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New lighting circuit cable colours (harmonised)

If the wiring in your house is newer than the 31st March 2004  you will have the following ‘new cable core colours’ (or harmonised) used in your lighting circuits:

Twin and earth cable

This is the standard format that will be used for the vast majority of your lighting circuit.

New colour twin and earth lighting circuit cable

New colour twin and earth lighting circuit cable

3 core and earth cable

If you have two way switching (e.g. landing lights with a switch upstairs and down) or intermediate switching the cable format below may be used.

New colour 3 core and earth lighting circuit cable

New colour 3 core and earth lighting circuit cable

If the wiring in your house is older than the 31st March 2004 and you have had some work done (e.g. an extension or partial rewire) you may have a combination of the old  and the new ‘harmonised’ colours shown above.

On the 31st March 2004 The IEE published Amendments No2. to BS 7671:2001 (the IEE Wiring Regulations) specifying new cable core colours for all new fixed wiring in UK electrical installations. These colours are called ‘harmonised’ colours as they are closely related to those used in mainland Europe.

NOTE: the bare earth wire in the cables above should be sheathed with a green/yellow earth sheathing wherever it is exposed e.g. in light switches, ceiling roses, junction boxes, fand use boxes.


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Old lighting circuit cable colours (unharmonised)

If the wiring in your house is older than the 31st March 2004 (and hasn’t been modified) you will have the following ‘old cable core colours’ used in your lighting circuits:

Twin and earth cable

This is the standard format that will be used for the vast majority of your lighting circuit.

Old colour twin and earth lighting circuit cable

Old colour twin and earth lighting circuit cable

3 core and earth cable

If you have two way switching (e.g. landing lights with a switch upstairs and down) or intermediate switching the cable format below may be used.

Old colour 3 core and earth lighting circuit cable

Old colour 3 core and earth lighting circuit cable

This is another variation of an older 3 core and earth cable

3 core and earth 0ld cable colour variation

3 core and earth 0ld cable colour variation

If the wiring in your house is older than the 31st March 2004 and you have had some work done (e.g. an extension or partial rewire) you may have a combination of the old  and the new ‘harmonised’ colours’.

On the 31st March 2004 The IEE published Amendments No2. to BS 7671:2001 (the IEE Wiring Regulations) specifying new cable core colours for all new fixed wiring in UK electrical installations. These colours are called ‘harmonised’ colours as they are closely related to those used in mainland Europe.

NOTE: the bare earth wire in the cables above should be sheathed with a green/yellow earth sheathing wherever it is exposed e.g. in light switches, ceiling roses, junction boxes, fand use boxes.


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Twin and earth cable current ratings

When considering cable ratings care needs to taken as this is not as straight forward as it may seem. For example, if you check the manufacturers current rating for 1.5 twin and earth cable (suitable for  most lighting circuits in a domestic environment) it will state that it is around 20 amps, but this value would only apply if the cable was in free air (as it can cool easily). If you put the same cable above a plasterboard ceiling covered by thermal insulation 100mm thick (17th edition regs reference method 100) its current carrying capacity is reduced to 16 amps (because it can no long cool as easily).

Consider the table below and the reference methods below that to see how the current carrying capacity is affected by installing the cable in different situations.

Conductor area (mm²)

Current carrying capacity (AMPS)
for various reference methods
(see below)

A

B

C

100

101

102

103

1

11.5

13

16

13

10.5

13

8

1.5

14.5

16.5

20

16

13

16

10

2.5

20

23

27

21

17

21

13.5

4

26

30

37

27

22

27

17.5

6

32

38

47

34

27

35

23.5

10

44

52

64

45

36

47

32

16

57

69

85

57

46

63

42.5

Examples of installation method

BS7671 17th Editon
Reference Method
Example of installation method
A In conduit in an insulated wall with the conduit close to or touching the inner skin.
B Enclosed in conduit or trunking on or in a wall.
C Clipped direct, or sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)
100 Above a plasterboard ceiling covered by thermal insulation, insulation thickness less than 100 mm.
101 Above a plasterboard ceiling covered by thermal insulation, insulation thickness greater than 100 mm.
102 In a stud wall with thermal insulation with the cable touching the inner wall surface.
103 In a stud wall with thermal insulation with the cable NOT touching the inner wall surface.

Cable sizing example

So lets say we wish to rewire a lighting circuit for the upstairs of our house. We have 3 bedrooms, a bathroom and a landing light. So lets assume we will be powering 5 100w lamps which gives us 500 watts in total.

We plan to use our standard 1.5mm twin and earth cable that the manufacturer rates at 20 amps, but the cables are run under thick (more than 100mm) insulation in the loft (installation method 101) so our cables current carrying capacity is reduced to 13 amps (from top table).

So if we now take our total wattage and divide it by the voltage of the circuit we will see how much current (amps) our lights will draw:

500(watts) / 240(Volts) = 2.08 amps

This is well within the 13 amp current carrying capacity of our cable so the 1.5 twin and earth cable will be fine for the job.

Note

This is quite a crude calculation, circuit designers will take additional factors into consideration when sizing cables, for example, in large installations the length of cable (and the resulting volt drop along it, due to its resistance) will need to be considered. Also my power calculation is not entirely accurate because we would need to consider the power factor of the circuit; inductive loads (like the chokes in fluorescent lamps and low voltage lighting transformers) have the effect of putting the current and the voltage in the circuit ‘out of phase’ which will affect the current drawn.

In summary, if you have an average size 3 -4 bedroom house with run-of-the-mill lamps and the odd fluorescent lamp, and after doing the calculation above you are well within the current carrying capacity of the cable (after allowing for the installation method as described) and you protect the circuit with a 6 amp circuit breaker you can’t go far wrong.

In a large dwelling (with potentially long cable runs) and a mix of exotic lighting with low voltage lighting transformers etc  the basic calculations above may not be sufficient and I would recommend consulting an expert.


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