House Wiring for Beginners

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Revision as of 16:31, 29 March 2011 by John Rumm (talk | contribs) (→‎Overview: New piccie - couple of wording changes as a result.)
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House Wiring for Beginners gives an overview of a typical basic domestic mains wiring system, then discusses or links to the common options and extras.

Further information on options is available in the Rewiring Tips article.


Safety

  • This article can not practically cover everything.
  • The writing of this article may be incomplete when you read it.
  • Almost anyone can edit wiki articles at any time.
  • Errors and omissions can occur in real life.
  • Laws and regulations change over time.
  • Interpretation of regulations and law may change over time
  • The article may assume knowledge that some readers might not possess
  • Unexpected situations may change the specific requirements for some circuits.

For these reasons and more, one should not carry out safety critical work based solely on wiki content. Information and plans should be independantly checked and verified before action.

Anyone installing wiring should also understand some basic safety issues not discussed here. This article is an introductory overview rather than a complete A to Z on rewiring, and assumes some basic electrical knowledge.

Regulations

Some requirements are mentioned in this article. These apply to new wiring, and in many cases are not requirements for existing wiring.

Word Meanings

The Electrical Glossary may be useful.


Overview

Typical house wiring diagram illustrates each type of circuit:

Click for bigger picture

In a typical new town house wiring system, we have:

  • L&N tails from the electricity meter to the CU
  • A split load CU
  • Ring circuits from 32A MCBs in the CU supplying mains sockets. 2 such rings is typical for a 2 up 2 down, larger houses will have more.
  • Radial lighting circuits from 6A CU MCBs. 2 or more circuits typical.
  • Earth connection from incomer to CU.
  • 10mm² Main equipotential bond to other incoming metal services (gas, water etc)
  • 4mm² Supplementary Equipotential bonding in bathroom, connecting all the main touchable metal objects together.

Systems may often have some of the following as well:

  • Dedicated circuit MCB & cable supplying cooker
  • Dedicated high current circuit MCB & cable supplying shower
  • 2 way lighting switching for stairs, large rooms & walk through rooms
  • Outdoor lighting supplied by a 6A MCB, often via a PIR automatic switch.
  • 16A MCB and cable supplying hot water immersion heater.
  • A high current MCB supplying storage heaters. Sometimes these are run from the main CU, but often from a timeswitch controlled dedicated CU (with either a separate "off peak" electricity meter, or a dual tariff meter).


The radial lighting circuit has 3 common wiring options, which may be mixed at will:

  • "loop-in" (as per diagram above). The circuit is fed to each lamp fitting in turn, and a separate switch wire connect from the fitting to the switch. (this is the most common techniques used in modern wiring)
  • Switch loop through (the circuit connects to each switch in turn, and a separate switched wire goes from the switch to each lamp)
  • Junction box loop in where the termination and feed connection are does at junction boxes, and wires run to switches and lamps from there.

The diagram is shown with 6A lighting fuse and 32A ring circuit MCB. Other options are also possible.

CU

The Consumer Unit, previously called a fusebox, contains these things:

  • A main isolating switch. This switches off everything.
  • In most cases an RCD
  • A fuse or MCB for each circuit. This cuts the power in the event of high fault current.
  • An earth connection block which connects earth to the earth wires of the various circuits
  • 1 or 2 neutral connection blocks which supply the neutral connection to the N wires of the various circuits

Each fuse or MCB supplies one circuit only. One circuit may supply anything from 1 to a large number of loads.


Split Load CU

Split load CUs have become popular in recent years, and offer significant advantages over the traditional unsplit CU.

A split load CU divides the MCBs into 2 or more separate banks. Each bank usually having its own RCD. There is usually a master switch that will switch the whole CU.

The split load CU has the following advantages:

  • Some wiring work can be carried out with just a section of a CU turned off, perhaps retaining access to light and power while working.
  • Almost all faults can only take out the power to part of the system
  • The split load arrangement means both RCDed and non RCDed loads can be supplied from the one CU.

Split load CUs are recommended, and this article will assume the use of a split load CU.

Earth Type

An electrical earth is simply a connection to wet ground. It is used to greatly reduce shock risk.

Most houses have an earth connection supplied by the electricity supplier. Those that don't (generally country houses several miles from the nearest town) use a local earth rod instead (TT systems).

An earth rod is a copper coated steel rod hammered into the ground. (Water pipes were used in the past, but this is no longer safe due to widespread use of plastic water supplies.)

The supplier's earthing terminal or own earth rod is connected to the CU earth block. A wire connects this point to each incoming service at its entrance to the house (ie gas, water, oil).

Each electrical circuit in the house takes its earth connection from the CU earthing block.

