Electrical wiring in the United Kingdom

There are a number of specific national practices, habits and traditions associated with consumer mains wiring in the United Kingdom (and Ireland) that differ significantly from other countries. These 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 (IET) wiring regulations (BS 7671), colloquially referred to as "the regs" (BS 7671: 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.

In Scotland the Building (Scotland) Regulations 2004 apply.

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 BS 7671 "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, Australia, and New Zealand 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 original colour scheme recommended by the IEE for fixed wiring was permitted until 2006. As a result, the international standard blue/brown scheme is as of 2006 found in most appliance flexes. In fixed wiring, the blue/brown scheme is only found in very new (post-2004) installations, and the old IEE black/red scheme is likely to be encountered in existing installations for many more decades.

	Pre-1977 IEE	Pre-2004 IEE	Current IEC
Protective earth (PE)	Green	Green/yellow bi-colour	Green/yellow bi-colour
Neutral (N)	Black	Black	Blue
Single phase: Line (L) Three phase: L1	Red	Red	Brown
Three phase: L2	Yellow	Yellow	Black
Three phase: L3	Blue	Blue	Grey

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.

Other colour schemes

Cables of USA origin will have White as Neutral and Black as live. This can occur on IEC mains leads and dual 220V/110V imported equipment.

On telecommunications nominal 48V DC supplies, the live is usually -42V (flat batteries) to almost -57V(float charge), the positive power wire is thus red (the wiring providing the earth/ground connection at the power source is conventionally-coloured as often will be earth connections not intended to carry power, i.e. on older installations being green); the same principles were also applied to equipment operating at lower voltages which used non-floating power supplies. It is now unusual to encounter such wiring in a domestic installation due to the absence of power supplies for modern systems involving a separate float-charged battery and the general non-provision of backup power; such wiring in domestic premises in any case was only required for the short distances between the rectifier or power unit, battery (if any) and the device receiving the main power feed.

Potential for confusion

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.

Direct current systems

Direct current mains are now only of historical interest in the UK but they also gave rise to possible confusion because the colours used were red for positive and black for negative. With a three-wire distribution system (similar to an AC split-phase electric power system) it was possible for either positive or negative to be live, creating an additional opportunity for confusion.

Circuit design

Main article: Ring circuit

UK electrical socket circuits are normally described as either radial or ring. Socket circuits are usually connected as a ring, with radially connected sockets being the exception.

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 practice unknown in 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. Since there are now two independent paths from the supply to every device, this practice allows more power to be supplied than could otherwise be carried with a given thickness of cable. It was adopted to save on copper during the post-WWII period of shortage.

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. Standard sizes have a conductor cross sectional area of 1, 1.5, 2.5, 4, 6 and 10 mm². Sizes of 1 or 1.5 mm² are typically used for 6 or 10 ampere lighting circuits and 2.5 mm² for socket circuits. The protective earth conductor in older cables was normally one standard size smaller than the main conductors but is now specified to be the same size.

The earthing conductor is supplied uninsulated since it is not intended to have any voltage difference to surrounding earthed articles. Additionally, if the insulation of a live or neutral wire becomes damaged, then the wire is more likely to earth itself on the bare earth conductor and in doing so either trip the RCD or burn the fuse out by drawing too much current.

Regulations

All new 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. British Standard BS 7671 (the "Wiring Regulations") is not statutory, thus someone doing electrical work is allowed to deviate from the wiring regulations to some degree, but it is generally accepted that it is best to follow the wiring regulations to the highest standard possible.

Some of the restrictions introduced with Part P were controversial, especially the rules surrounding work carried out by 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 enterprises which meet the minimum agreed criteria for Scheme entry:

(The minimum criteria for Scheme entry is set by the EAS Committee, on which all of the commercial enterprises running Competent Persons Schemes are actively represented).

Scheme membership allows an enterprise to "self-certify" work that they carry out without the requirement to have undergone any formal installation training or to hold relevant qualifications in electrical installation practices - since practical competence can be assessment-based only.

The building control authority must be informed of any notifiable work carried out by someone not registered 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".

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.

In some cases the installation of 12V downlighters is notifiable whereas the installation of 230V mains downlighters is not. This is because 12V downlighters draw high currents, in comparison with a mains voltage lamp with the same power rating, and that combined with the wrong choice of cable could lead to a fire.

Additionally, whilst the Building Regulations apply equally to anyone carrying out electrical work in dwellings, without adequate knowledge and test equipment it is not possible to ensure that the work carried out is safe. Registered Scheme members must issue appropriate certification, yet the householder will almost certainly be unable to.

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

British 13A double socket

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 4662:2006 - Boxes for flush mounting of electrical accessories – Requirements, test methods and dimensions, 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 mm or 40 mm long) located on a horizontal centre line, 60.3 mm apart. Double face plates for BS 4662 boxes measure 147 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. 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 "20 A 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 circuit overcurrent protection device. The protection device used in such accessories is a 1" (25.4mm) ceramic cartridge to the requirements of BS 1362:1973 fuse, commonly 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 Connection Unit, 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 circuit, 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 small 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 circuit current. This is likely to cause a fire.

