# explain 120/208v



## guitarboyled

Hi everyone
Thanks in advance for your help

If I understand things correctly 120/208v implies the _Line voltage_ (voltage measured between any two line conductors) is 208 volt and the phase voltage (voltage measured between a line an the neutral) is 120 volt.

Where does this 208v come from? How does 120v lines add up to make 208v?

Are they any books out there that you would recommend?


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## william1978

................


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## william1978

Welcome to the forum.


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## wildleg

http://www.electricalknowledge.com/forum/forum.asp?FORUM_ID=11


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## nick

guitarboyled said:


> Hi everyone
> Thanks in advance for your help
> 
> If I understand things correctly 120/208v implies the _Line voltage_ (voltage measured between any two line conductors) is 208 volt and the phase voltage (voltage measured between a line an the neutral) is 120 volt.
> 
> Where does this 208v come from? How does 120v lines add up to make 208v?
> 
> Are they any books out there that you would recommend?



Well its time rotation of sine wave and the wye connection kinda if A phase is going thur its peak cycle B & C are at different levels in voltage . 

Meaning there not at peak voltage in there cycle moments in time .

the difference is the measured 208 volts between these phases A B / B C / C A .

The wye common point connection makes this a neutral point three phase cycle has 120 moments in any second in time when a sine wave is crossing zero volts in the cycle of the sine wave each phase is at a point the voltage will not be 240 volts but 208 volts on a wye transformer . basically there not at the same peaks or points in time so its less voltage between phases . Take care


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## william1978

Well said Nick.


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## Richard Rowe

Very well said, explainations like that is why I joined this forum!


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## guitarboyled

Could this graph I made explain it?

Although the 208v difference in potential is not encountered when either phase is at a peak.

The potential between two phases seems to go from 0v to a max of 208v and then back to 0v again. Could this explain where the 208v comes from?

Measuring potential between blue and yellow phase:

∆ = 0v at point A and D 
∆ = 208v (104v + 104v) at point C

My next question is: Do I have 240v total potential in position B (if all 3 phases are used)?


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## drsparky

Bingo


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## running dummy

phase to phase voltage is found by taking the single phase voltage and multiplying it by 1.73.

120V phase A-120v Phase B= 120 X 1.73= 207.6

If you really want to know where 1.73 came from I can find it, I really just remembered the "magic" number.

EDIT: this would obviously be for a wye connection


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## running dummy

this is also how you get the 480V with 277V phases.

277V (277phase A- 277V phase B) X 1.73= 479.21

Like I said before, I kind of lost the explanation of this number over the years but 1.73 as a multiplier always stuck with me.


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## guitarboyled

The 1.73 comes from the square root of 3. That must some how relate to the graph. 

The next thing about 3 phase current I would like to figure out is if 208v is available for a 3 phase motor or 240v (position B of the graph: blue line 120v, yellow line 60v, red line 60v)


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## nick

guitarboyled said:


> The 1.73 comes from the square root of 3. That must some how relate to the graph.
> 
> The next thing about 3 phase current I would like to figure out is if 208v is available for a 3 phase motor or 240v (position B of the graph: blue line 120v, yellow line 60v, red line 60v)


Well look at this http://www.windstuffnow.com/main/3_phase_basics.htm


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## guitarboyled

Thanks great site

I think I also need to improve my basic alternator and motor knowledge.

Any recommended reading?


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## guitarboyled

I understand the fact that in a single phase unit the power falls to zero three times during each cycle. E, F and G in the following graph. Power oscillates constantly between 0v and 120v











But the web site also claims that in a 3 phase unit the power delivered to the load is the same at any instant. Is it really? Doesn’t the power oscillate constantly between 208v and 240v as illustrated in the previous graph?


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## raider1

The peak voltage is not 120 volts. 120 volts is the effective (RMS) voltage. The peak voltage is 120 volts multiplied by 1.414. So from 0 to the peak, the voltage would be 170 volts.

Chris


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## Toronto Sparky

I'm surprised at the number of licensed guys that don't understand 3 phase systems.. 
However even more surprised at those who can't get 3way and 4ways switches to work.


