Electrical wiring (UK)

Electrical wiring (UK)

The modern UK standards and regulations for electrical wiring no longer differ substantially from those in other European countries. However, there are a number of noteworthy national peculiarities, habits and traditions associated with domestic electrical wiring in the UK (and Ireland) that differ significantly from other countries. These, along with their historic background, are the focus of this article. They include:
*ring circuits
*fused plugs
*switched sockets
*absence of normal switches and sockets in bathrooms (except for special pull-cord ceiling switches and "shaver sockets" with built-in isolation transformer)
*historic wiring colours
*asymmetric supply-voltage tolerances

Legal basis

In England and Wales, the Building Regulations (Approved Document: Part P) require that domestic electrical installations are designed and installed safely according to the "fundamental principles" given in British Standard BS 7671 Chapter 13. These are very similar to the fundamental principles defined in international standard IEC 60364-1 and equivalent national standards in other countries. Accepted ways for fulfilling this legal requirement include
* the rules of the IEE wiring regulations (BS 7671), colloquially referred to as "the regs" (BS7671: 2008, 17th Edition).;
* the rules of an equivalent standard approved by a member of the EEA (e.g., DIN/VDE 0100);
* guidance given in installation manuals that are consistent with BS 7671, such as the IEE On-Site Guide and IEE Guidance Notes Nos 1 to 7.

Installations in commercial and industrial premises must satisfy various safety legislation, such as the Electricity at Work Regulations 1989. Again, recognised standards and practices, such as BS7671 "Wiring Regulations", are used to help meet the legislative requirements.

Wiring colours

The standard wiring colours in the UK are (as of 2006) the same as elsewhere in Europe and Australia and follow international standard IEC 60446. This colour scheme had already been introduced for appliance flexes in the UK in the early 1970s, however the IEE recommended a different scheme for fixed wiring until 2006. As a result, the international standard blue/brown scheme is as of 2006 found in all but the oldest appliance flexes. In fixed wiring, the blue/brown scheme is only found in very new (post-2004) installations, and electricians are likely to encounter the old IEE black/red scheme in existing installations for many more decades.

The standard colours in fixed wiring were harmonized in 2004 with the regulations in other European countries and the international IEC 60446 standard. For a transitional period (April 2004 – March 2006) either set of colours were allowed (but not both), provided that any changes in the colour scheme are clearly labelled. From April 2006, only the new colours should be used for any new wiring.

The colour change has been controversial and was delayed for three decades, because the colour blue which was previously used as a phase colour is now the colour for neutral, and the colour black which was previously used for neutral now indicates a phase. While confusion in identification of these conductors could be dangerous, the combinations of colours used usually resolves ambiguities. The installation of cables with the 'new' colours in an installation where the 'old' colours exist, could leave the way open for confusion; it is important in such situations that consideration is given to correct identification of the cables – with the use of marker tags if necessary. It has also been suggested that the new phase colours are difficult to distinguish in low-light conditions, but the same can be claimed for most colour combinations, including the old British phase colours. A mnemonic advantage of the new colours is that the first two letters of "BLue" and "BRown" match the corresponding positions on the BS 1363 socket face: "bottom left" (neutral) and "bottom right" (live).

Ring circuits

UK electrical power circuits are normally described as either radial or ring.

A radial circuit is one where power is transmitted from point to point by a single length of cable linking each point to the next. It starts at the main switch or fuse and simply terminates at the last connected device. It may branch at a connection point. Lighting circuits are normally wired in this way, but it may also be used for low power socket circuits.

A British peculiarity unknown in many other national wiring codes is the ring circuit. A cable starts at the supply point and goes to each device in the same way as with a radial. The last device is connected back to the supply so that the whole circuit forms a continuous ring. The primary advantage is to allow more power to be supplied than could otherwise be carried with a given size of cable.

Cables are most commonly a single outer sheath containing separately insulated live and neutral wires, and a non-insulated protective earth to which sleeving is added when exposed. There is some confusion as to why the third earthing conductor is supplied uninsulated, however the general consensus is for ease of terminating. Standard sizes have a conductor cross sectional area of 1, 1.5, 2.5, 4, 6 and 10 square millimetres. Sizes 1 or 1.5 are typically used for 6 or 10 ampere lighting circuits and 2.5 for socket circuits. The protective earth conductor is normally one standard size smaller than the main conductors.