For more information on earthing types and earth wiring, see Earthing Types


Sockets

Sockets may be wired on ring circuits or radial circuits. Mostly rings are used, as they use less copper for most circuit layouts, they have safety advantages over radial circuits (sometimes debated), can provide more power, and cover more floor area per circuit.

Ring

Sockets are on 32A ring circuits in most house installations. These use a ring of cable (ie a loop), so that at the CU 2 cables are connected to the MCB instead of 1. An unlimited number of sockets may be connected on each ring.

One ring circuit per floor is a fairly common arrangement, but by no means the only option. Larger houses will generally have more rings than this. It is also common to have a ring dedicated just for sockets in the kitchen since that is where you will find many of the highest power consuming appliances in a modern house.

2.5mm² cable is usually used for ring circuits. 4mm² is used when cable will be under insulation or bunched with other cables.

For more information on rules and practices governing socket circuits, see another unwritten article on socket circuits.

Radial

Radial socket circuits are used less often. These use a single cable from CU to socket, then a single cable to the next socket along the line etc. Radials use more copper on most circuits, though less cable on physically long narrow shaped circuits. Connection faults have greater consequences than with ring circuits. (Confusion over the relative safety of ring & radial circuits is widespread.)


Lighting

Radial circuits are used for lighting. There is one lighting circuit on each lighting MCB. Lighting circuits are usually on a 6A MCB or 5A fuse, though 10A can be used (with some extra restrictions) for large circuits. However if the area served is large, more 5A or 6A circuits would in most cases be preferable.

lighting uses 1mm² or 1.5mm² cable. 1.5mm² removes the restrictions that apply with 1mm², and the cost difference is trivial.

Fuse or MCB

Light bulb failures can trip MCBs. fuses have a significant advantage over MCBs for lighting circuits, as they rarely nuisance trip on bulb failure.

Loop-in Wiring

The power feed cable may go to either the switch or the bulbholder. If it goes to the bulbholder, this is called loop-in wiring, and the ceiling rose then uses 4 connections instead of 3, the extra one being unswitched live.

With loop-in wiring, the cable from rose to switch has 3 conductors, namely earth, unswitched live and switched live. Regs conformance requires that brown sleeving be fit over the neutral coloured conductor at each end of the switch cable.

The following wires are present at the bulb holder: 


       N -----------------------+--------------------- N  to next
supply                          |                         light 
       L -------------------+---)--------------------- L  fitting
                            |   |
                            |   |
       L -------------------+   +--- N
switch                                    bulb holder
       SW L ------------------------ SW L



       N -----------------------+--------------------- N  to next
supply                          |                         light 
                                 \
       L -------------------+-----)------------------- L  fitting
                            |    /
                            |   |
                            |   |
       L -------------------+   +--- N
switch                                    bulb holder
       SW L ------------------------ SW L


All cable colours are as expected except for the switched live. Light switches are usually wired with standard T&E, which means the switched live wire will be black (existing installs) or blue (new installs).

If you connect all the blacks together, your fuse or MCB will trip.

For a pretty diagram of lightingt wiring, see letsfixit lighting diagram

For more information on lighting see

2 Way Switching

2 way switching means having 2 or more switches in different locations to control one circuit. They are wired so that operation of either switch will control the light(s).

The method is explained in 2 Way Switching


Compulsory Low Energy Lighting

Installing some lighting fittings that are only compatible with low energy lamps is now compulsory during a rewire. These are sometimes replaced by the householder once the system has been signed off.

For more information on compulsory low energy lighting, see


Lighting earthing

A lot of metal light fittings require earthing, but those marked with the double insulated square in square symbol Logo do not need an earth connection.

The earth wire in the T&E must be run to all switches, junction boxes & light fittings, including those that are plastic or don't need an earth connection. It is not permitted to borrow an earth or neutral from another circuit.

For more information about unearthed lighting circuits see Lighting Circuits Without an Earth

Cookers

All in one electric cookers (oven, hob & grill in one unit) are fed by a high current cable from the CU, typically on a 45A MCB.

Single cavity ovens with no hob are more often put on a 13A plug.

Most hobs require their own high current feed, but some are available that incorporate load limiting switching, and are designed to be run on a 13A plug.

Combi cookers (microwave & fan oven, with or without grill) are always on a 13A plug.

Cables

Bathroom Electrics

All bathroom electrics are supplied from the RCD side of the CU.

Equipotential Bonding

All major metal items in the bathroom are connected together using 4mm² green/yellow insulated wire. This is called equipotential bonding. The wire is connected to metal pipes using BS 951 earth clamps. The wire is connected to radiators using connectors. It is permitted to place equipotential bonding connections immediately outside the bathroom if necessary.


  • Radiators are bonded with radiator bonding clamps.
  • Earth wire is connected to earth rod with a BS7430 clamp.

Zones

Bathrooms are divided into 3 zones, with different rules for each zone.

