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Why Is American 220v Not Considered 2 Phase?

Superburban

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#1
A little thing that has always been bugging me. Here we have 220 volts by using 2 110 volt legs that are 180 degrees apart. Ie, load on 2 legs.

Much of Europe has 220 volts, but only on one leg, the other is neutral. Like our 110V setup, but 220 volts.

3 phase has power on three wires, makes sense to call it 3 phase.

Single phase 110V here (or 220V in much of Europe), only has the load on one wire. Again, it makes sense to call in single phase.
Following all that, it seems that our 220 Volts (110 on each leg), should be called 2 phase.
 

Superburban

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#4
Ok, Thanks that makes sense. Is there any place that uses 2 phase?

Another similar question, how do they get the two legs in phase, when the power being generated is in 3 phase, and has the legs 120 degrees apart?

Thanks for the quick reply, and illustrations, they help.
 

Superburban

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#5
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.
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
 

Tony Wells

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#6
No, the primary feed to the transformer on the pole (at least mine) is 7200 VAC, single phase. There is one hot wire and a neutral run to it, with the neutral also being grounded by a copper wire running down the pole to a round plate on the bottom of the pole. So mos residential services, on my rural coop, run a simple 2 winding transformer, although I should mention that on the secondary, there are I believe 3 hot taps, so they can get the proper or closest voltage out on the secondary. This allows for some variation in the primary feed. It could be that one of the possible secondary taps runs at 260, one close to 240 (mine is close to 237), and one could be 190. So when they bring the service drop in, they make the best selection then.

If you look at the poles, some will have 3 hots on the top crossbar. That is all three phases. But in residential areas, normally you won't see all three, but often 2, so they have twice the capacity since they can use either one, whichever is loaded least, to add the next customer. But that represents 2 separate windings in a transformer upstream. So they are both full wave, single phase.
 

rwm

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#7
This came up before and someone said 2 phase was available in Philly?

R
 

Tony Wells

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#8
Yes, 2 phase is still in limited use. Wiki lists parts of Pensilvania as well as part of Connecticut as still using it. I think part of the confusion is that true 2 phase requires 4 conductors, 2 for each phase, whereas single phase is delivered with only 2, a hot and a neutral.

I think there is kind of a crossover in descriptive terms. When we have a center tapped transformer, we can properly call it "split phase", because half the winding provides "phase A" and the other half "phase B", so it's technically proper to call it "dual phase"or "split phase". Easy transition to "2 phase", but truly a little different animal. When you split a single phase, which contains the entire sine wave of a "phase", you only get either the voltage swing from zero to positive, with the peak of the sine wave at whatever voltage half the whole winding is providing, in our case here in the US, that's about 120VAC. Or, you could get the "other" half of the sine wave, which swings from zero to the "negative" peak, which reaches the same voltage as the other half. They are symmetrical. Then enter the time domain. There is a period on split single phase during which the voltage is zero. That's while the power swings to the other side of zero. So that pair of wires just waits while the other gets the other half of the sine wave. Think of zero being represented by the neutral wire. It's always at zero potential. Then all our comparisons reference that neutral as the mid-point of our entire phase cycle. So half the time each "hot" leg of our single phase system is at zero volts while the other leg gets the other peak and hence is hot. Then when you are using 240, and not using the neutral as a reference zero, there is only an infinitely small time when the voltage drops down the sine wave and crosses] the zero voltage line, but various factors make this "zero crossing" insignificant for most applications, like motors for instance. Then you properly measure the available voltage from "peak to peak" or "P-P". The reasons behind this are a little complicated, and I don't know enough about them to truly provide a good explanation.

Now, all the real electrical engineers can come in and clear up any details I have gotten wrong, or shoot the whole things down. If I have a gross misunderstanding of the subject, I hope they do. I'm sure they could at least add something, or maybe give a better explanation, but that's how I have it in my mind, anyway, for what that's worth .
 

CluelessNewB

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#9
Personally I think waveform diagrams like Jim Dawsons 240/120 although technically correct are maybe adding to the confusion. Typically 240 V motors have no connection to the neutral. Since the motor only has 2 connections if we were to arbitrarily choose one leg to call our zero volt reference, the waveform will look like a sine wave and except for the amplitude would be identical to what you would see on a 120V line. Voltage is relative and is always measured between two points, just like distance. To show a signal on a oscilloscope that looked like that diagram would require 3 connections (and an oscilloscope with multiple channels) because you would really be measuring 2 voltage signals referenced to neutral. Note that the motor itself only has 2 connections and only "sees" one voltage signal and that signal looks like a sine wave.

I have either made this clearer or totally muddied the waters, I'm not sure which :)
 

Keith Foor

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#10
Well, to further complicate matters I have heard US power referred to as Edison 2 phase.
Now that makes little to no sense since Edison HATED AC and wanted DC in everyone's house with a small generator in a barn out back.
Tesla and Westinghouse partnered to create the system we all know today in the US so how Edison got involved is anyone's guess and may be totally incorrect. But I have heard that said a couple times referring to the 2 leg 180 degree out of phase 120/240 60Hz system we currently enjoy.
 

Keith Foor

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#11
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.
 

