Handbook of Electrical Installation Practice - BeKnowledge

548 Handbook of Electrical Installation Practice
The fill-factor, FF 1758 ¥ 100/9448 = 18.6%.
For 110 ¥ 100 trunking C = 16.25W/m (Table 20.7).
Maximum permitted power for trunking = 16.25 ¥ 18.6 0.345 W/m.
= 44.59W/m.
The losses for circuits Nos 2, 7 and 9, marked (*), are too high. This could be put
right by increasing this conductor sizes for those circuits. However, as the total
power loss W t is well within the maximum permitted value W p it is feasible to try
the optimising procedure on these circuits.
Wt
35. 77
= = 0. 803
W 44. 54
p
M 2 = 395 . - 295 . ¥ 0803 . = 1. 58
For circuit Nos 2, 7 and 9, Wa ¥ M 2 is greater than W c, hence these sizes can be
retained.
The optimising procedure is based on experimental work which showed that a
limited number of circuits can dissipate a certain amount more than the average
UHG power, provided that the overall losses of the group are less than the permitted
maximum. It can be seen that, as the total power loss of the group increases
towards its maximum permitted value, the optimising factor M 2 decreases to
unity.
The maximum permissible value for M 2 is 2.4, irrespective of the ratio W t/W p.
The optimisation procedure is applicable only to groups in conduit or trunking.
Increasing the conductor sizes of circuit Nos 2, 7 and 9 would also provide a
thermally acceptable design, but the cost would be higher.
Comparison with sizes derived by an across the board application of a conventional
group rating factor, C g = 0.48, would show, as before, that savings can be
made on some of the larger conductors. Voltage-drop limitations tend to mask the
comparison for smaller conductors but, where a direct comparison is available, the
conventional group factor may lead to sizes which are thermally inadequate.
Groups on trays
The procedure, given in Fig. 20.3, is initially much the same as that for conduits and
trunking, with the width and depth of the group replacing the size of the enclosure
and the fill-factor. Because a group on a tray may be of such a width that heat losses
from the edges do not affect the temperature rise at the centre, the initial calculations
are therefore based on the assumption that there is no heat loss from the sides
of the group.
On the other hand, a shape factor Cs may be applied which corrects for the effect
of added heat loss from the edges of a group. This tends to compensate, at least in
part, for the rapid decrease in cable current-carrying capacity as the depth of a group
increases. Further comment on this is made when discussing the example.
A further difference is introduced, due to the fact that groups on trays and racks
are laid in an orderly manner so as to facilitate fixing with binders or clips. This
usually results in an approximately rectangular cross-section for the group and the