical degradation, which produced what became known as green goo at terminals.[3]IET, Estimating the age of an electrical installation
Example
Imran buys a 1960s house in Morley. The loft cable is grey, the lighting cable is red and black and the consumer unit still has rewireable fuses. His surveyor recommends an inspection. The electrician reports that the age of the wiring is a reason to test it, not proof that it has failed, and the EICR shows where it needs replacing. Imran learns that the cable colours told him the age of the wiring but not its condition.
What people get wrong
Many people think that if a cable is flat, grey and called twin and earth, it must be modern. The IET says grey became near-universal after 2004 to 2006, but older cable also came with a grey sheath, and earlier cable had red and black cores. Colour tells you roughly when it was put in, and only testing tells you its condition.
The other mistake is changing cable without understanding its rating. A size is chosen to suit the load and the protective device, which is work for a registered electrician. Never extend or join cable yourself.
The bottom line
Twin and earth is the flat three-conductor cable behind most UK sockets and lights. The live is brown, the neutral blue and the earth bare copper, with older cable using red and black. Its colours and earth size help date an installation. An inspection, not the cable’s looks, shows whether it is still fit for use.
Common questions
Why is it called twin and earth?
It has two insulated cores, the live and the neutral, which are the twin, plus a bare copper conductor for the earth. Prysmian describes it as a flat PVC insulated and sheathed cable with two insulated cores and an uninsulated bare copper protective conductor.
Is red and black twin and earth dangerous?
Not by itself. The IET says red and black cores were used from the 1950s until 2004 to 2006, so the colours tell you the age. Age is a reason to have the wiring inspected, and an EICR shows whether the cable is still in good condition.
Is the earth wire thinner on purpose?
Yes. In Prysmian's range, a 2.5 mm² cable has a 1.5 mm² earth wire. The earth conductor only needs to carry fault current briefly, so it can be smaller than the live conductors.
Can I use twin and earth outdoors?
Prysmian says twin and earth is designed for indoor applications. Garden and outdoor circuits use different cable and need a registered electrician.
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. ↩
- Prysmian. Live, neutral, earth - is it that simple?. Checked 11 November 2026. ↩
- IET. Estimating the age of an electrical installation. 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. ↩
