Electrical wiring in the United Kingdom

The detailed rules for end-use wiring followed for practical purposes are those of BS 7671 Requirements for Electrical Installations.

However, there are a number of specific national practices, habits and traditions that differ significantly from other countries, and which in some cases survived harmonisation.

Direct current mains supplies are only of historical interest in the UK but the colour coding was red for live and black for earthed (regardless of the polarity).

(This was one of the recommendations of the Electrical Installations Committee, convened in 1942 as part of the Post War Building Studies programme, which in 1944 determined that the ring final circuit offered a more efficient and lower cost method to support a greater number of sockets.

In practice, it is impossible to ensure the load does split evenly, so regulations require a thicker wire, of at least 2/3 the current capacity of the fuse or circuit breaker.

Cables are most commonly a single outer sheath containing separately-insulated line and neutral wires, and a non-insulated protective earth to which sleeving is added when exposed.

In installations fed from low-voltage public supplies in the UK this means of earthing can be any of the methods TN-S, TN-C-S or TT defined in BS 7671.

In the event of an insulation fault from a live conductor to an appliance's metal frame (an exposed conductive part), the frame could—if not so connected—be dangerous if touched by someone who is also for example standing outside on the ground, or standing inside on a concrete floor, or holding a tap whose pipe connects it electrically into the ground.

In conventional installations in the UK the voltage between an appliance frame and the Earth itself during a zero-impedance fault has a dangerous magnitude: it might be reduced to about half of the 230 V line-earth voltage, which is well above the 50 V usually accepted as safe for an AC system, or it might be nearly 230 V in a TT system with a poor earth electrode for the installation.

In TT systems it is almost always necessary to have an RCD, as earth electrodes usually have many times higher resistance than a typical supply cable, so earth-fault currents are relatively low.

Supplementary bonding is particularly used in situations such as bathrooms, where body resistance is low and therefore requires magnitude and duration of touch voltages to be very limited.

3021) following a European-wide agreement reached in 1988 to unify the various national voltages, which had ranged between 220 and 240 V, to a common European standard (CENELEC Harmonization Document HD 472 S1:1988).

Though in practice this new standard may have made little difference to the voltage provided to premises by distributors, owing to the allowed tolerance range, there was however a significant change in terms of device manufacturing – in order to get a CE compliance marking devices must correctly operate across a voltage range of 230 V ±10%, enabling pan-European compatibility.

[16] Note that 240 V AC RMS means a momentary peak voltage of plus or minus 339 volts (see this explanation), occurring 50 (either positive or negative) or 100 (both, opposing) times per second.

For larger loads like air conditioners, cookers, water heaters and other fixed appliances a double-pole switch is used, which also isolates the neutral for greater safety.

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.

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.

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.

Conductor size and voltage drop tables are available to determine the selection, which will be based on the load current supplied and factors such as building insulation.

Since BS 7671:2008 normal domestic sockets are permitted, at distances greater than 3 m from the edge of the zones, providing the circuit is RCD protected.

As the new regulations 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.

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.

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.

In sites where hazardous flammable gases or liquids are present, special wiring rules are applied to reduce the probability of a spark igniting a fire or explosion.

All new electrical work in England and Wales within a domestic setting must comply with Part P[26] of the Building Regulations first introduced on 1 January 2005, which are legally enforceable.

Some of the restrictions first introduced with the 2005 version of Part P of the Building Regulations were highly controversial, especially the rules surrounding work carried out by unregistered electricians, builders and DIYers.

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 local authority's building control must be informed of any notifiable work carried out by someone who is not registered under this scheme before it is started (unless it is an emergency) and must subsequently be approved by them.

This is because 12 V 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.

Due to uncertainty about who then becomes responsible for any hidden wiring, very few electricians are happy to sign off an installation that they have not been party to from the outset, and been able to agree stages to inspect and test before any covering in.