  • Electrical fittings in the bathroom in zone 0 must conform to IPX7 or better.
  • Electrical fittings in the bathroom in zones 1 & 2 must conform to IPX4 or better.
  • Electrical fittings in the bathroom outside of the zones do not need to confirm to any specific IP rating, but must be appropriate for the circumstance in which they are used.

There are also limitations to the type of electrical equipment permitted in each zone. See the Bathroom electrics article for more details.

Sockets

Until the introduction of the 17th edition of the wiring regulation, sockets were not permitted in a bathroom at all, unless they were either a transformer isolated shaver socket, or sockets to power extra low voltage devices, both of which are permitted in Zone 2 or outside.

For more information on bathroom electrics, see unwritten article on Bathroom Electrics.

Showers

An electric shower will be fed on its own high current cable, fed from its own MCB on the RCD protected side of the CU.


For more information on electric showers, see

Outdoor Lighting

Outdoor lighting is run on its own radial circuit off its own MCB in the CU. Usually this is a 6A MCB, but lighting is less likely to cause problems if run on a 5A fuse.

Exterior cabling is now required to be armoured and uv proof.


For more information on outdoor lighting, see


RCD

NOTE - The current set of IEE Wiring Regulations, the 17th Edition are more onerous in the requirements to install RCD (or RCBO) protection than the previous 16th Edition. In general, ANY cable which is less than 50mm below the wall surface AND is NOT mechanically protected as defined by the IEE Regulations (which generally means armoured cable [SWA], pyro cable [MICC, mineral insulated copper clad], cable in steel heavy guage conduit or a limited use of highly specialised cables incorporating an earthed foil screen MUST HAVE a 30mA non timed delayed RCD protecting that circuit, in addition to all other requirements. Such circuit protection may be derived from either an RCD protecting several circuits, or individual RCD/RCBOs on each circuit. This statement and it's implications now overrides anything else in this article which was written to the 16th Edition, pending more detailed updates to this article.

New installations will have one or more RCDs.

RCDs reduce shock risk (they don't eliminate it), but they also introduce reliability and safety issues, so RCDs are usually used on some but not all circuits.

With a supplier provided earth connection, the most common arrangement is a split CU with an RCD on one side, and no RCD on the other. Generally the RCD side is used to supply sockets and shower, with most other items on the non-RCD side.

With a local earth rod, the situation is different in that all circuits must be RCD protected, since a local earth rod is not a sufficiently good earth on its own. An electrical system must not rely on one RCD only (such installations have proven too unreliable), so 2 RCDs are used, with more than one possible arrangement,

A common option is to have the supply fed through a 100mA time delayed RCD, the output of which goes to a split CU with RCD on one side. This is not an ideal arrangement, as an earth leakage fault on the non-CU side will cause complete power failure, and sometimes inability to reset the power.

A better option is to have a split CU with an RCD on each side. Now any single earth fault can only kill half the power, so you will still have power.

For more information on RCDs & RCBOs, see


RCBOs

An RCBO is a combined RCD and MCB, and is fitted in place of an MCB. RCBOs offer significant advantages over RCDs, but they add cost.

Where RCBOs are used, they are fitted in the non-RCD side of the CU, and supply circuits needing RCD protection.


Neutral Connections & RCDs

Neutrals on RCD and non-RCD sides must not be mixed. If any neutral wire is connected to the wrong side, the RCD will trip.

The same princple is true for RCBOs, each RCBO'ed circuit needs to have its neutral connected to the RCBO neutral and not elsewhere.


RCD or non-RCD side

Installations using an earth rod will run all circuits off RCDs.

Installations using a supplier earth connection will run some circuits off an RCD, and some with no RCD:

Sockets

Usually all sockets are run on the RCD side. Plug-in appliances are the source of almost all electrocutions, so this is where RCDs are beneficial.

However an RCD is not a regs requirement for most sockets. Only sockets likely to be used by outdoor equipment are required to be powered via an RCD.

A dedicated socket feed for a fridge frezer is best run from the non-RCD side.


Lighting

lighting is run off non-RCD side.

  • lighting on RCDs can cause early lighting failure in a fire, and has resulted in death due to failure to escape.
  • lighting on RCD can also cause lighting failures which can result in stair falls, which claim many lives every year.
  • lighting results in 0 deaths by electrocution per year, so addition of RCD to lighting is not of value.


Immersion Heater

Immersion Heaters are run off the non-RCD side.

  • Immersion Heaters are a common cause of earth leakage, and can thus make an installation unreliable if on an RCD.
  • Contrary to what we were taught about electricity and water as children, earth leakage in immersion heaters does not lead to electrocutions in real life. The reasons why are beyond the scope of this article.


Fixed Appliances

Fixed appliances such as cooker etc are not a significant shock risk, and are usually powered from the non-RCD side.

