Handbook of Electrical Installation Practice - BeKnowledge

Table 20.20 Maximum length of circuits.
Selection of Wiring Systems 577
Voltage Length for 2.5%
Size drop voltage drop
Cable (mm 2 ) (mV/A/m) (m)
Single-core PVC in conduit 2.5 18 16
PVC/PVC two-core + earth, clipped direct 2.5 18 16
XLPE/SWA/PVC, clipped direct 1.5 31 9.3
Single-core 85°C rubber in conduit 1.5 31 9.3
Light duty MICC two-core PVC covered 1.5 28 10.3
Single-core thermosetting in conduit 1.5 31 9.3
cable will not be running at its maximum permitted temperature. The lower operating
temperature will reduce the voltage drop to the extent that all of the cables
will be suitable.
It is clear that the apparent advantage of the higher current-carrying capacities
of certain cables is lost under voltage drop considerations. However, voltage drop
is not subject to grouping or ambient temperature correction factors and for
groups of cables the advantages of higher temperature cables once more become
apparent.
Table 20.20 shows the maximum lengths of certain different cables which will give
a 2 1 /2% voltage drop on a 230V system with a 20A load, single-phase, selected on
their current-carrying capacities. Similar tables can be prepared from BS 7671 for
other types of load and 4% voltage drop.
It can be seen that voltage drop is in many cases more limiting than current rating
and that advantages gained in current rating are lost in voltage drop penalties.
Earth-loop impedance
For any voltage to earth, fuse and disconnecting time, there is a maximum earthloop
impedance. Some of that impedance is external to the circuit or installation
concerned; a typical value for a TN-C-S (PME) service from the public supply
network is 0.35 W.For a circuit at 230V to earth a 20A Type C mcb requires a current
of 200A to disconnect either socket outlet or fixed apparatus circuits so that the
maximum earth-loop impedance cannot exceed 1.15 W. If the external impedance is
0.35W then the total impedance of the phase and protective conductors in the circuit
must not exceed 0.8 W.Table 20.21 shows the length of various types of cable to give
a phase and circuit protective conductor resistance of 0.8 W (assuming a circuit load
of 20A). It is based on the resistance of conductors at their maximum permitted
operating temperature. Similar tables can be prepared for other types of protective
devices and ratings.