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
|
----------
|
0.90
|
|
* ‘Spaced’ means a gap
between cables at least equal to cable diameter.
|
