Notice that 240.21(C) does not give blanket permission to not protecting the secondary in a two-wire to two-wire transformer connection. The transformer ratio must be taken into account.
As far secondary grounding, if your choose not to ground the secondary 250.21(B) would come into play.
I think an argument can be made that 240.20(B)(1) would require you to use the mid-point as a ground, however, I have seen it ignored and a two-wire secondary only being used.
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