Cable Sizes & current carrying capacity

Cable Sizes & current carrying capacity (stranded copper conductors) BS 5467 XLPE Insulation

Conductor area (mm²)

Current carrying capacity (A)
for cables “clipped direct”

Overall diameter (mm)

2 core
70°C PVC

3 or 4 core
70°C PVC

2 core
90°C XLPE

3 or 4 core
90°C XLPE

2 core

3 core

4 core

1.5

21

18

27

23

12.1

12.8

13.3

2.5

28

25

36

31

13.6

14.1

15.0

4

38

33

49

42

14.7

15.3

16.4

6

49

42

62

53

15.9

16.6

18.7

10

67

58

85

73

18.0

19.5

21.1

16

89

77

110

94

20.4

21.6

23.4

 

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.

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 / SWA)

Conductor

Old UK Colour

Harmonised Colour

2 Core
(T&E or SWA)

3 Core
(3&E or SWA)

4 Core
(SWA)

5 Core
(SWA)

Live 1

Red

Brown

X

X

X

X

Live 2

Yellow

Black

X

X

X

Live 3

Blue

Grey

X

X

X

Neutral

Black

Blue

X

X

X

Earth (CPC)

Green / Yellow Stripe

Green / Yellow Stripe

X[1]

[1][2]

[2]

X

 
Notes

  1. Earth wire in cable will be a bare conductor and will need green/yellow sleeving to be applied at point of termination.
  2. Earth wires are present in T&E or 3&E cables. Other cable types such as SWA or MICC will still need to be earthed, however this will be carried using the cable armour / screen.

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

Installations that are wired using cables to both colour schemes should carry a warning sticker (see example) on or near the consumer unit that statesand looks something like this:

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.

 

Voltage Drop

Any design must ensure the maximum voltage drop allowed between source and point of use is not exceeded when at full load. In the 16th edition this as 4% of the nominal supply voltage (about 9.2V at 230V AC). The 17th edition (in force from 1 July 2008) limits this to:

  • 3% for lighting
  • 5% drop for other loads
  • (but 6% and 8% respectively are permitted for private supplies)

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 slightly for cables with higher operating temp limits, such as MICC. The length figure above is per length of cable, not per each conductor.

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 official I.E.E. “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 any following 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.

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.

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 / SWA)

Conductor

Old UK Colour

Harmonised Colour

2 Core

(T&E or SWA)

3 Core

(3&E or SWA)

4 Core

(SWA)

5 Core

(SWA)

Live 1

Red

Brown

X

X

X

X

Live 2

Yellow

Black

X

X

X

Live 3

Blue

Grey

X

X

X

Neutral

Black

Blue

X

X

X

Earth (CPC)

Green / Yellow Stripe

Green / Yellow Stripe

X[1]

[1][2]

[2]

X

 
Notes

  1. Earth wire in cable will be a bare conductor and will need green/yellow sleeving to be applied at point of termination.
  2. Earth wires are present in T&E or 3&E cables. Other cable types such as SWA or MICC will still need to be earthed, however this will be carried using the cable armour / screen.

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

Installations that are wired using cables to both colour schemes should carry a warning sticker (see example) on or near the consumer unit that statesand looks something like this:

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.

 

Single & earth

  • Like T&E but with no neutral conductor.
  • Not a popular cable.
  • Supplier

Usually used for lighting circuits, the wiring works like this:
The permanent lives and switched lives of the circuit use the single core and earth cable (6241Y if you want to google it).

This run starts from the MCB and loops between the lightswitches to provide a permanent live and earth to the lightswitchs. Another length of 6241Y is then used from the lightswitch to the light fitting to provide a switched live and earth at the lightfitting.

The neutral cable is a double sheathed cable (6181Y with a blue inner sheath) that runs from the CU neutral busbar and from lightfitting to light fitting (there will only be one neutral at the end of the circuit).


It makes it easier to put light fittings up as there are less cables to mess with at the fitting.

Concentric cables

Neutral and earth conductors are arranged distributed around the central live conductors. Mainly used for incomers.

FP 200 and other Soft Skin Fire Alarm Cables

Introduced as an alternative to MICC cable, but not as robust (the sheathing will burn away but circuit integrity is maintained).

  • Silicone compound insulation
  • Overlapping internal metal screen and drain wire (CPC)
  • Low smoke sheathing material

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.

Mineral Insulated Copper Clad Cable(MICC)

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.
  • Picture shows 1.5 mm² MICC with outer diameter 7.2 mm

MICC is one of the toughest of cables because:

  • its metal cased
  • it handles very high temperatures
  • it still works even if you hammer it quite flat – the conductors and mineral insulation all compress in the same proportion, and you have to go a long way before it shorts out

Steele Wire Armoured (SWA)

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 bedding, then by a spiral screen of galvanised 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). PVC Sheathed version if to BS 6346, XLPE-sheathed version is to BS 5467, and the low smoke version is to BS 6724.

  • For outdoor & garden use.
  • Available in 2, 3, 4, & 5 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.

Cable Sizes & current carrying capacity (stranded copper conductors) BS 5467 XLPE Insulation

Conductor area (mm²)

Current carrying capacity (A)
for cables “clipped direct”

Overall diameter (mm)

2 core
70°C PVC

3 or 4 core
70°C PVC

2 core
90°C XLPE

3 or 4 core
90°C XLPE

2 core

3 core

4 core

1.5

21

18

27

23

12.1

12.8

13.3

2.5

28

25

36

31

13.6

14.1

15.0

4

38

33

49

42

14.7

15.3

16.4

6

49

42

62

53

15.9

16.6

18.7

10

67

58

85

73

18.0

19.5

21.1

16

89

77

110

94

20.4

21.6

23.4

 

LSF / LSH / LSZH

Low Smoke & Fume, Low Smoke & Halon, or Low Smoke Zero Halogen.

Most network cables are insulated with polyethylene, PVC or Thermoplastic Urethane (TPU) . In a fire, a halogen-containing plastic material releases a poisonous gas, e.g. hydrogen chloride, that forms hydrochloric acid when it comes in contact with water. Designated Halogen-free cables, on the other hand, do not produce a dangerous gas/acid combination or toxic smoke when exposed to flame.

Low smoke zero halogen cable reduces the amount of toxic and corrosive gas emitted during combustion. This type of material is typically used in poorly ventilated areas such as aircraft or rail cars. Low smoke zero halogen is becoming very popular and, in some cases, a requirement where the protection of people and equipment from toxic and corrosive gas is critical.

Other benefits of halogen free cable include:

  • It is often lighter, so overall cable network system weights can be reduced.
  • The environmental impact of halogen free cabling can be lower if there are fewer toxic chemicals.
  • Low smoke versions of commonly available PVC cables are readily available.
    e.g. BS 7211 for the PVC-sheathed BS 6004, and BS 6724 for the PVC-sheathed BS 5467.