3.8.4 -  Protection of conductors in parallel

The most common application of cables in parallel is in ring final circuits for socket outlets, whose special requirements will he considered in {6.3.2}. Cables may otherwise be connected in parallel provided that they are of exactly the same type, run together throughout their length, have no branches and are expected to share the total circuit current in proportion to their cross-sectional area. It is not recommended that cables are connected in parallel at all except in ring final circuits..

Overload protection must then be provided for the sum of the current-carrying capacities of the cables. If, for example, two cables with individual current ratings of 13 A are connected in parallel, overload protection must be provided for 26 A.

Account must also be taken of a short circuit which does not affect all the cables: this is made less likely by the requirement that they should run close together. In {Fig 3.25}, for example, the 30 A cable with the short circuit to neutral must be able to carry more than half the short circuit current without damage until the protection opens.

Fault current sharing in these circumstances depends on the inverse of the ratio of the conductor resistance's. If, for example, the fault on one cable were to occur close to the connection to the protective device, almost all of the fault current would be carried by the short length from the protection to the fault.

In these circumstances there would be little protection for the faulty cable, and it would be prudent to provide protection with the installation of a suitable RCD.

Fig 3.25 Cables in parallel The lower resistance path will carry the higher fault current