Tony Wells

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#13
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.
 

CluelessNewB

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#14
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!
 

John Hasler

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#16
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.
 
Last edited:

abrace

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#17
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.
 

rwm

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#18
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
 

John Hasler

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#20
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.
 

Tony Wells

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#21
Yup, you guys are spot on. That's what I get for post at 1 in the morning. My apologies for any confusion.


Oh, and unless you have an isolation transformer, or your scope is so coupled internally, best not to probe the socket your scope is plugged into. Or use a battery operated portable scope. So I've heard.....never smoked a scope yet.
 

GA Gyro

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#22
Hmmm...
Letting the magic smoke out of a scope...
Would be a real mistake... :eek:
Much worse than letting the smoke out of a sawzall or a drill... :(
 

rwm

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#23
I hope ya'll know that was a joke about measuring the 7200 V lines!
Actually we live in a neighborhood built in 1920 and we have tons of overhead high voltage lines just waiting to break. Last year I had to call about a tree on fire from arcing. Fortunately the rain put it out 'cause they showed up the next day to check it out. Good ole Duke Power.
R
 

olduhfguy

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#24
It's easy to see that 3 wires could be considered 3-phase but it's a US problem. Most countries that provide only 240 volts have 2 service wires, but if they had 3 service wires it would most likely be 3 phase. In the US we couldn't decide whether we liked 120 or 240 volts so we decided to have both - we have one phase and 3 wires (120v to neutral, 240v between L1 and L2 - the third one is a neutral/ground). To confuse things further if you order 3 phase in the US you may get grounded delta 3 phase with one leg neutral/grounded, 3 phase delta with all phases above neutral, or 3 phase wye all phases above neutral.
 

GA Gyro

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#25
I hope ya'll know that was a joke about measuring the 7200 V lines!
Actually we live in a neighborhood built in 1920 and we have tons of overhead high voltage lines just waiting to break. Last year I had to call about a tree on fire from arcing. Fortunately the rain put it out 'cause they showed up the next day to check it out. Good ole Duke Power.
R
Yeah... Definitely not a good idea to climb the pole to measure voltage...
As I understand it... it is a federal offense (terrorism) for an un-authorized person to climb a utility poll...

About a year or so ago... they changed the Xformers in my neighborhood... was chatting with the line guys...
They said the current service 'was' 8700 volts, however it was gonna change to 12K volts.
There is 3 phase at the head of my subdivision on the boulevard... none in the neighborhood.
Surprising... one leg (one line) serves close to 100 homes.
 

Superburban

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#26
Ahh man, I really wanted to check out the high voltage lines. I downloaded the oscilloscope app for my I phone, and duct tapped my 20 foot aluminum ladder to the end of my 30 foot aluminum ladder. I figured with the light rain we are having, no one would see me climb up a,d check out the wave forms.

Ohh well, I'll just drink another beer instead.

:big grin:
 

Keith Foor

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#27
OK, some additional info on 3 and split phase. 208 3 phase is 208 and NOT 240 because the phases are not 180 degrees out of phase. Since the phases are 120 degrees out of phase the maximum leg to leg RMS voltage is 208. RMS is something else. Since we have now drug o-scopes into this discussion. The actual PEAK voltage to neutral is actually 169 volts and not 120. 120 is the RMS voltage that what was adopted to refer to it. Did you ever wonder why you needed to use 370 volt caps for a RPC? That would be it. the Peak voltage on 240 is actually 338 volts. So while a regular volt meter will show 240 a scope which shows peak voltage would indicate 338 volts.

More confusion
 

ddickey

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#28
OK, some additional info on 3 and split phase. 208 3 phase is 208 and NOT 240 because the phases are not 180 degrees out of phase. Since the phases are 120 degrees out of phase the maximum leg to leg RMS voltage is 208. RMS is something else. Since we have now drug o-scopes into this discussion. The actual PEAK voltage to neutral is actually 169 volts and not 120. 120 is the RMS voltage that what was adopted to refer to it. Did you ever wonder why you needed to use 370 volt caps for a RPC? That would be it. the Peak voltage on 240 is actually 338 volts. So while a regular volt meter will show 240 a scope which shows peak voltage would indicate 338 volts.

More confusion
Not quite, or maybe I don't understand what you're saying. 208 three phase will always be a wye connected secondary with 120v phase to ground. If the voltages are the difference of 1.73 (√3) they are always wye connected. 240/120 is always delta connected as it is 1/2 ratio. phase to phase is 240 and phase to ground is 120, it's a 4 wire system. Phase angles are 120° apart in all instances. RMS has nothing to do with 208v systems.
 

Eddyde

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#29
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.
Here in NYC its not uncommon for residential buildings to have 3 phase service, especially new or renovated buildings. The main reason is for elevators and air conditioning. However, most apartments will only have a single phase (two of the three phases) sub panel. 208v phase to phase 120v to the neutral.
 

John Hasler

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#30
Here in NYC its not uncommon for residential buildings to have 3 phase service, especially new or renovated buildings. The main reason is for elevators and air conditioning. However, most apartments will only have a single phase (two of the three phases) sub panel. 208v phase to phase 120v to the neutral.
By residential I meant single-family.
 
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