LT Distribution Cables for Non-linear Loads - 3.5 Core, 4 Core or 5 Core ?

This is an excerpt from a article by a veteran electrical consultant .

Conventionally we have been using 3.5 Core cables for LT distibution and it has worked fine for linear and balanced 3 phase systems. The neutral only carried the unbalance of current and would be lightly loaded . However, we live in Information age today with single phase electronic equipments dominating the load profile . These non-linear load bring in the unaccounted factor ' Harmonics' or more precisely ' Triplen Harmonics ' .

Non-linear loads like UPS, Computers, Electronic Ballasts, SMPS, Drives in industries produce third harmonic currents . Let us have a look at third harmonic currents 

As you can see, the third harmonic currents in three phases do not cancel out but add up to produce a very heavy 3rd Harmonic Current . In a star connected 3 Ph 4 Wire System , this current flows through the neutral . In such cases, neutral is made to carry currents equal or sometimes more than the phase currents . So we need a 4 Core or 5 Core Cable for LT distribution. Selection of 4 Core or 5 Core Cable must be decided based on Load Profile , PFC and Harmonic Filter so that neutral rating can be calculated precisely .

There are multiple cases where customers have been adding non-linear loads without considering ill-effects of Triplen Harmonics . Many problems like Transformer Failure, Fire Hazards and Equipment failure have been observed at such places . Thus wherever non-linear loads are prevalent, strengthening the neutral and changing Cables to 4 Core or 5 Core must be undertaken on priority .

Electrical Energy is like a wild beast . Only a skilful hunter can tame it . It is recommended to take guidance and help from a competent electrical consultant while executing any projects .