Get Tech Tips
Subscribe to free tech tips.
Why is 240V called “Single-Phase”?
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 we called two 120v opposing phases “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 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-phase service 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.