Handbook of Electrical Installation Practice - BeKnowledge

Table 20.6 Cable data for singlecore
unsheathed PVC cable in
conduit or trunking.
† Note: All values apply to one singlecore
PVC cable to BS 6004, at 70°C.
rectangular outline of the cables is the outline of the heat-dissipating surface of the
group.
Under these circumstances it is convenient to assume that each cable occupies a
square area with sides equal to the diameter of the cable. The total cross-sectional
area of the group is then equal to the sum of these squares. The depth, calculated
by dividing this area by the width, approximates to the visual depth.The unit system,
used to calculate the area occupied by each circuit, differs therefore from that
used for cables in conduit and trunking by being based on the square of the cable
diameter and not on p/4 times that value.
Example 3 Group of SWA cables on a perforated steel tray
Ambient temperature 25°C, C a = 1.
(Note: The reference ambient temperature for cables on trays is 25°C, this
being a more usual temperature for industrial installations. For any other ambient
temperature:
C
a =
Ê 70 - Ambˆ
Ë 45 ¯
Selection of Wiring Systems 549
Table 20.7 Trunking and conduit data
for groups of cables.