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## guitarboyled

I'm not an electrician; I'm a HVAC technician who just graduated. Our training was pretty good but I want to learn more about electricity. 

Where does the 1.414 come from?

Just trying to understand the science behind it all.


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## alimaj

Hi just briefly ac voltages are vector quantities , their calculations include both magnitude and direction. 1.732 is the sq. root of 3; 1.414 is the sq. root of 2.


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## guitarboyled

I came across those values a few times but where do they come from?


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## nick

Well If a lamp was connected first to a dc voltage and then to a ac voltage the lamp will light up more brilliantly when connected to dc .This is do to dc voltage its constant it remains at lets say 100 voltages continuously whereas the ac voltage reaches 100 volts peak only at two points during the cycle .
In order for the lamp to light with equal brilliance on ac as well as dc we must raise the ac voltage to 141 peak volts . 
Effectively then 141 peak volts of ac will light up a lamp as does 100 volts of dc . Thats were they came up with the formula E peak = 1.41 x Eeff 


EFFECTIVE VALUE of an ac wave can be calculated from the peak value by the following formula Eeff= 0.707 x E peak 


For all sine waves whether voltage or current the value given to an ac wave will always be the effective value unless stated otherwise .
AC voltmeters and ammeters will always read the effective value of the ac wave unless it is indicated otherwise .





Take care


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## Buddha In Babylon

To expound on what nick is explaining so brilliantly...
Effective voltage is also known as RMS which is an acronym for RootMeanSquare (which nick also mentions)....which is a laborious calculation of voltage readings at every angle of rotation in a sine wive...all 360 instantaneous values....The Root squared of the averages will give you effective voltage.... or is it the average root squared??? or the squared average root??? i dont know...its a complicated calculation but it works out, that 1.414 times rms will give you peak voltage...and peak voltage times .707 will give you rms... that is WHERE those values come from. RMS


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## nick

Well yes and yes RMS is what is read and its the effective value . 

The meter takes many readings it averages these many readings to calculate the true R M S value . Take care


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## guitarboyled

According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipating the same power (heat) by an equivalent resistance. Your example explains it better.

But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).


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## nick

guitarboyled said:


> According to wikipedia, the efficient value (also known as RMS) of an alternate current equals the value of a direct (constant) current dissipatingthe same power (heat) by an equivalent resistance. Your example explains it better.
> 
> But when do I need to know this value, when is important to know for instance that 141v AC is equivalent to 100v DC? And what is the relation of this value with the nominal current provided by my service company. I know for instance they try to keep the residential current between 106v and 127v (for 120v nominal).


Well you dont need to know unless you just want to know your meter does that for you .
The same value is current or voltage they are both treated as the same in the same circuit .
power companys voltage changes hundreds of different values during the normal day up or down .

Heres how i know some times we install a voltage current monitor kit day/week logger on a breaker or switchboard to monitor its resent problem or history like lots of tripping.

Our service dept will monitor a service and look at the results of the week to check out stuff the voltage changes on a paper graph of record voltages can vary from 480 volts down to say 450 then back up to 495 this is the power company loading the lines or switching the lines or just plain old generator generating .

Its lots of different voltage changes up or down my personal home service is coming in at 126 volts and 245 volts single phase its my poco supply .

Take care


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## John

John said:


> Try this.....:thumbsup:
> 
> Electricity is actually just simple geometry.
> 
> Here is how to explain a 277/480 VAC, 3 phase electrical system visually using geometry.
> 
> Using the scale of .1” equals 1 volt
> 
> On a floor draw a circle with a radius of 27.7” (27 ¾”) that is equal to 277 volts and mark the center of the circle with an “X”
> 
> View attachment 1473
> 
> 
> 
> For this next step a magnetic compass or a drawing protractor and a straight edge is required.
> 
> View attachment 1476
> 
> View attachment 1477
> 
> 
> 
> Draw a line from the center ”X” of the circle to the outside line of the circle. Then draw another line 120 degree from the first line from the center “X” of the circle to the outside line of the circle. Then draw another line 240 degrees from the first line from the center “X” of the circle to the outside line of circle. Label the lines A, B & C where they intersect the outside circle. Now it should look like this.
> 
> View attachment 1474
> 
> 
> Now measure the distance from “A” to “C”
> 
> View attachment 1475
> 
> 
> The measurement should be about 48” (converted to scale would equal 480 volts) depending on how accurate you draw and measure. The measurements from “A” to “B” and “B” to “C” should be about the same.
> 
> There are more complex mathematical equations but this is the simplest and a neat thing to do for someone new to the trade.
> 
> Carry On!


