International standard IEC 60446 Basic and safety principles for man-machine interface, marking and identification - Identification of equipment terminals, conductor terminations and conductors defines basic safety principles for identifying electrical conductors by colours or numerals, for example in electricity distribution wiring. The standard has been withdrawn; the fourth edition (IEC 60446:2007) was merged in 2010 into the fifth edition of IEC 60445 along with the fourth edition, IEC 60445:2006.
The standard permits the following colours for identifying conductors:
(The colours green and yellow on their own are only permitted where confusion with the colouring of the green/yellow protective conductor is unlikely. Combinations of the above colours are permitted, but green and yellow should not be used in any of these combinations other than as green/yellow for the protective conductor.)
Use of colours
Neutral or mid-point conductor
If a circuit includes a neutral or midpoint conductor, then it should be identified by a blue colour (preferably light blue). Light blue is the color used to identify intrinsically safe conductors, and must not be used for any other type of conductor.
AC phase conductors
The colour combination green/yellow is always and exclusively used to identify the protective conductor. On any 15 mm length of the conductor, one of these two colours should cover between 30% and 70% of the area and the other the remaining area.
Insulated PEN conductors (a single wire that serves as combined Neutral and Earth) should be marked either green/yellow along their entire length with light blue markings at their ends, or light blue along their entire length with green/yellow markings at the ends. The cable must have a cross section of 16 mm2 (5 AWG) or greater.
United States, Canada and Japan
The three countries United States, Canada and Japan are mentioned in a note in the standard for using different colours:
- white or natural grey for mid-wire or neutral conductor (instead of light blue)
- green for the protective conductor (instead of green-and-yellow)
British Standard BS 7671:2001 Amendment No 2:2004 adopted the IEC 60446 colours for fixed wiring in the United Kingdom , with the extension that grey can also be used for line conductors, such that three colours are available for three-phase installations. This extension is expected to be adopted across Europe and may even find its way into a future revision of IEC 60446.
Where conductors are in addition identified by numbers, then these numbers must be written in Alpha numerals, digits 6 and 9 must be underlined, and green-and-yellow conductors must not be numbered.
- IEC 60446:2007:  Basic and safety principles for man-machine interface, marking and identification - Identification of conductors by colours or numerals, International Electrotechnical Commission, Geneva.
- IEC 60445:2010:  Basic and safety principles for man-machine interface, marking and identification - Identification of equipment terminals, conductor terminations and conductors, International Electrotechnical Commission, Geneva.
- Paul Cook: Harmonised colours and alphanumeric marking. IEE Wiring Matters, Spring 2006.