Difference between revisions of "House Wiring for Beginners"
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Revision as of 14:33, 24 April 2007
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.
- 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.
Typical house wiring diagram illustrates each type of circuit:
__ Consumer unit | | Electricity Meter _____________________. |__| | |______// <-- L&N tails | 5A 20A 45A 32A |______/ |_____________________|____________ Supplier's earth terminal | | | || \___________ Earth to water & gas pipes | | | || sw / | | || (O)---o/o \ _ | | \ _ light \ [_] | _| [_] | /| | [_] | / | |_ | | | (O)---o/o | [_] | | | \ | spur | _| | | \ _ | [_] |_ | [_] | | [_] o/o____ | / _|_ | |\ _ \/ | | | | | \____[_] (O) | |___| | | spur \ | | | | \ Cooker \/_ (O)--o/o--(J) \ _ [_] /| [_] (O)--o/o-- | Socket / Radial Ring (O)--o/o-- Sockets Lighting
In a typical new town house wiring system, we have:
- L&N tails from the electricity meter to the CU
- A split CU, with 2 independantly switched halves. One half has RCD followed by MCBs, the other half has a main switch & MCBs.
- 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.
- 4mm^2 Equipotential bonding in bathroom, connecting all the main touchable metal objects together.
Systems may often have some of the following as well:
- High current mcb & cable supplying cooker (non-RCD side of CU)
- High current mcb & cable supplying shower (RCD side of CU)
- 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. (non-RCD side of CU)
- A high current mcb supplying storage heaters. (non RCD side of CU) Sometimes these are run from the main CU, and sometimes a separate white electricity meter supplies a separate CU for them.
The diagram's radial lighting circuit shows the 3 wiring options, which may be mixed at will:
- loop-in, 3rd light on diagram
- (J)unction box used to provide multiple feeds
The diagram is shown with 5A lighting fuse and 32A ring circuit MCB. Other options are also possible.
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 separate banks, one on each side. One bank uses a main isolating switch, the other has an RCD in place of the switch. Each side is switched separately.
The split load CU has the following advantages:
- Most wiring work can be carried out with the other half of the system switched on.
- Almost all faults can only take out the power to half 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.
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.
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 cicuit 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 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), and can provide more power and cover more floor area per circuit.
Sockets are on 32A ring circuits in most house installations. These use a ring or loop of cable, so that at the CU 2 cables are connected to the mcb instead of 1. A large 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.
2.5mm^2 cable is used for ring circuits. 4mm^2 is used when cable will be under insulation or is bunched with other cables.
For more information on rules and practices governing socket circuits, see another unwritten article on socket circuits.
20A 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 less cable on physically long narrow shaped circuits, but connection faults have greater consequences than with ring circuits. (Confusion over the relative safety of ring & radial circuits is widespread.)
20A radials use 4mm^2 cable.
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.
Light bulb failures can trip MCBs. Fuses have a significant advantage over MCBs for lighting circuits, as they rarely nuisance trip on bulb failure.
Lighting uses 1mm^2 or 1.5mm^2 cable. 1.5mm^2 is recommended, as it removes the restrictions that apply with 1mm^2, and the cost difference is trivial.
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.
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).
To achieve 2 way control the usual single switch is replaced with either of the following circuits:
2 way switching:
o------------------------------------o / ------O O------ / o------------------------------------o a switch cable run switch b
3 way switching:
o---------------o o----------------o / \ / ------O X O------ / \ / o---------------o o----------------o a switch cable crossover sw cable switch b
- a & b are the 2 connections that are used in place of the usual single switch
- the switches at each end are 1 pole 2 way switches
- Any number of crossover switches may be added to give 3 or more way switching.
- the cable runs are from one switch location to another
- 3 core & earth cable is usually used for 2 way switch circuits.
For more information on 2 way switching, see
Compulsory Low Energy Lighting
Installing some lighting fittings that are only compatible with low energy lamps is now compulsory. These are sometimes replaced by the householder once the system has been signed off.
For more information on compulsory low energy lighting, see
- CFL Lamps
- unwritten article
A lot of metal light fittings require earthing, but those marked with the double insulated square in square symbol 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.
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.
All bathroom electrics are supplied from the RCD side of the CU.
All major metal items in the bathroom are connected together using 4mm^2 green/yellow insulated wire. This is called equipotential bonding. The wire is connected to metal pipes using earth clamp connectors. The wire is connected to radiators using connectors. It is permitted to place equipotential bonding connections immediately outside the bathroom if necessary.
- Interior pipe earthing is done with EC14 earth clamps to BS951.
- Outdoor pipe earthing is done with EC15 earth clamps.
- ?earthing is done with EC16 earth clamps.
- Radiators are earthed with earth clamps.
- Earth wire is connected to earth rod with a BS7430 clamp.
Bathrooms are divided into 3 zones, with different rules for each zone.
- Light fittings in the bathroom in zone 1 must conform to IP__ or better.
- Light fittings in the bathroom in zone 2 must conform to IP__ or better.
- Light fittings in the bathroom in zone 3 must conform to IP__ or better.
Most bathrooms have no sockets. Sockets are permitted, but there are various extra rules that apply.
For more information on bathroom electrics, see unwritten article on Bathroom Electrics.
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
- Cable Sizes
- unwritten article on electric showers
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
- Rewiring Tips#Outdoor Security Lighting
- Dimmed PIR Lights
- unwritten article on outdoor lighting
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 earth fault can only kill half the power, so you will always have power.
For more information on RCDs & RCBOs, see
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: RCD (usually)
- Lighting: non-RCD
- Immersion heater: non-RCD
- Coooker: non-RCD
- All bathroom electrics: RCD
- Outdoor electrics: RCD
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 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 of little use.
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 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 are a cause of nuisance trips when powered from RCDs.
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.
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.
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.
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 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:
- 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.
- Minimum: none
- Recommended: 1 double socket for a short corridor, 2 or possibly more in a long one.
- 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.
- 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
- 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.
- 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.
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
- Red = Live
- Black = Neutral
- Bare or green/yellow = Earth
- Brown = Live
- Blue = Neutral
- Bare or green/yellow = Earth
For more information on cable colours, see Wiring colour codes