For those of you that haven't been around too long this is a rerun of the best of John. :thumbsup:

And this will work for most of all 3 phase voltages. :whistling2:


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## Buddha In Babylon

hahaha the best of john...
yeah but i did get a kick out of that. never seen it before.


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## John

Buddha In Babylon said:


> hahaha the best of john...
> yeah but i did get a kick out of that. never seen it before.


Glad you liked it...but can you do it?

I did on lunch break on a large jobsite one time and most of the electricians on the job didn't understand it....:no:


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## guitarboyled

Great explanations guys thanks, I’m getting a pretty good idea of the relation between the line current and the phase current.

120 / 208
277 / 480
347 / 600

But to get back to what Raider1 said, is the peak voltage really 170v in a 120v nominal supply line? If my service provider makes sure the voltage is never higher than 127v how can it reach 170v?

Wouldn’t be the other way around, the peak value supplied is 120v therefore :

Efficient Value (RMS) is only 85v = 120v/1.414 or 120v x 0.707


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## alimaj

Hi guitarboyled I would suggest you re read nick's posts they have the info you want. But just to rspond to your last post which was answered by nick in a previous post, you have NO CONTROL what the peak voltage is , the peak voltage is RMS *1.414 SO WHAT EVER IS THE RMS the peak voltage would be 1.414 times higher. If you rectify an AC voltage and connect a capacitor the capacitor voltage would rise to approx. the peak voltage.


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## guitarboyled

I understand that I have no control over the peak voltage but are you saying that when the supply company guarantees the voltage will always be between 106v and 127v, they are talking about Effective Value (RMS) and not Peak value?

That would mean that the peak can go up to 179v (127v x 1.41).


If this is the case, when do we use the peak value?


Therefore I also presume that all motor voltage are in RMS


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## alimaj

As stated in other posts AC voltages and current are RMS values unless stated otherwise, your multitester measures RMS values. Voltages going to motors are RMS values.


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## raider1

> If this is the case, when do we use the peak value?


Peak value is not used in most applications, it is more a theroretical principal than a usable measurement.

The utility will supply you with a nominal 120/240 volt system for most residential systems. This voltage is expressed as the effective (RMS) voltage and not the peak.

Chris


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## guitarboyled

Thanks a million guys, sorry for being slow at grasping these concepts. We almost did no electrical at school, reason why I’m here acquiring as much knowledge as I can.

If there is any good book to learn basic electrical please feel free to recommend it.

I’m still not sure about the web site claiming that in a 3 phase unit the power delivered to the load is the same at any instant. Is it really? Doesn’t the RMS voltage oscillate constantly between 208v (position C) and 240v (position B) as illustrated in the previous graph? Or is that not how it works.


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## drsparky

You can read peak value with an oscilloscope, and then convert it to RMS. I don't know many who need to do it, but I have found problems with the AC power buy looking at the sign wave.


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## guitarboyled

For instance, when I need to dimension or size conductor (wiring), breakers, power consumption, etc. do I simply use the 208v value?


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## raider1

guitarboyled said:


> For instance, when I need to dimension or size conductor (wiring), breakers, power consumption, etc. do I simply use the 208v value?


Yes, when doing calculations for loads you would use the nominal voltage.

Here are a couple of NEC definitions and sections dealing with voltages.



> Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.





> Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.





> Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.





> 110.4 Voltages.
> Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.





