What do the colours of electrical wires mean?

At a glance

International

IEC 60445:2021 with Amendment 1 (2026)

UK wiring rules

BS 7671:2018 (18th edition), amended through A4:2026

Single-phase (IEC/UK)

Line brown · neutral blue · earth green-and-yellow

Old UK fixed wiring

Line red · neutral black

One colour with one job: green-and-yellow

The most important rule in the scheme is also the simplest: the green-and-yellow bicolour is reserved for the protective conductor and must not be used for anything else. It was chosen because a two-colour stripe is distinctive, and because both colours are reasonably separable for most forms of colour-vision deficiency — the stripe pattern is recognisable even when the hues are not. The UK moved from plain green to green-and-yellow earths in the 1970s. Blue for neutral and brown for line were introduced on flexible appliance cords across Europe at about the same time, which is why UK plugs have been brown and blue for decades even though fixed wiring in walls kept red and black much longer.

Harmonisation in the UK: the red–brown and black–blue trap

In 2004 an amendment to BS 7671 brought UK fixed wiring into line with the IEC colours, and after a transition period the harmonised colours became the norm for new work from 2006. The awkward result is that black changed meaning. In an old installation black is neutral; in a new three-phase installation black is a line conductor. An extension to an older circuit can therefore contain red and brown lines and black and blue neutrals side by side, which is why the rules require a warning notice at the distribution board where old and new colours coexist. Colour alone cannot tell you which convention a given black wire follows.

Three-phase circuits went from red, yellow and blue to brown, black and grey — so blue moved from being a line to being the neutral. That is the change most likely to catch someone out.

North American practice

The United States and Canada developed separately. Under the US National Electrical Code the grounded conductor, the equivalent of neutral, is white or grey, and neutral terminals on sockets and plugs must be substantially white. The equipment grounding conductor, the equivalent of the protective earth, is bare copper or green, sometimes green with a yellow stripe. Live (ungrounded) conductors are commonly black and red, but the colours for them are governed by local practice and the code's exclusions rather than a single mandated scheme. A European visitor who assumes 'blue is neutral' or 'black is neutral' in an American panel will be wrong on both counts.

Colour identifies; it does not prove

Conductor colours are identification, not isolation. Insulation fades and discolours with heat, some cables are re-marked with coloured sleeves at their ends, and mistakes during installation are not rare. Grey and black can be hard to tell apart in poor light, and brown and red look similar under warm or low-colour-rendering lamps. The colour tells an electrician what a conductor is supposed to be; only testing tells them what it is. This page explains the conventions; any electrical work should be carried out by a competent person following the current wiring rules where they are.

The data

What goes wrong, and what to do

How to check it yourself

1. Identify which convention the installation was built to: look for a mixed-colours notice at the consumer unit.
2. Use colour only to form a hypothesis about each conductor's function.
3. Leave identification and testing to a competent person using the current wiring rules.

What the evidence says

Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.

• FactStrong evidence

  IEC 60445, which covers the identification of conductors by colour and alphanumeric marking, is currently the 2021 edition with an amendment published in 2026.

  Source: International Electrotechnical Commission (IEC) webstore and catalogue; European Standards online catalogue (EN, ISO, IEC, BSI, ASTM listings)

• FactModerate evidence

  In UK fixed wiring, line was red and neutral black before harmonisation; the harmonised brown line and blue neutral apply after the 2004 change, with the old colours retained in fixed wiring until 2006, and the green-and-yellow earth dating from the 1970s.

  Caveat: Summarised from a secondary source and not yet checked against the standard itself; confirm with the standard before relying on it.

  Source: Electrical wiring in the United Kingdom (Wikipedia)

• FactStrong evidence

  BS 7671, the UK wiring regulations, is in its 18th edition (2018), now amended through Amendment 4 of 2026.

  Caveat: Checked against the IET's own BS 7671 page on 30 September 2026; the IET amends the regulations every few years.

  Source: BS 7671 — IET Wiring Regulations

• Colourwise interpretationModerate evidence

  Because black means neutral in pre-harmonisation UK wiring but a line conductor in harmonised three-phase wiring, conductor colour cannot be used on its own to identify a conductor's function in mixed installations.

  Based on: Direct consequence of the old and new colour schemes described by the cited sources co-existing in extended installations.

Related questions

Why did UK wiring change from red and black to brown and blue?

To harmonise with the IEC colours already used across Europe and on appliance flexes. An amendment to BS 7671 introduced them in 2004, and the old fixed-wiring colours were phased out for new work by 2006.

Is a blue wire always neutral?

In harmonised European and current UK wiring, yes by convention. In a UK three-phase installation built to the old colours, blue was a line conductor — one reason colour must be confirmed by testing.

What colour is earth in American wiring?

Bare copper or green insulation, sometimes green with a yellow stripe. White or grey is the grounded (neutral) conductor, which has no European colour equivalent.

Related topics

Go further

Sources

• International Electrotechnical Commission (IEC) webstore and catalogue — IEC (Standards body)
• European Standards online catalogue (EN, ISO, IEC, BSI, ASTM listings) — European Standards s.r.o. (Reference work)
• Electrical wiring in the United Kingdom (Wikipedia) — Wikipedia (Reference work)
• BS 7671 — IET Wiring Regulations — Institution of Engineering and Technology (IET) (Standards body)

Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.