The earthing conductor is supplied uninsulated for a number of reasons, primarily because it maybe considered safer that way. If the insulation of the live/neutral wire becomes damaged due to intrusion by a foreign object (such as a nail/screw) or by fire or heat (caused by short circuit or poor installation), then the wire is more likely to earth itself and in doing so either trip the RCD or burn the fuse out by drawing too much current. Some may argue that insulating the earth conductor would unnecessarily increase the cost, weight, size and rigidity of the cable.


All electrical work within a domestic setting must comply with Part P of the Building Regulations in England and Wales introduced on 1 January 2005., which are legally enforceable. One way of achieving this is to apply British Standard BS7671 (the "Wiring Regulations"), including carrying out adequate inspection and testing to this standard of the completed works.

Some of the restrictions introduced with Part P were controversial, especially the rules surrounding work carried out by non-professional (or, more accurately, unregistered) people such as DIYers. Under the new regulations, commencement of any work other than simple changes becomes notifiable to the local building control authority; "other than simple" in this context means any work in a kitchen or bathroom other than like-for-like replacement, work in other areas more than just adding extra lights or sockets to an existing circuit or meeting certain other criteria, such as outdoor wiring.

To coincide with the new regulations, the Government approved several professional bodies to award "competent persons" status to individuals and this allows them to "self-certify" work they carry out. The building control authority must be informed of any notifiable work carried out by someone not qualified under this scheme before it is started (unless it is an emergency) and must subsequently be approved by them. Originally, it was widely understood that inspection by a qualified person (leading to authority approval) must be organised and paid for by the home-owner or person responsible for the site and this caused some considerable criticism.

On 6 April 2006, Part P was amended to clarify the actual requirements around certification of DIY work (or work completed by someone otherwise unable to self-certify) and to "make enforcement more proportionate to the risk". [cite web|title=Note on Part P to the Building Regulations|publisher=IET|url=http://www.theiet.org/publishing/wiring-regulations/part-p/index.cfm|accessdate=2007-10-28]

The 2006 amendment makes it clear that it is the responsibility of the building control authority to issue the necessary certificate (a Building Regulations Completion Certificate) once work has been completed. Any inspection required to safely issue that certificate must be determined by, and paid for by, the building control authority. This can be done "in house" or they may contract the work out to specialist body. Note that although any inspections are at the expense of the building authority, notification of building work is a formal process and a building control fee is payable.

One of the quirks of Part P is that in some cases the installation of 12 V downlighters is notifiable where as the installation of 240 V mains downlighters isn't. This appears strange but it's because 12 V downlighters draw high currents and that combined with the wrong choice of wire could lead to a fire.

Another element of confusion is that the term "Special Locations" has different meanings in Part P of the Building Regulations and BS7671 (the "Wiring Regulations").

Installation accessories

Many accessories for electrical installations (e.g., wall sockets, switches) sold in the UK are designed to fit into the mounting boxes defined in BS 4662, with an 86 mm × 86 mm square face plate that is fixed to the rest of the enclosure by two M3.5 screws (typ. 25 or 40 mm long) located on a horizontal center line, 60.3 mm apart. Double face plates for BS 4662 boxes measure 143 mm × 86 mm and have the two screws 120.6 mm apart.

Accessories in the BS 4662 format are only available in a comparatively limited range of designs and lack the product diversity and design sophistication found in other European markets. The UK installation-accessory industry is therefore occasionally criticized for being overly conservative. [Pat Delaney: [http://www.emconline.co.uk/story.asp?storyType=106&sectioncode=281&storyCode=1024689 True grid] . EMC, February 2003.] As many modern types of electrical accessories (e.g., home automation control elements from non-UK manufacturers) are not available in BS 4662 format, other standard mounting boxes are increasingly used as well, such as those defined in DIN 49073-1 (60 mm diameter, 45 mm deep, fixing screws 60 mm apart) or — less commonly in the UK — ANSI/NEMA OS-1.

The commonly used domestic wall-mount socket used in the UK for currents up to 13 A is defined in BS 1363-2 and normally includes a switch. For higher currents or three-phase supplies, IEC 60309 sockets are to be used instead.

Note that many high load non-UK-sourced appliances need IEC 60309 connectors (or wiring via a British Standard "20A connection unit") in the UK because of the lower plug rating.