> 220.5 Calculations.
> (A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
> (B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.


Chris


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## guitarboyled

My next question concerns the Delta 3 phase configuration 

In the Y (Wye or star) configuration:

Line Current = Phase Current
Line Voltage = Phase Voltage x √3

120 / 208
277 / 480
347 / 600

But in the Delta configuation:
 
Line Current = Phase Current x √3
Line Voltage = Phase Voltage

So why do we use the 120/240 nomenclature? Since there is no neutral in a delta configuration how do you go from 120 to 240?


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## nick

guitarboyled said:


> My next question concerns the Delta 3 phase configuration
> 
> In the Y (Wye or star) configuration:
> 
> Line Current = Phase Current
> Line Voltage = Phase Voltage x √3
> 
> 120 / 208
> 277 / 480
> 347 / 600
> 
> But in the Delta configuation:
> 
> Line Current = Phase Current x √3
> Line Voltage = Phase Voltage
> 
> So why do we use the 120/240 nomenclature? Since there is no neutral in a delta configuration how do you go from 120 to 240?


Well check this there is a neutral http://en.wikipedia.org/wiki/File:CenterTappedTransformer.svg


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## guitarboyled

Thanks Nick

So the center tap acts as a neutral… 

I’m confused, you can get 120v, 208v and 240v all from the same configuration. I presume in the 3 phase mode voltage is 240v. When would get 208v? Is there such a thing as two phase?


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## steelersman

guitarboyled said:


> Thanks Nick
> 
> So the center tap acts as a neutral…
> 
> I’m confused, you can get 120v, 208v and 240v all from the same configuration. I presume in the 3 phase mode voltage is 240v. When would get 208v? Is there such a thing as two phase?


 
No you get 208v in 3 phase from any line to line combination whether it's A-B, A-C, or B-C. 240v is single phase line to line.


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## RePhase277

guitarboyled said:


> Thanks Nick
> 
> So the center tap acts as a neutral…
> 
> I’m confused, you can get 120v, 208v and 240v all from the same configuration. I presume in the 3 phase mode voltage is 240v. When would get 208v? Is there such a thing as two phase?


In a 240 V delta, with a center tap, you would get 120, 208, and 240. Usually, the coil between A and C is center tapped for a neutral (N), and you get 120 V from A to N, and from C to N. You get 240 V between any two phases, and you get 208 V from B to N.

So, between A and N or C and N, you only have 1/2 a coil. But between B and N you have 1-1/2 coils. The vectors work out such that some of the voltage is canceled and you get 208. But hat voltage is really just an artifact, and doesn't really serve any purpose. Many appliances and electronics have been smoked because someone didn't understand the 208 V high leg.

P.S.: Yes, there was such a thing as 2-phase, but it is ancient history. There was no benefit to it that couldn't be done with single phase or done better with 3-phase.


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## guitarboyled

Therefore, between A and N and C and N there’s 120v single phase

And Between B and N wouldn’t I presume there's 208v single phase

I also understand there’s 240v between any two phases but I'm guessing we never use only two phases but always connect all three.


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## RePhase277

guitarboyled said:


> Therefore, between A and N and C and N there’s 120v single phase
> 
> And Between B and N wouldn’t I presume there's 208v single phase
> 
> I also understand there’s 240v between any two phases but I'm guessing we never use only two phases but always connect all three.


Yes, B to N would be single phase 208, but it is virtually never used. Any device that requires 240 V single phase, like a window A/C or table saw, could be connected between any two phases. So really, with a 120/240 V delta, you get 120 V single phase, 240 V single phase, 120/240 V single phase, and 240 V three phase. Plus single phase 208 V.


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## frenchelectrican

guitarboyled said:


> Therefore, between A and N and C and N there’s 120v single phase
> 
> And Between B and N wouldn’t I presume there's 208v single phase
> 
> I also understand there’s 240v between any two phases but I'm guessing we never use only two phases but always connect all three.


 
Inphase277 did expain very clear on that and speaking of 208 volt *line to netural* that is what we called wild leg , bastard leg , red leg , hot leg , crazy leg.

