Cables

This article describes the main characteristics of most of the different types of cable that you are likely to encounter in a domestic installation, both those currently in use, and those you are likely to find in older installations.

Conductor Colours and Harmonisation

Historically the UK used its own set of colour codes for fixed and flexible wiring. In the early 1970s the colours used for flexible wiring were harmonised across Europe, however the colours for fixed wiring remained unchanged until recently. In March 2004 the fixed wiring colours were also harmonised.

During a transition period that began on 31st of March 2004 and ended on the 31st March 2006, use of either colour scheme was acceptable. Now only the harmonised colours may be used.

The result is that it is now possible to encounter installations that use both colour schemes, and hence great care must be taken to ensure conductors are correctly identified when carrying out any work.

For more details see Wiring colour codes

Conductor Colour Coding (single phase T&E)

Conductor	Old UK Colour	Harmonised Colour
Live (Phase)	Red	Brown
Neutral	Black	Blue
Earth or CPC	Green / Yellow Stripe	Green / Yellow Stripe

Conductor Colour Coding (three phase / Triple and Earth)

Conductor	Old UK Colour	Harmonised Colour
Live 1	Red	Brown
Live 2	Yellow	Black
Live 3	Blue	Grey
Neutral (if used)	Black	Blue
Earth or CPC	Green / Yellow Stripe	Green / Yellow Stripe

There is an IEE leaflet explaining these changes. That is available here

In theory, installations that are wired using cables to both colour schemes should carry a warning sticker (see example) on or near the consumer unit that states:

CAUTION This installation has wiring colours to two versions of BS7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified.

Modern Mains Cables

T&E

Twin & Earth cable. This is the most common cable used for domestic wiring today.

• L&N are individually insulated in PVC, the earth conductor is bare
• There is an overall sheath of grey or white PVC
• Not suitable for unprotected use outside (outer PVC insulator vulnerable to attack by UV radiation)
• 1mm² 1.5mm² & 2.5mm² have non-stranded conductors
• 4mm², 6mm², 10mm² & 16mm² have stranded conductors

Cable Sizes

Conductor area / CPC area (mm²)	Installation method (see table below)					Conductor dia (mm)	Outer PVC size (h x w)(mm)
	A	B	C	D	E
1 / 1	11.5	12	16	13	10.5	1 x 1.13	4.5 x 8.2
1.5 / 1	14.5	15	20	16	13	1 x 1.38	4.7 x 8.2
2.5 / 1.5	20	21	27	21	17	1 x 1.78	5.3 x 9.9
4 / 1.5	26	27	37	27	22	7 x 0.85	6.1 x 11.4
6 / 2.5	32	35	47	34	27	7 x 1.04	6.8 x 13.1
10 / 4	44	47	64	45	36	7 x 1.35	8.4 x 16.8
16 / 6	57	63	85	57	46	7x 1.71	9.6 x 19.5

Installation Methods

The current carrying capacity of any cable is dictated by its maximum conductor temperature, and this in turn will be affected by its ability to dissipate heat. The way it is installed can have a significant effect on this ability. The following table explains the methods cited above. Note that this is only a small subset of the 26 or so methods described in BS7671.

Installation Method	Description	BS7671 16th Edtn reference method
A	in conduit in insulated wall	method 6
B	directly in insulated wall	method 15
C	clipped direct, or sheathed cables embedded directly in masonry, brickwork, concrete, plaster or the like (other than thermally insulating materials)	method 1
D	above plasterboard ceiling covered by thermal insulation, insulation thickness <100 mm	n/a
E	as D but with insulation thickness >100 mm	n/a

• Ratings (BS 7671 Table 4D5A) apply for ambient temperature of 30 deg.C and conductor temperature of 70 deg.C

• Note that ratings for installation methods D & E are from the draft 17th edition and since this is not yet in force, should be regarded as provisional.

• All the usual derating factors apply (see below and appendix 4 of BS 7671 or appendix 6 of the OSG).

• Current ratings are continuous.