The appliance itself should have its own protection measures, such as another fuse, if the plug fuse does not provide the required overcurrent protection (this is often the case with electronic items).

Most fixed-wiring double socket outlets to BS1363 are rated 20A total, 13A per socket, and do not include a 13A fuse. In practice, they are only protected by the ring circuit overcurrent protection of 30A. However multiple sockets on extension leads are usually rated at 13A in total simply because the plug of the extension cable is fused at 13A, and this rating applies to the total rating of all the sockets together. Some wall mounted multiple sockets are also 13A rated in total, these types having an inbuilt 13A fuse.

Consumer supply, metering and distribution

A domestic supply typically consists of a large cable connected to a service head, a sealed box containing the main supply fuse. This will typically have a value from 40–100A. Separate live and neutral cables ('tails') go from here to an electricity meter, and often an earth conductor too. More tails proceed from the meter into the consumer side of the installation and into a consumer unit/distribution board, or in some cases to a Henley block (a splitter box used in low voltage electrical engineering) and thence to more than one distribution board. The distribution board (aka fusebox) contains one or more main switches and an individual fuse or miniature circuit breaker (MCB) for each final circuit. Modern installations may alternatively use devices combining overcurrent and additional residual current protection in place of fuses and MCBs or provide additional protection through the inclusion of one or more residual current device.

Special locations

Bathrooms

The installation of electrical devices in bathrooms and shower rooms is regulated in Section 701 of BS 7671:2008, and Part P of the Building Regulations. For such rooms, four special zones are defined, 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.

Older regulations defined Zone 3 as 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; from BS7671:2008, this is replaced by the term 'outside the zones'. This includes any space under the bath or shower that can only be accessed with a tool.{ref bs7671:2008}

Within Zone 0, no devices are allowed apart from suitable equipment and or insulated pull cords. In Zone 1, only separated extra low voltage (SELV) 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 Zones 1 and 2 must be protected by a 30 mA residual current device (RCD).

Shaving sockets (with isolating transformer) are permitted in Zone 2 if direct spray from a shower is unlikely, even if they are only IP20. Before the 2008 regulations, such shaving sockets were the only sockets permitted in a bathroom or shower room. Since BS7671:2008 normal domestic sockets are permitted, at distances greater than 3m from the edge of the zones, providing the circuit is RCD protected. As the new regulations also require all general purpose sockets not for use by skilled or instructed persons to be RCD protected, this effectively permits normal wiring in the larger bathroom. (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. )

Swimming 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. Since 2008, all sockets for general use should be RCD protected, removing the questions that used to arise, such as if a socket by the door might power a lawnmower does it need an RCD?

Supply 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 unify the various national voltages, which ranged at the time from 220 V to 240 V, to a common European standard of 230V (CENELEC Harmonization Document HD 472 S1:1988).

The standard nominal supply voltage in domestic single-phase 50 Hz installations in the UK is still 240V AC (RMS), but since 1 January 1995 (Electricity Supply Regulations, SI 1994, No. 3021) this has an asymmetric voltage tolerance of 230 V+10%-6% (253-216.2 V), which covers the same voltage range as continental 220v supplies to the new unified 230v standard. This was supposed to be widened to 230 V ±10% (253-207 V), but the time of this change has been put back repeatedly and as of 2007 is set for 2008 (BS 7697). The old standard was 240 V ±6% (254.4-225.6 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.

See also

• Consumer mains wiring
• BS 1363
• BS 546
• Pattress
• Technical standards in colonial Hong Kong

Further reading

• BS 4662:2006 Boxes for flush mounting of electrical accessories – Requirements, test methods and dimensions. British Standards Institution.

External links

• IEE Wiring Regulations
• Electrician's Course notes for the 17th Edition IEE Regulations
• Electrical Safety at Work, Health & Safety Executive
• Electrical standards and approved codes of practice, Health & Safety Executive
• General Cable - Imperial / Metric Conductor Size Comparison Chart
• Meeting the requirements of EaWR 1989
• Approved Document P for guidance on Part P of the Building Regulations
• Local Authority Building Control Part P Competent Persons Register
• Text of the The Electrical Equipment (Safety) Regulations 1994 as in force today (including any amendments) within the United Kingdom, from legislation.gov.uk.
• Text of the The Plugs and Sockets, etc. (Safety) Regulations 1994 as in force today (including any amendments) within the United Kingdom, from legislation.gov.uk.
• Text of the The Electricity Safety, Quality and Continuity Regulations 2002 as in force today (including any amendments) within the United Kingdom, from legislation.gov.uk.
• Text of the The Building Regulations 2000: Electrical safety as originally enacted or made within the United Kingdom, from legislation.gov.uk.
• Text of the The Electricity at Work Regulations 1989 as originally enacted or made within the United Kingdom, from legislation.gov.uk.