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klik.system | Specification guide to prefabricated wiring systems
Current-carrying capacity
BS 8488 identifies that
prefabricated wiring systems
may employ flexible cables.
It should be noted that the
calculations related to
flexible cables require
particular attention.
The current-carrying
capacities tabulated in
appendix 4 of BS 7671 are
based on cables having
solid conductors (Class 1), or
stranded conductors (Class
2), except for Tables 4F1A
to 4F3B. Therefore, to obtain the correct current-carrying
capacity for cable types similar to those covered by Tables
4D1, 4D2, 4E1 and 4E2 but with flexible conductors (Class 5),
the tabulated values in 4D1 etc are multiplied by factors. The
factor for up to and including 16mm
2
is shown in table 1.
Cable Size
Current-carrying capacity factor
≤ 16.0mm
2
0.95
Table 1 Current-carrying capacity factor
Loaded core factor
An important design aspect identified by BS 8488 is the
loaded core factor, which is critical to the correct design of
say a multicore SWA cable used for home runs.
A loaded core is where the conductor carries more than 30%
of its rating, after applying the rating factor for the total
number of current carrying cores.
This was specifically addressed in amendment 1 to BS 7671:
2008 with the introduction of TABLE 4B5 – Rating factors for
cables having more than 4 loaded cores and the equation:
I zlc = I t2c x C
gN
. It is important to note, that for single phase
circuits, more than “
3
loaded cores” applies; regulations
523.6.1 and 523.6.2 refer.
This means that the current carrying capacity of the multicore
cable is derived by applying the rating factor for the number
of loaded cores CgN to the tabulated current-carrying
capacity of the equivalent 2-core cable. When the exact
number of loaded cores is not identified in table 4B5, the
next higher number of loaded cores is used.
Figure 5 - Prefabricated
wiring system using flexible
cable