I forgot about 300.3(B)(1) exception as well.
Direct current circuits are theoretically allowed to be isolated phase, neutral, and equipment ground in aboveground applications. This used to be an explicit exception but would now fall under 90.4 alternative methods.
Advantages of isolated phase, neutral, and equipment ground for underground installations is that it is easier to make paralled conductors the same length and conductors do not have to cross each other at terminations. If say 500 KCM copper wires are crossing each other at terminations a they are allowed to press against each other, insulation can cold flow and create a short circuit. Some people do not pay attention to that when running say 3/0 through LB fittings leading to premature failure. Usually takes 5 to 40 years to happen. Michael R.Cole
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