  • Many fixed appliances contain heating elements, which sometimes cause nuisance trips when powered from RCDs.

Kitchens

There is no requirement to RCD kitchen sockets, except where they are liable to be used for outdoor equipment, ie are near an outside door. However, countertop plug-in appliances do occasionally electrocute people, so above-counter sockets are best fed by RCD.

Under-counter Appliances

Opinion has always been divided as to whether to feed under-counter appliances off the RCD. They are not a significant electrocution risk, and running them on the RCD significantly reduces system reliability, so I would not RCD them. Some do however. Either option is permissible.

Freezer

Freezers are best on a non-RCD feed, as a trip when the house is unattended is liable to cause defrosting, and RCDs are prone to nuisance trips. Deaths by food poisoning are orders of magnitude higher than electrocutions, and a non-RCD feed already has several layers of protection against shock & electrocution. The ideal is to put a freezer on its own dedicated feed, then an overcurrent fault in another appliance can not cut the power to the freezer either.

Fixed appliances do not require an RCDed feed.

Bonding

Kitchens do not need equipotential bonding. Sinks should not have earthing added.

Number of Rings

Most kitchens are supplied by one ring circuit. However this may be insufficient for some larger or all-electric kitchens.

Load & Diversity calcs


Spurs

Spurs are permitted, but sockets should be included in the ring rather than spurred wherever practical. Spurring is best only used for later additions to circuits.

Restrictive rules apply to number of spurs, cable size, loading and number of sockets on the end of a spur.

Spurring sockets prevents the easy later addition of more sockets in some positions, as a spur may not be spurred off a spur. Spurs also prevent the addition of more sockets at existing spurred positions, whereas a practically unlimited number of sockets can be added where a socket is in the ring. Bear in mind the number of sockets wanted has risen greatly over the years, and can only be expected to rise further.


Number of Sockets

Minimum and desirable numbers of sockets recommended per room are given. Recommended numbers are inevitably a matter of opinion, and are only recommended as a starting point for consideration.

  • Bedroom
    • Minimum: 1 double socket at each of 2 locations
    • Recommended: 2x double sockets at each of 4 locations (in or near corners) + a double socket at side of single bed, or a double socket at each side of double bed. 2 way lighting switches controllable at door & bedside.
  • Corridor
    • Minimum: none
    • Recommended: 1 double socket for a short corridor, 2 or possibly more in a long one.
  • Kitchen
    • Minimum:
    • Recommended: Under worktop: 4 or 5 double sockets. Over worktop: 1 double socket per 2 metres. All sockets on ring circuit, no spurs. 2A or 5A sockets on lighting circuit: one above each set of cupboards, one below each set of cupboards, one away from cupboards & worktop.
  • Lounge:
    • Minimum: 1 double socket at each of 2 locations
    • Recommended: 2x double sockets at each of 4 locations (in or near corners) + 1-3 double sockets where PC or AV equipment is to be used. If the room has 2 or more doors, 2 way lighting switches controllable at each door. 4-6x 2A or 5A sockets on lighting circuit.
  • Utility room:
    • Minimum: 1 double socket
    • Recommended: 2 or 3 double sockets, all on ring circuit
  • Bathroom:
    • Minimum: no sockets
    • Recommended: none. 1 socket somewhere out of easy reach in zone 3 if you wish to use an appliance in the bathroom (eg washing machine or dehumidifier). A shaver socket at the sink is an option, but plugging items in outside the room is probably better practice.
  • Home Office:
    • Minimum: a double socket at 2 locations
    • Recommended: 2x double sockets at each of 4 locations (generally near corners), plus anything from 2 to 6 double sockets where computer or other business appliances will go.
  • Small shed:
    • Minimum: no electricity supply
    • Recommended: if far from the house, a double socket can be useful.
  • Large shed:
    • Minimum: no electricity supply
    • Recommended: plenty of lighting & sockets according to size & proposed use. Given the tendency for electricity use to rise over the years, an overrated feed cable may prove useful in time.
  • Greenhouse:
    • Minimum: no electricity supply
    • Recommended: A splashproof double socket above head height can be handy. If serious about cultivation, fluorescent lighting, a couple of splashproof double sockets positioned at head height or above, and a 13A socket for discharge lighting can all come in useful.


Part P

Complete rewires are now covered by Part P. This means testing and paperwork is required once an installation is complete, if it was not installed by a suitably qualified and guilded tradesperson.

For more information on Part P, see Part P


Cable Colours

Old colours:

  • Red = Live
  • Black = Neutral
  • Bare or green/yellow = Earth

New Colours:

  • Brown = Live
  • Blue = Neutral
  • Bare or green/yellow = Earth

For more information on cable colours, see Wiring colour codes


See Also

Wiki Contents

Wiki Subject Categories

Category:Electrical

Category:Lighting

Electrical Glossary

Rewiring Tips

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