• Outer cable sizes are guideline figures and can vary for different brands of cable of the same conductor size.

T&E current carrying capacity

Note The current carrying capacity values given for the various cable sizes above are only correct for the fixing methods listed. They are also only correct for a single cable not in close proximity to any other cables. For all other installation methods not covered above, the values stated will be incorrect. To find the correct rating in these circumstances the quoted current carrying capacity listed in columns C (i.e. method 1) will need to be "de-rated". This means that correction factors need to be applied to the current carrying capacity in order to factor out these environmental effects that otherwise could result in cable overheating (and hence overloading) if ignored.

In addition to the fixing method, there are other correction factors that will effect the rating:

• Ambient Temperature
• Covering in thermal insulation
• Grouping cables together
• Use of semi enclosed (rewireable) fuses
• Encapsulation in conduit or trunking

For full details of the de-rating factors and more detailed version of current carrying capacity table, see table 4D1A in appendix 6 of the On Site Guide.

Typical T&E Cable Applications

Cable Size (mm²)	Typical Applications
1.0	lighting circuits
1.5	high power lighting circuits, 16A Radial power circuits
2.5	32A Ring final circuits, 20A Radial circuits
4.0	32A Radial circuits, low power electric shower
6.0	Small Sub mains, Radial circuits for showers, cookers, and other high power devices
10.0	Sub Mains, Radial circuits for high power showers, cookers, and other very high power devices
16.0	Sub Mains

3&E

Three core and earth. Has three insulated conductors and a bare earth conductor. In all other respects as T&E above. Typically used for two way lighting circuits, or other applications needing a permanent and switched live supply as well as neutral (e.g. feeding light switch activated extractor fans)

Singles

• PVC insulated single conductors (larger sizes are stranded as per T&E table above)
• Standard cable for use in conduit
• Typically used in domestic work only for main & equipotential bonding

Tails

Very heavy gauge singles with two layers of insulation. Typically used for connections between electricity incomer, meter, and consumer units. Comon sizes include 16, 25, and 35 mm²

LSF / LSH

Low Smoke & Fume, or Low Smoke and Halon. A cable designed to minimise the amount of toxic fumes released when burnt.

• Available in pretty purple

SWA

Steel wire armoured. A robust cable frequently used for exterior wiring, where it may be buried directly into the soil, or suspended from a catenary wire. It consists of a number of individually insulated conductors covered next with a flexible rubberised sheath, then by a spiral screen of steel wires, and finally by a tough outer sheath. The insulating materials used typically being PVC or XLPE thermosetting plastic (the latter having a higher temperature rating, hence allowing the cable to carry a larger maximum current for a given conductor size)

• For outdoor & garden use
• Available in 2, 3, & 4 core versions.
• Must be terminated using the correct glands.
• May use the steel wire armour for the circuit earth / CPC in place of or in addition to a core.

MICC

Mineral Insulated Copper Clad Cable. Renowned for having good fire resistance and also a smaller cross section for a given current carrying capacity than other cable types

• aka pyro (from makers name Pyrotenax)
• Copper tube sheath with magnesium oxide insulation
• Fireproof
• Rigid
• Occasionally seen in domestic premises, mainly in blocks of flats
• Widely used for fire alarm systems in commerce
• Unterminated ends prone to absorbing moisture from the air
• Special cable terminations required
• Ideal for flammability risk areas, eg traversing a thatched roof.

Hi Tuff

A PVC cable that is more robust than standard PVC sheathed cables and is suitable for general power, control and fixed wiring uses. An alternative to SWA for outdoor applications when direct burial of the cable is not required.

• Temporary wiring on open sites.
• Lighter and more manageable than steel wire armoured cables and requires only nylon glands to install - factors which contribute to speed of installation and cost savings.
• Provided there is no specific requirement for additional metal protection such as steel wire armouring, it will meet the demands of the most arduous installation conditions.
• Will not ignite under extreme applied heat.

