(I'd edit my post, but its long since too late for that... mea culpa)
There's a couple of things being mixed up here. Stranded wire versus bar is a mechanical issue. But what you were talking about is the separators you sometimes see to run several separate cables alongside each other in the same phase. In this case, there are a couple of things being overcome. For starters, the skin depth of Al and Cu is only about 10mm at 50-60 Hz. However, the form of the limit is not the reactive power loss, its the resistive power loss of the conductor. Eventually, making it thicker just stops decreasing its resistance at that frequency.
The article hinted at this in the discussion on cable construction, when the author was describing wraps of aluminum conductor around a steel core. Now, imagine an A/B comparison, where the A side is a very large diameter cylinder of Al strands around a large steel core. The B side is three smaller such cylinders separated by air, with the same total circumference of Al. The B side is much lighter overall for the same current-carrying capacity. You might like to hollow out the A side, except that you need to endure the compressive stress of the outer sheath of Al conductors bearing down on the structural core somehow. Enter the periodically spaced triangular separators that actually are present on the B side.
There's a couple of things being mixed up here. Stranded wire versus bar is a mechanical issue. But what you were talking about is the separators you sometimes see to run several separate cables alongside each other in the same phase. In this case, there are a couple of things being overcome. For starters, the skin depth of Al and Cu is only about 10mm at 50-60 Hz. However, the form of the limit is not the reactive power loss, its the resistive power loss of the conductor. Eventually, making it thicker just stops decreasing its resistance at that frequency.
The article hinted at this in the discussion on cable construction, when the author was describing wraps of aluminum conductor around a steel core. Now, imagine an A/B comparison, where the A side is a very large diameter cylinder of Al strands around a large steel core. The B side is three smaller such cylinders separated by air, with the same total circumference of Al. The B side is much lighter overall for the same current-carrying capacity. You might like to hollow out the A side, except that you need to endure the compressive stress of the outer sheath of Al conductors bearing down on the structural core somehow. Enter the periodically spaced triangular separators that actually are present on the B side.