Handbook of Electrical Installation Practice - BeKnowledge

of the group. The calculated UHG rating may be increased by the factor Fw derived
from the following equation, where A is the aspect or width/depth ratio of the
group:
036 .
Fw = + 097 . , for A > 6
139 . A
Selection of Wiring Systems 553
Table 20.10 Types of cables and flexibles, UHG increases in conductor size.
Because the position of any cable in the group is not usually known, it is not
possible to take full advantage of the ‘spare capacity’ in a group where the total
losses are less than the total permitted losses. This is because all of the heavily
loaded cables may be concentrated at the centre of the group. However, a partial
advantage may be taken of such under utilisation of the total permitted losses.
The curves shown in Fig. 20.4 can be used to determine the factor by which the
UHG rating of any individual cable can be increased. This rating factor is a
function of the total area occupied by cables carrying more than their maximum
calculated UHG rating and the overall utilisation of the group. The utilisation
of the group, X, is the ratio of the total circuit losses over the total permitted
losses.
If the utilisation of the group is less than 80%, the rating factor for an 80%
utilisation should be used unless the position of the heavily loaded cables within the
group is known.
For the larger cables, which may be mounted on supports so that the cables are
spaced apart to permit better heat dissipation, information for calculating rating
factors is given in ERA Reports 74–27 and 74–28. Rating factors for certain specific
cases have been derived from such information and are given in BS 7671 and in