That one reason why anytime you run into delta system always test and I mean test them very carefull as due not always be marked in correct spot so that something you have to watch out on that part. 

{ French verison }

Inphase277 a vraiment exfait de la peine très clair et la conversation de 208 *volts ligne à netural* c'est ce que nous avons appelé la jambe sauvage, la jambe de bâtard, la jambe rouge, la jambe chaude, la jambe folle.

Cette une raison pourquoi n'importe quand vous vous heurtez au système de delta teste toujours et je veux dire les testent très carefull comme dû non être toujours marqué dans la tache correcte pour que quelque chose vous doive observer de sur cette partie. 


Merci,Marc


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## guitarboyled

Also how would using two phase result in 240v single phase?

Doesn’t the Delta 3 phase also follow the 120° shift between each phase?










To get true 240v single wouldn’t the two phase need to be in sync?

From wikipedia:

Another system commonly seen in North America is to have a delta-configured secondary, with a centre tap on one of the windings supplying the neutral. This allows for a 240 V three-phase supply and three different single-phase supplies from the same secondary: 120 V between either of two (non-"high leg") phases and the neutral, 208 V between the third phase (the "high leg") and the neutral, and 240 V between any two phases. 

Steelersman says 208v in 3 phases and InPhase277 says 240v in 3 phases. Can we get a consensus on this one?


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## RePhase277

guitarboyled said:


> Also how would using two phase result in 240v single phase?
> 
> Doesn’t the Delta 3 phase also follow the 120° shift between each phase?


Yes, it does. But it is just convention that when two of the three phases is used it is called single phase. And when using only two of the three phases, the resulting waveform is that of a single sine wave, a composite of the two. 






> Steelersman says 208v in 3 phases and InPhase277 says 240v in 3 phases. Can we get a consensus on this one?


He is talking about a 120/208 V wye connected system. Which is 208 between phases. Then we got on the subject of 3 phase deltas, which is what I described.


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## guitarboyled

Thanks,

And could I use that 240v on a convential oven let's say?


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## Toronto Sparky

You still need the neutral on the oven.. for timer/lamp/receptacle 
Older ovens needed it for the range top elements. 
(Remember the old 7 position controls everyone?)

Keep in mind Ovens are normally 240 but one can also buy a 208v oven for use in apartment buildings that are connected wye.


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## guitarboyled

What can I do with that specific 240v? I'm guessing lighting also from a previous post.

Couldn't I use the centered tap neutral for that oven connection? Just curious.


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## RePhase277

guitarboyled said:


> What can I do with that specific 240v? I'm guessing lighting also from a previous post.
> 
> Couldn't I use the centered tap neutral for that oven connection? Just curious.


Yes. What Toronto was saying is that most stoves aren't just 240 V. They are 120/240, and require a neutral. So if you have just a straight 240 V delta, then you couldn't use anything that required a neutral. But if you had a 120/240 V delta with a center tap providing a neutral, then it would be fine.

Specifically, the single phase 240 could be used for anything that required it. You will usually find a 120/240 V delta in heavy commercial buildings, like machine or cabinet shops. Very often, table saws are 240 V single phase. Many window unit and through-the-wall A/C units are 240 V single phase.


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## guitarboyled

The delta configuration with center tap concept seems harder to grasp. 

If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?


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## RePhase277

guitarboyled said:


> The delta configuration with center tap concept seems harder to grasp.
> 
> If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?


Nope. Figure A represents how 208 V appears when the single phase voltage is 120 V in a wye configuration. In a wye system, the line-to-line voltage comes from multiplying the phase voltage by 1.732. In a delta system, the line-to-line voltage and the phase voltage are one in the same. In other words, the single phase voltage of a delta system is the same as the phase to phase voltage, due to how the coils are connected.

If you had only only one coil, it would be 240 V. What does change is the current. The line current divide by 1.732 is the current in the coils, because there are multiple paths for the current.


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## guitarboyled

Oups! I wrote Fig A 277/480 when obviously it's 120/208v

So is it possible to represent the delta 120/240 configuration with the 3 phase sine wave graph?