Plug and accessory fuses

Some accessories require protection at a lower current than that provided by the ring main protection device. The protection device used in such accessories is a 25 mm ceramic cartridge fuse, rated at 3 A, 5 A, or 13 A.

In the case of permanently connected equipment the fuse is contained in a holder mounted in an accessory known as a fused spur box, which usually includes an isolator switch and often a neon bulb to indicate if the equipment is powered. In this case the fuse protects the spur (equipment supply) cable and any switch contacts.

In the case of non-permanently connected domestic equipment, a socket rated at 13 A is attached to the ring main, into which a fused plug may be inserted. The fuse protects the contacts (including any switch contacts) and the equipment flex. There are two benefits to this arrangement. Firstly with low power equipment a flex with a low current rating (and therefore minimal diameter) can be used. Secondly, if the equipment is moved to a different socket, it will remain protected by the same (hopefully correct) fuse. The disadvantage is that despite warnings to the contrary people often use a fuse rated at too high a current, or even wrap a blown fuse in aluminium foil, meaning that under fault conditions the contacts and flex will be subjected to anything up to the maximum ring main current. This is likely to cause a fire.

Note that the equipment itself should have its own protection measures, such as another fuse, unless the plug or accessory fuse affords all required protection (as is the case with most table lamps, for example). It is also important to be aware that when a double socket is rated at 13A this applies to the total rating of the two sockets together, and in this case the use of a 13 A-rated double socket (e.g. a "free double socket" on an extension lead) for powering more than one highly rated appliance, such as an electric fire, is dangerous. (This is not the case for most fixed-wiring outlets to BS1363, which states that the standard rating of a double socket for use as part of the fixed wiring is 20 A.) Other "high-current" appliances such as washing machine and clothes dryer should ideally be fed from separate sockets for various reasons, but note that they do not use their full rated current at all times (in the cases of the Washing Machine and Dishwasher, for example, only when heating water which is a small percentage of the cycle).

Consumer unit

A domestic supply typically consists of a large cable entering the house which is connected to a sealed box containing the main supply fuse. This will typically have a value from 60–100 A. Separate live and neutral cables go from here to a meter, and from there proceed to one or more consumer units. This contains a main switch and individual fuses or Miniature Circuit Breakers (MCBs) for each circuit.

pecial locations


The installation of electrical devices in bathrooms and shower rooms is regulated in Section 701 of BS 7671:2008, and Part B of the Building Regulations. For such rooms, four special zones are defined [http://diydata.com/electrics/bathroom_electrics/bathroom_electrics.php] , in which additional protection is required for electrical facilities:

* Zone 0 is the smallest cuboid volume that contains the bathtub, shower basin, etc.
* Zone 1 is the area above Zone 0, up to a height of 2.25 m above the floor.
* Zone 2 is the area above Zone 1 up to a height of 3 m, as well as the area that is horizontally within 0.6 m from Zone 1.
* Zone 3 is the area above Zone 2 up to a height of 3 m, as well as the area that is horizontally within 2.4 m from Zone 2.

Within Zone 0 no devices are allowed apart from suitable equipment and or insulated pull cords. In Zone 1 only Separated Extra Low Voltage devices are permitted. Any AC transformer supplying such a device must be located outside Zones 0–2. The minimum required ingress protection rating in Zone 0 is IPX7 and IPX4 in Zone 1 and 2. If water jets are likely to occur, at least IPX5 is required in Zone 1–3. Otherwise, in Zone 3 and beyond, an ingress protection rating of IP20 is the minimum required. Equipment in Zone 1–3 must be protected by a 30 mA residual-current circuit breaker (except for shower pumps and shower heaters, where the use of an RCD is so far only recommended).

Shaving sockets (with isolating transformer) are permitted in Zone 2 if direct spray from a shower is unlikely, even if they are only IP20. In a bathroom or shower room, such shaving sockets are the only sockets permitted in the entire room. In any other room with a bathtub or shower, normal sockets are permitted as long as they are outside Zone 3.

(Earlier British wiring rules in bathrooms used to be far more restrictive, leading to British peculiarities in bathrooms such as the use of cord switches. The 2001 edition of the Wiring Regulations is more flexible now, placing restrictions on bathroom installations that are now more similar to those in other European countries. The forthcoming 17th Edition of the Regulations is likely to allow 13A sockets in bathrooms, subject to zoning and protection by a 30mA Residual Current Device.)

wimming pools

For swimming pools, Section 603 of BS 7671 defines similar zones. In some of these zones, only industrial sockets according to IEC 60309 are permitted, in order to discourage the use of portable domestic appliances with inappropriate ingress protection rating.

