to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Cables, conduits and trunking
  4.1 - Cable insulation materials 4.4 - Cable supports, joints and terminations
  4.2 - Cables 4.5 - Cable enclosures
  4.3 - Cable choice 4.6 - Conductor and cable identification
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4.3.5 - Cable grouping correction factors


4.3.5 - Cable grouping correction factors

If a number of cables is installed together and each is carrying current, they will all warm up. Those which are on the outside of the group will be able to transmit heat outwards, but will be restricted in losing heat inwards towards other warm cables. Cables 'buried' in others near the centre of the group may find it impossible to shed heat at all, and will rise further in temperature {Fig 4.9}.

Fig 4.9 The need for the grouping correction factor Cg
a) widely spaced cables dissipate heat easily
b) A closely packed cable cannot easily dissipate heat and so its temperature rises

Because of this, cables installed in groups with others (for example, if enclosed in a conduit or trunking) are allowed to carry less current than similar cables clipped to, or lying on, a solid surface which can dissipate heat more easily. If surface mounted cables are touching the reduction in the current rating is, as would be expected, greater than if they are separated. {Figure 4.9} illustrates the difficulty of dissipating heat in a group of cables.

For example, if a certain cable has a basic current rating of 24 A and is installed in a trunking with six other circuits (note carefully, this is circuits and not cables), Cg has a value of 0.57 and the cable current rating becomes 24 x 0.57 or 13.7 A. The symbol Cg is used to represent the factor used for derating cables to allow for grouping. {Table 4.4} shows some of the more useful values of Cg.

The grouping factors are based on the assumption that all cables in a group are carrying rated current. If a cable is expected to carry no more than 30% of its grouped rated current, it can be ignored when calculating the group rating factor. For example, if there are four circuits in a group but one will be carrying less than 30% of its grouped rating, the group may be calculated on the basis of having only three circuits.

The grouping factor may also be applied to the determination of current ratings for cables as explained in {3.8}.

Table 4.4 - Correction factors for groups of cables
Number of circuits
Correction factor Cg
-
Enclosed or clipped
Clipped to non-metallic surface
-
-
Touching
Spaced*
2
0.80
0.85
0.94
3
0.70
0.79
0.90
4
0.65
0.75
0.90
5
0.60
0.73
0.90
6
0.57
0.72
0.90
7
0.54
0.72
0.90
8
0.52
0.71
0.90
9
0.50
0.70
0.90
10
0.48
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0.90
* ‘Spaced’ means a gap between cables at least equal to cable diameter.

 

 

 

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations