Handbook of Electrical Installation Practice - BeKnowledge

544 Handbook of Electrical Installation Practice
Because they have to deal with the subject in a general way, these factors must
be derived on the assumption that all the cables in a group are of the same size and
are equally loaded. This situation does not often arise in practice, but it is usually
assumed that the application of such factors to a practical group of mixed sizes of
cables would be on the safe side. In fact, this is not true.
A simple example will illustrate the degree of overheating which can arise with
the smaller conductor sizes. Consider a group of mixed sizes of cable which contains
a small bundle of seven 1.5mm 2 cables and a 25mm 2 cable. The seven 1.5mm 2
cables occupy approximately the same space in the group as the 25mm 2 cable and
present about the same surface area for heat dissipation. However, if they are rated
using a common grouping factor, the 1.5mm 2 cables would have a temperature rise
of about 3.4 times that of the 25mm 2 cable. This would lead to the smaller cables
running hot. Overheating of smaller cables is accompanied by oversizing of the
larger ones.
Extensive work on this matter has resulted in the issue of several new ERA
reports in the 69–30 series, which provide ratings for cables in groups of mixed sizes
carrying different loads. These reports deal with cables made to BS 6004 and BS
7211 installed in trunking, and to BS 6346, BS 5467 and BS 6724 installed in multilayer
groups (groups where the cables are in contact with each other) on trays and
ladder racks.
It is obvious that, because the ratings for cables in such groups are specific to the
cable and load content of each group, rating factors and current-carrying capacities
cannot be tabulated in the usual way. In fact, a cable ceases to have a specific rating
under these circumstances and, dependent on the size of the group and the loading
of the other cables, its permissible load may be even greater than that for the cable
in isolation. The method of conductor size selection given in the ERA reports provides
for the adjustment of individual sizes (subject to other constraints such as
voltage drop) so that the most economical mix of sizes can be selected.
Overcurrent protection is also influenced by the thermal effect of such groups
and the ERA reports include formulae for checking that each conductor will meet
the requirements of BS 7671.
The ERA reports on methods for designing groups with mixed loads use the well
documented and proven uniform heat generation (UHG) principle. Design based
on UHG allocates correctly the permitted power losses between small and large
cables, generally provides a more economical combination of cable sizes and
may permit the addition of further circuits later without requiring a change in the
original circuits.
The following descriptions of applications to groups in conduit, trunking and on
a tray illustrate the method and its advantages. The procedures described are applicable
to circuits which must comply with given voltage-drop requirements. If this is
not the case reference should be made to the ERA reports.
Cables in conduit and trunking
The procedure starts by selecting conductor sizes to comply with voltage drop
requirements. These sizes are minima and the size of a conductor will be increased
only if it is thermally too small. An application of the UHG principle checks that