Historic Mains Cables

Paper

• Paper insulation
• From the WW1 era
• Very rare now

PBJ

PolyButyl Jute insulated cable, with a rubberised Hessian appearance to the sheath.

• Commonly used for mains incomer insulation,
• Lots of old PBJ is still safely in service

Lead sheathed

• Common in 1930s for socket circuits
• Used as exterior farm cable well after that
• Lead sheath does not make good earth connections
• Rubber interior insulation

VIR

Vulcanised India Rubber insulated cable.

• Two layers of rubber insulation, one on each conductor, and an overall sheath.
• Typically does not includ an earth wire, which is usually run as a separate uninsulated single.
• The most common historic wiring
• Twisted pair cotton/rubber was also very common
• Rubber wiring uses thinner conductors than PVC, since the rubber is higher temperature rated
• Rubber insulation perishes, cracks & falls off
• Most VIR wiring is now in a very bad way, especially at termination points
• Properties still having VIR insulated cable are highly likely to be in urgent need of rewiring, and may represent a significant safety risk.

Aluminium

• Cheaper alternative to copper
• Used at one time until its risks were realised
• Aluminium cable creeps, oxidises & fractures.
• Fire risk
• Requires special connections, do not connect to old ali cable using connectors intended for copper.
• Al requires a larger conductor size than Cu for the same current rating

Copper clad aluminium

• An attempt to improve the properties of ali cable
• Significantly better than al, surface oxidation is eliminated, creep reduced & the cracking risk more or less eliminated

Imperial T&E

Older T&E wiring having similar construction to today's cables.

• 7/.029 T&E
  • imperial stranded version of 2.5mm² T&E
• Ashathene T&E
  • Precursor to PVC T&E
• PVC outer VIR inner
  • an early T&E cable
• 2 core T&E
  • no earth, used for lighting circuits, or power circuits with a separate (usually uninsulated) single run alongside to provide an earth.

Mains Flexes

• Round 3 core PVC
• Round 2 core PVC
• Oval 2 core PVC
• Rubber & cloth insulated - higher temp rated flex for irons & heaters

Voltage Drop

Any design must ensure the maximum voltage drop allowed between source and point of use is not exceeded when at full load. This is usually defined as 4% of the nominal supply voltage (about 9.2V at 230V AC).

Conductor CSA (mm²)	PVC (max 70° C) Voltage drop mV/A/m	XLPE (max 90° C) Voltage drop mV/A/m
1.0	44	46
1.5	29	31
2.5	18	19
4.0	11	12
6	7.3	7.9
10	4.4	4.7
16	2.8	2.9

These figures are presented for SWA cable here. The values quoted will usually also be correct for other copper cable types. Figures will differ for cables with higher operating temp limits, such as MICC.

These figures assume that the cable is operating at or near its maximum operating temperature. In designs where the maximum current load is significantly less that the capacity of the cable then these figures will be somewhat pessimistic. For these circumstance, it would be more appropriate to calculate voltage drops based on table 9A of the On Site Guide.

Calculation Examples (PVC SWA):

1) 20m of 4mm², maximum load of 30A would drop 20 x 0.011 x 30 = 6.6V
2) 40m of 6mm², maximum load of 45A would drop 40 x 0.0073 x 45 = 13.14V
3) 10m of 1.5mm², maximum load of 16A would drop 10 x 0.029 x 16 = 4.64V

(1) and (3) are adequately specified with respect to voltage drop. 
However (2) is out of  spec and a larger cable will need to be selected, 
even though the current handling capacity of the 6mm² cable has not been exceeded. 
Upgrading to 10mm², gives a result of 40 x 0.0044 x 45 = 7.92V which is acceptable. 
It initially appears that this still only leaves just over 1V of remaining drop 
available for the outbuilding wiring, however since we will not be operating the 
larger cable anywhere near its maximum temperature, the situation is actually less
tight than the calculation suggests.

See Also

• Wiki Contents
• Wiki Subject Categories
• Cable Sizes
• Wiring colour codes
• Cable Resistances