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. The UK rules require each of the two colours to cover between 30 and 70 per cent of the surface, so that both can be seen from any angle and the conductor reads as a pattern rather than a single hue. Britain adopted it in stages. Regulations in force from 1 July 1969 made appliance leads brown for live, blue for neutral and green-and-yellow for earth, which is why UK plugs have been brown and blue for decades. In fixed wiring the 14th Edition of the wiring regulations, as amended in 1970, allowed either green-and-yellow or plain green for earthing, and the 15th Edition of 1981 dropped plain green and reserved green-and-yellow exclusively for protective conductors. Fixed wiring in walls kept red and black for line and neutral much longer.

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

Amendment No 2 to BS 7671:2001, published on 31 March 2004, brought UK fixed wiring into line with the harmonised colours. An installation starting on site after that date could use the old colours or the new, but not both; one starting after 31 March 2006 had to use the new. Existing installations did not have to be rewired or re-marked. 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 amendment requires a warning notice at or near the distribution board where old and new colours coexist, and its guidance calls for old and new conductors to be marked L1, L2, L3 and N where they meet in a three-phase installation. 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

North American practice developed separately. OSHA summarises the US National Electrical Code position as follows: the grounded conductor, the equivalent of neutral, is generally white or natural grey in colour or marking, and the equipment grounding conductor, the equivalent of the protective earth, is bare or green. OSHA's own rule adds only that each of those two conductors must be identifiable and distinguishable from all others. Neither source fixes one colour for the live (ungrounded) conductors, so this page does not give one. 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)

• StandardStrong evidence

  Amendment No 2 to BS 7671:2001, published on 31 March 2004, replaced red (phase) and black (neutral) in UK fixed wiring with brown and blue, and made the three phases brown, black and grey. Installations starting on site after 31 March 2004 could use either the old or the new colours; those starting after 31 March 2006 must use the harmonised ones.

  Caveat: Colours and dates as published by the IEE in 2004 when the amendment came out; the current text of BS 7671 was not read.

  Source: IEE Wiring Matters, Issue 14, Spring 2004: 'All change — new colours agreed' and 'Harmonised colours and alphanumeric marking'; The history of colour identification of conductors (Wiring Matters, Issue 84, March 2021)

• FactStrong evidence

  In the UK, green-and-yellow earth, brown live and blue neutral were required in appliance leads from 1 July 1969; fixed wiring allowed green-and-yellow or plain green earths under the 14th Edition as amended in 1970, and the 15th Edition of 1981 reserved green-and-yellow exclusively for protective conductors.

  Source: The history of colour identification of conductors (Wiring Matters, Issue 84, March 2021)

• StandardStrong evidence

  In US practice under the National Electrical Code, grounded (neutral) conductors are generally white or natural grey and equipment grounding conductors bare or green, and OSHA requires each to be identifiable and distinguishable from all other conductors.

  Caveat: As summarised by OSHA in a 2011 letter of interpretation; the National Electrical Code itself was not read, and the letter says 'in general'.

  Source: Standard interpretation: electrical grounding of overhead cranes and hoists (5 December 2011)

• 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 or green, as OSHA summarises the National Electrical Code. White or natural grey is the grounded (neutral) conductor, so neither of the European neutral colours, blue or the older black, carries over.

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)
• IEE Wiring Matters, Issue 14, Spring 2004: 'All change — new colours agreed' and 'Harmonised colours and alphanumeric marking' — Institution of Electrical Engineers (now the IET) (Standards body)
• The history of colour identification of conductors (Wiring Matters, Issue 84, March 2021) — Institution of Engineering and Technology (IET) (Standards body)
• Standard interpretation: electrical grounding of overhead cranes and hoists (5 December 2011) — US Occupational Safety and Health Administration (Official standard or government)
• BS 7671 — IET Wiring Regulations — Institution of Engineering and Technology (IET) (Standards body)

Reviewed 1 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.