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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When we talk about the phase of an electric current, we’re referring to the timing of the voltage waveform. In a single-phase electrical system, the voltage waveform has a single sinusoidal shape. In other words, the voltage changes in a regular and predictable way over time, moving back and forth between positive and negative values.
In a 240V single-phase system, there are two 120V “hots” that are 180° out of phase with each other. This means that when one hot is at its maximum positive value, the other is at its maximum negative value, and vice versa. This allows the system to provide a consistent 240V of power, even though the individual hots are only 120V each.
“single phase” is because the voltage waveform only has a single shape.
A three-phase electrical system has three voltage waveforms that are offset from each other by 120°. This allows for even smoother and more efficient power delivery, but it also requires more complex equipment to generate and distribute the electricity. So, even though a 240V single-phase system uses two hots, it is still considered to be a single-phase system because the voltage waveform only has one shape.
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