Handbook of Electrical Installation Practice - BeKnowledge

578 Handbook of Electrical Installation Practice
Table 20.21 Length of circuits to give a total impedance of 0.8 W.
Line +
protective Length for
Size conductor 0.8 W
Cable (mm 2 ) (W/m) (m)
Single-core PVC + equal sized protective 2.5 0.018 44
conductor
PVC/PVC two-core + earth 2.5 0.021 38
XLPE/SWA/PVC, clipped direct 1.5 0.027 30
Single-core 85°C rubber + equal sized protective 1.5 0.031 26
conductor
Light duty MICC two-core PVC covered 1.5 0.018 44
Single-core thermosetting + equal sized 2.5 0.019 42
protective conductor
INSTALLATION METHODS
Guidance is given in the IEE Guidance Note No. 1, Selection and erection, on
methods of installation including spacing of supports for conduit and trunking and
current-carrying capacities of cables housed in them.
Conduit and trunking capacities
The cable capacity of conduit is dependent not only on the percentage cross-section
of the conduit which is occupied by the cables, but on the number of cables which
can be pulled in without excessive force. The length between draw boxes and the
number of bends also affects the drawing-in tension required. Excessive force
damages the insulation, or in extreme cases stretches the cable and so reduces its
cross-sectional area. Hence to IEE Guidance Note provides two sets of cable and
conduit factors, one for short straight runs (less than 3m) and one for long runs or
runs incorporating bends; the total of the cable factors must be less than the conduit
factor.
For instance, 20mm conduit, with a factor of 460 for short straight runs, can
accommodate 16 ¥ 1.5mm2 PVC single cables. However, eight circuits must be
derated by a group factor to 0.51 times their full rated current, so that 6mm2 cables
are required to carry 20A. These need 25mm conduit. This is still more economical
than the alternative of 2.5mm2 cables in three separate 20mm diameter conduits. If
a 3m run with two bends of 20mm conduit is used, only ten 1.5mm2 cables can be
accommodated.