Portable outdoor equipment

For use outdoors or in other wet locations (but not bathrooms) special sockets are made. These can be divided into three main groups, industrial sockets which are totally different from the standard sockets, sockets with the same pinout as normal sockets but that will only seal properly when the correct plug and socket are used together (e.g., the 5 A 13 A and 15 A variants of Lewden sockets) and sockets that completely enclose a normal plug with a seal around the flex (e.g., MK masterseal).

Sockets that are outside or can "feasibly supply equipment outside the equipotential zone" (a wording that is fairly ambiguous and the exact interpretation of which is subject to some controversy) should be protected by a 30 mA or lower RCD to provide additional safety.

upply voltage

Since 1960, the supply voltage in UK domestic premises has been 240 V AC (RMS) at 50 Hz. In 1988, a Europe-wide agreement was reached to change the various national voltages, which ranged at the time from 220 V to 240 V, to a common European standard of 230 V (CENELEC Harmonization Document HD 472 S1:1988).

As a result, the standard nominal supply voltage in domestic single-phase 50 Hz installations in the UK has been 230 V AC (rms) since 1 January 1995 (Electricity Supply Regulations, SI 1994, No. 3021). However, as an interim measure, electricity suppliers can work with an asymmetric voltage tolerance of 230 V +10%/−6% (216.2 V to 253 V). This was supposed to be widened to 230 V ±10% (207 V to 253 V), but the time of this change has been put back repeatedly and currently sits in 2008 (BS 7697). The old standard was 240 V ±6% (225.6 V to 254.4 V), which is mostly contained within the new range, and so in practice suppliers have had no reason to actually change voltages.

The continued deviation in the UK from the harmonised European voltage has been criticised in particular by light bulb manufacturers, who require tighter voltage tolerances to optimise the operating temperature and lifetime of their products, and who currently have to continue producing separate 230 V and 240 V versions.

ee also

* Electric power
* Technical standards in colonial Hong Kong


* [http://www.communities.gov.uk/pub/910/ApproveddocumentPElectricalsafetyPDF901Kb_id1130910.pdf Part P of Approved Documents] , The Building Regulations 2000: Electrical safety.
* [http://www.opsi.gov.uk/si/si1989/Uksi_19890635_en_1.htm The Electricity at Work Regulations 1989]
* BS 4662: Specification for boxes for the enclosure of electrical accessories. British Standards Institute, 1970.

External links

* [http://www.theiet.org/publishing/wiring-regulations/index.cfm IEE Wiring Regulations]
* [http://www.tlc-direct.co.uk/Book/1.1.htm The Electrician's Guide to the 16th Edition IEE Regulations] (N.B This guide refers to a depreceated version of the regulations)
* [http://www.domlec.co.uk/ Electrician's Course notes for the 17th Edition IEE Regulations]

* [http://www.hse.gov.uk/electricity/ Electrical Safety at Work] , Health & Safety Executive
* [http://www.hse.gov.uk/electricity/standards.htm Electrical standards and approved codes of practice] , Health & Safety Executive
* [http://www.hmso.gov.uk/si/si1994/Uksi_19943260_en_1.htm The Electrical Equipment (Safety) Regulations 1994]
* [http://www.opsi.gov.uk/si/si1994/Uksi_19941768_en_1.htm The Plugs and Sockets etc. (Safety) Regulations 1994]
* [http://www.opsi.gov.uk/si/si2002/20022665.htm The Electricity Safety, Quality and Continuity Regulations 2002]
* [http://www.generalcable.co.nz/Technical/ General Cable - Imperial / Metric Conductor Size Comparison Chart]
* [http://www.nectasafety.co.uk/duty-holder.htm Meeting the requirements of EaWR 1989]
* [http://www.cappershaw.com/index.php?option=com_content&task=view&id=14&Itemid=30 Wiring a standard UK plug]
* [http://www.communities.gov.uk/publications/planningandbuilding/partp Approved Document P for guidance on Part P of the Building Regulations]
* [http://www.competentperson.co.uk/ Local Authority Building Control Part P Competent Persons Register]

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