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This is a common misconception. US homes are not two phase, they are single phase 240 volts, called split phase because the single transformer leg is tapped in the middle to form two 120 volt lines, each 180 degrees out of phase with the other.

https://en.wikipedia.org/wiki/Split-phase_electric_power

Three phase circuits are 120 degrees out of phase.

But completely agreed that three phase 400V seems crazy!

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But isn’t it just “two phase”? I.e. how “true” two phase would be different?

Ok, help from our new common friend:

Yes, your statement is essentially correct. In the United States, residential electrical supply is typically a single-phase, 240-volt system that is "split" into two 120-volt legs. Each of these legs are 180 degrees out of phase with each other, which allows for both 120-volt and 240-volt circuits within the home. This arrangement is often referred to as "split-phase" electricity. It's not technically two-phase because a true two-phase system would have two waves offset by 90 degrees, not 180.

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