Why is it called single-phase 240 when there are two opposing phases (or legs)?

I wondered why we called two 120v opposing legs “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.

That freaked me out. How can a transformer primary be one phase, a SINGLE sine wave, and put out two perfectly opposing sine 120v legs?

It's just two separate winding wraps in OPPOSITE directions on the secondary, or more simply, the neutral is tapped in the center, so OBVIOUSLY they wrap in opposite directions from the center out. Stupid simple, but I just never knew it.

So, unlike a three-phase service that uses all three power phases from the power supply, the single-phase service only uses one. The second leg is “created” in the secondary of the distribution transformer itself and is the same “phase” but is split due to the center tap (more on center-tapped transformers in our short podcast episode).

Pretty cool.

—Bryan

P.S. — We also have an old podcast episode that breaks down the differences between single-phase, three-phase, and split-phase power. That may also be useful.