Both national and international standards for the conductor sizing of cables do not adequately take into account the additional heat load arising from harmonic currents. Some standards prescribe the maximum current values for four-conductor and five-conductor cables under the assumption that only two or three conductors are loaded. However, today’s harmonic situations may give rise to the fourth conductor (neutral) being fully loaded or even overloaded simultaneously with a balanced load on the three phase conductors. Other standards provide a general instruction that under a particular harmonic impact on the phase conductors, a certain additional load has to be taken into account for sizing the neutral conductor. However, the practitioner will usually not know how much harmonic impact arises from a particular load or group of loads.
In the following application note, an approach will be given to estimate the additional thermal impact due to harmonic currents in the LV power supply system of a building. Based on this estimation, it provides a methodology on how to dimension and select three-phase cables that are supposed to feed single-phase final circuits containing distorting loads.
The Highlights from this publication:
- The harmonic impact on the neutral current can be substantial. Cables have to be over-dimensioned accordingly.
- Assuming that all harmonics accumulate in the neutral results in an approach that falls on the safe side.
- It is good to keep this safety margin, because similar loads can have a very different harmonic behaviour.
- It is also good to provide some reserve for imbalance coming on top of the harmonics.