One hot conductor and a neutral is called single phase; that makes complete sense. It continues to make sense with three phase power where you have three separate hot conductors. Why does the logic fall apart when you have two hot conductors? And if there is a reason why those two hot conductors are called single phase, why is the first example of a single hot and a neutral also called single phase?
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Many of the answers here seem to depend on specific uses of terminology and subtle differences between them. But the way I heard the answer to this question is simply that when counting phases in AC power, it’s customary not to count phases that are just a flipped polarity from a phase that you’ve already counted. So two-phase power would have phases at 0° and 90° (and hence maybe 180° and 270°). Apparently that system is or has been in use somewhere at some point.
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