Simply put (without explaining how alternating current waves work) it is important to remember that nominal voltages are the average of the voltages sign wave. I believe typical max volts is around 169v for 120v system, but my math may be wrong on that as I have been out of school for quite some time.
Three phase is 3 electrical supplies that are 120 degrees apart from each other (in relation to their sign wave pattern) that results in an efficient system that allows 3 “hot” or supply conductors to feed circuits while only using 1 neutral. With the phases being 120 degrees out of phase with each other, it does not result in a voltage that doubles similar to single phase. An example is 1 conductor + neutral = 120v and 2 conductors = 208v
Single phase has 2 electrical supplies that are not out of degree with each other, which results in doubling the voltage when using two conductors. An example, 1 supply conductor + neutral = 120v and 2 supply conductors = 240v this example applies to North America.
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