Why is 240V called “Single Phase”?

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This is a quick article from the archives that got a big response 4 months ago. I also just did a Facebook Live video this morning baring my soul on the topic of flowing nitrogen in response to an Email.


Why is it called single phase 240 when there are two opposing phases?

I wondered why two 120v opposing phases was called “Single phase 240” for years.

Then someone pointed out to me that a typical “single phase” pole transformer only has one power leg entering and two coming out.

This freaked me out. How can a transformer primary be one phase, a SINGLE sine wave and output two perfectly opposing sine phases?

It’s just two separate winding wraps in OPPOSITE directions on the secondary. Stupid simple, but I just never knew it.

So unlike a three phases services that uses all three power phases from the power supply, the single phase service only uses one. The second phase is “created” in the secondary of the distribution transformer itself and is the same “phase” but opposite.

Pretty cool.

— Bryan


  1. Avatar Gary L Reecher says:

    It’s single phase because the two voltages are 180 degrees out of phase with each other. At one time there was two phase voltage where the two voltages were 90 degrees out of phase with each other.

  2. Avatar Mark Kw says:

    “Phasing” requires having two sine waves occurring at some point other than 180° opposite each other. Single phase is such because the opposing sine waves are exactly 180° opposite each other, whereas “two phase” systems operate with the sine waves occurring at 90° apart and “three phase operates with the sine waves 120° apart. Phased systems allow motors to be self-starting, but single-phase motors require a start winding to create the electrical separation of 30° to facilitate starting rotation.

    1. Avatar Mark Kw says:

      I should have added that the starting phase shift on a capacitor-start single phase motor is 90°

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