Why is residential wiring known as single phase?

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TimWA

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Every sparky knows he's working with 2 phases in a standard residential panel, and better not get them mixed up! If I have to be careful with tandem breakers, and not getting my multi-wire legs on the same phase etc., how does "single phase" apply?

Because they pull off opposite ends of the same utility transformer (ie. one winding)? Because they cancel out on the neutral with 240V? Or is it a misnomer. I've always been curious about this.
 
There are two ways to handle this.

For me I just accept 'because we do'

The other way takes 600 forum posts.


Good luck! :D
 
Haha, that "because we do" seems to come up discomfortingly often doesn't it! "Well, um...because it works that way!"
There's no relatively simple answer? Not to worry.
 
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Why is residential wiring known as single phase?

The transformer is single phase but center tapped to give each leg a phase shift of 180? to each of the 120v legs.
 
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TimWA said:
Every sparky knows he's working with 2 phases in a standard residential panel, and better not get them mixed up! If I have to be careful with tandem breakers, and not getting my multi-wire legs on the same phase etc., how does "single phase" apply?

Because they pull off opposite ends of the same utility transformer (ie. one winding)? Because they cancel out on the neutral with 240V? Or is it a misnomer. I've always been curious about this.
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If said sparky is working on a 120/240 volt single phase system and thinks he is working with 2 phases, he doesn't know his subject very well.

It's known as single phase because that is what it is. Just because there are two voltages, doesn't mean there are two phases.

Connect two car batteries in series. You will have both 12 and 24 volts. Does that make DC 'two phase'?

If you (or anyone) insists that 120/240 single phase is actually two phase, please, I would like to see both phases on an oscilloscope.
 
There is a true 2 phase system that is not the same as a 120/240 center tapped single winding.

Pretty much all single phase induction motors operate as 2 phase motors. They use capacitors or different values of inductance in windings to create a shift in phase from one winding to the other. They would not be able to develop rotation without a shift in phase.
 
This is similar to the situation when you have to correct a homeowner when he incorrectly calls a 3-way switch, a 2-way switch. I've explained the reason numerous times, but they just seem to always get glassy-eyed after I tell them why it's a 3-way switch.
 
ajohn@voyager.net said:
The transformer is single phase but center tapped to give each leg a phase shift of 180? to each of the 120v legs.
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If they were 180 off, the net sum of the two would be zero, not 240.

The two 120 volt legs are in phase and additive. They both reach + 120 volts at the same time. If they were 180 degrees shifted, one leg would be + 120 at the exact same time the other was - 120. If you plotted the three wave forms, you would see that the two 120 volt waves occupy the same space in a two dimensional plane as represented by an oscilloscope. If they were 180 off, both 120 volt waves would be visible, but mirror images of each other (which isn't the case).
 
K8MHZ said:
If they were 180 off, the net sum of the two would be zero, not 240.

The two 120 volt legs are in phase and additive. They both reach + 120 volts at the same time. If they were 180 degrees shifted, one leg would be + 120 at the exact same time the other was - 120. If you plotted the three wave forms, you would see that the two 120 volt waves occupy the same space in a two dimensional plane as represented by an oscilloscope. If they were 180 off, both 120 volt waves would be visible, but mirror images of each other (which isn't the case).
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If the two 120V legs are in phase and additive, how do multiwire circuits work? Isn't a two-hot-single-neutral permissable because the legs cancel (ie. not additive) on that neutral? If they cancel, does this not mean they are opposite phase? Just asking!
 
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TimWA said:
... If they cancel, does this not mean they are opposite phase?
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A 120/240V 1Ph 3W circuit can be created from a single 240V source which is cut in half (i.e. a center tap) or it can come from two 120V sources connected in series.

Almost all of the hundreds of other postings all deal with the use of the neutral as the reference point.
 
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K8MHZ said:
If they were 180 off, the net sum of the two would be zero, not 240.

The two 120 volt legs are in phase and additive. They both reach + 120 volts at the same time. If they were 180 degrees shifted, one leg would be + 120 at the exact same time the other was - 120. If you plotted the three wave forms, you would see that the two 120 volt waves occupy the same space in a two dimensional plane as represented by an oscilloscope. If they were 180 off, both 120 volt waves would be visible, but mirror images of each other (which isn't the case).
Click to expand...


Simple answer from another forum:

You said:
Quote:
"Let me assure you if you take an oscilloscope and measure the two 120volt legs you can see that they are 180 degrees out of phase and that they are two distinct phases."

For a residential panel this is very true. There is a reason that it is true. There is two different ways to achieve this. One is to center tap one phase. This will give you two sine waves that are 180 degrees out of sink. The other is to use two transformers.

Think about three people standing side by side. The one on the left is person (A) the one in the middle is person (N) and the one on the right is person (B).
Now person (A) walks 120 steps to the left and person (B) walks 120 steps to the right. Both (A) and (B) are 120 steps from person (N) but (A) is 240 steps from (B).

Now let?s look at a delta transformer. It is drawn as a triangle with three corners that can be labeled (A), (B) and (C) from (A) to (B) is one phase of the three phases of the delta transformer, thus when it is taped in the center it is 120/240 volt single phase. Only one phase is being used.

When we refer to two phase electrical we are referring to two phases that are 90 degrees out of phase to each other. These will read the same to ground but this will not be doubled when reading phase to phase as in the residential panel.

Let?s say that the polyphase circuit (two phase) is 120 volts to ground for each phase then the voltage between the two phases would be about 170 volts.
 
And this is two phase. Anyone who calls 120/240 single phase "two phase" needs to put down the screwdriver, and the lineman's, and pick up a book.
 

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TimWA said:
If the two 120V legs are in phase and additive, how do multiwire circuits work? Isn't a two-hot-single-neutral permissable because the legs cancel (ie. not additive) on that neutral? If they cancel, does this not mean they are opposite phase? Just asking!
Click to expand...

If they canceled the voltage would be 0, not 240 from hot to hot.

Again, think of two 12 volt car batteries connected in series. The voltage across the two open terminals will be 24 volts. The voltage from either open terminal to the center will be 12 volts. Just like the two 12 volt batteries combine to give us 24 volts, the two 120 volt transformer windings combine to give us 240 volts.

A single phase transformer can have many taps. Adding taps does not make phases appear. If we tapped between the center of a 120/240 single phase transformer and the two hot legs, we could then get more multiple voltages. We could have 60, 120, 180 and 240 volts, plus we could have 4 different 60 volt sources, three different 180 volt sources, two different 120 volt sources and a single 240 volt source, all from a single phase. And this tapping could continue forever and the system would still be single phase system.
 
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