Which cable has better electrical properties, A multiplex cable (e.g., 4 runs of single core) or Multicore cable (e.g., 4 core cable with overall sheath)?
Be aware that the response to your question s not a trivial issue and there are large amount of cases and cable configurations to consider. For simplification, the following comparison is limited to a flat & trefoil cables configurations considering using the same shielding grounding, same insulation, and cable size, the multiplex cable performs worst terms of losses and ampacity. This comparison could be extended for similar configurations of multiplex or multicore cable arrangements as shown in the figure below.
The closer proximity and the skin effects in a multiplex cable make the heat-dissipation somehow less and increasing the cable operating temperature, therefore lowering the current-carrying capacity. Also, the electrical stresses are higher than separated single conductor cables.
If both 3 phase cables configuration is forced to carry the same current, the electrical losses per cable(I2R) for the multiplex configuration is also larger since the resistance increase with the temperature. However, the losses per phase in a flat configuration is not uniform since the center phase tend to experience larger losses than the outer phase. It should be noted that sheath losses in some cases could be greater than conductor losses particularly for shield grounded at both ends of the cable.
Considering the example in the Link, should be fair to say that the trefoil configuration provides lower cable reactance XL is lower (Link ) causing a larger charging VARs per cable as can be calculated per the following Link.
