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.
You may prefer to call it split phase. The source is one phase of the 3 phase network. There is a center tap in the transformer which is a 0 Volt or Neutral reference point. If you take a meter and measure the voltage you will get this: L1 to N = 120V and L2 to N = 120V and L1 to L2 = 240V.
Edit: assuming you are talking about North America.
In the UK the 3 phase supply is transformed down to 415V per phase on the secondary side of the transformer. The secondary windings are in a star configuration and the star point is used for a neutral conductor. The measurement between any one phase and neutral is 220V and is hence known as a single phase.
>Why is 240V electric called “single phase” despite being **two 120V hots 180° out of phase**?
This is exactly why it’s called single-phase.
240V single-phase power comes from the power grid, and then it’s split in half at the transformer to give you two 120V single-phase buses. These buses can then be bonded together if you need 240V power.
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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