Why Is American 220v Not Considered 2 Phase?

It's single phase because it comes off one winding in the transformer. We just center tap it to get the 120. 3 phase comes from 3 separate windings on the secondary of the transformer.


Well, it's generated as 3 phase at the power plant.
 
Yes it is, Keith, and the generators have 3 winding sets to do it. That's how we have 3 separate waveforms and there are 3 hots to the one neutral. The transmission transformers are generally separate, per phase, because of size. That way they are simpler and cheaper to build also.

When it get to the distribution stations and sub stations it's still 3 phase, all the way out into the fields, where it is divided up by selection of conductors needed and stepped down to the voltage needed. It still only take one hot and one neutral to provide a single secondary winding that is center tapped to give household 120/140. Pole pigs may well have more than one secondary winding if they are serving more than one business or residence. But even those transformers are fed generally with a single primary hot. In fact they could stack on secondaries until they run out of amperage, but they would always be single phase like the primary.

I could accept using the term split-phase for what we have, since the 120 outlets really only receive half-wave AC. That seems fair to me because it's half of the complete sine wave that we see on our 240. But I don't recall anyone calling it that.
 
I could accept using the term split-phase for what we have, since the 120 outlets really only receive half-wave AC. That seems fair to me because it's half of the complete sine wave that we see on our 240. But I don't recall anyone calling it that.

Actually if you were to put an *oscilloscope probe on a 120 outlet you would still see a full sine wave, the shape would not be changed, just it's amplitude (and peak-peak voltage) would be 1/2 the height of the 240V. I wish I still had an oscope available so I could take some pictures.

wave.gif

* Caution most oscilloscopes can't handle that high of an input voltage without using a 100x probe!
 
Actually if you were to put an *oscilloscope probe on a 120 outlet you would still see a full sine wave, the shape would not be changed, just it's amplitude (and peak-peak voltage) would be 1/2 the height of the 240V. I wish I still had an oscope available so I could take some pictures.
This is quite correct.

. ...........____ . . . _____________ L1
. ................... ) || (
. 7200 ......... ) || ( 120
. ................... ) || (______________NEUTRAL
. ................... ) || (
. ................... ) || ( 120
. .............___) || (_____________ L2


(The dots are to fight off HTML space compression)

You can think of the center-tapped secondary as two separate 120V windings connected in series. Each gives half the total voltage, not half the waveform.
 
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I could accept using the term split-phase for what we have

It is commonly referred to as 240V split single phase around here.

The multi wire branch circuit 'exploits' this fact, which is what allows us to have (2) 120V circuits share a single neutral provided they are on separate poles. I use that all the time. Want a pair of 20A circuits in the shop? You can run 12-3 to them and split off the hots when you get into the double gang box. One hot to one receptacle, the other to the other, and share the neutral. The neutral won't ever get overloaded as it only carries the difference in load between the 2 circuits. One receptacle pulling 5 amps, and the other is pulling 7, there is only a 2A load on the neutral.

This was a lot more commonly done before the arc fault invention.
 
I'm not buying that explanation. Someone go put a scope on the 7200 v line and see what the wave form is there.....
R
 
I get it now, so residential only gets one leg from the power station, and it is converted to two hot legs at the transformer. I always thought residential got two legs out of the three phase.

Thanks
That's actually one way to do it: you get 208 phase to phase and 120 phase to neutral. It isn't common in residental distribution but you will find it in some large buildings such as hospitals. In residential distribution they normally just connect the primary of your pole pig phase to neutral (with adjacent pole pigs going on different phases to balance the load). If you are out near the end of a branch like we are there may only be one phase up on the poles.
 
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