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## RePhase277

guitarboyled said:


> Oups! I wrote Fig A 277/480 when obviously it's 120/208v
> 
> So is it possible to represent the delta 120/240 configuration with the 3 phase sine wave graph?


Yes, but the amplitude of each sine wave is 240 V, whereas in the 120/208 V wye, each sine wave has an amplitude of 120 V.

In the delta, coinciding with one of the sines, is a 120 V sine that represents the center tapped coil.


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## Larry Fine

guitarboyled said:


> Where does this 208v come from? How does 120v lines add up to make 208v?


We get 240v from two 120v secondaries in series on single phase because the positive and negative peaks occur at the same time.

In a 3-phase wye system, the positive peak of one phase occurs midway between the negative peaks of the other two, and vice versa.

The 208v voltage between phases occurs when neither polarity is at peak, but between them, when the positive and negative voltages are equal.


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## slowforthecones

smells like green horn in here...


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## Toronto Sparky

Larry Fine said:


> We get 240v from two 120v secondaries in series on single phase because the positive and negative peaks occur at the same time.
> 
> In a 3-phase wye system, the positive peak of one phase occurs midway between the negative peaks of the other two, and vice versa.
> 
> The 208v voltage between phases occurs when neither polarity is at peak, but between them, when the positive and negative voltages are equal.


Love this short and sweet explanation... Perfect


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## zappy

frenchelectrican said:


> Inphase277 did expain very clear on that and speaking of 208 volt *line to netural* that is what we called wild leg , bastard leg , red leg , hot leg , crazy leg.
> 
> That one reason why anytime you run into delta system always test and I mean test them very carefull as due not always be marked in correct spot so that something you have to watch out on that part.
> 
> { French verison }
> 
> Inphase277 a vraiment exfait de la peine très clair et la conversation de 208 *volts ligne à netural* c'est ce que nous avons appelé la jambe sauvage, la jambe de bâtard, la jambe rouge, la jambe chaude, la jambe folle.
> 
> Cette une raison pourquoi n'importe quand vous vous heurtez au système de delta teste toujours et je veux dire les testent très carefull comme dû non être toujours marqué dans la tache correcte pour que quelque chose vous doive observer de sur cette partie.
> 
> 
> Merci,Marc


One more name for it, stinger leg.


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## zgozvrm

guitarboyled said:


> Where does this 208v come from?


This is a result of vector addition.

First of all, the turns ratio of the transformers are such that 120 volts is obtained from the primary voltage. For instance if your primary voltage is 480V, 3-phase (480V can be obtained by tapping off of any two lines: leg A and leg B, leg A and leg C, or leg B and leg C), there would be a 4:1 ratio in each transformer (you need 3 to create a wye) since 480 divided by 4 is 120.

Remember that, since the primary voltages are 120 degrees out of phase, so are the secondary voltages.

In order to understand vector addition, think about how planes react to wind currents. Both the plane and the wind current have their own specific speed and direction. This can be represented by vectors, by drawing line segments of lengths that represent the speed and direction of each.

For instance, if the plane is heading, say due East at 50 mph (it's a slow plane), we would draw a line segment 50 units long going from left to right with an arrowhead on the right end. Suppose the wind was blowing due north at 40 mph, this would be represented by another vector; this one 40 units long pointing upward. These 2 vectors would be 90 degrees "out of phase." In other words, the phase angle between the 2 vectors is 90 degrees. Obviously, the wind will throw the plane off course, unless it is blowing in the same direction as the plane (or opposite). It will also cause the plane to either speed up (with a tailwind) or slow down (with a headwind). The result of this (the resultant vector) will be the vector sum of the 2.

In this case, we have a crosswind, that will cause the plane to veer off-course in a north-easterly direction. It will also cause the plane to slightly speed up, so the resultant vector, will be slightly longer than the plane's speed vector. If you put one source vector (the plane's for instance) head-to-tail to another source vector (the wind's), the resultant vector will have a length equal to the tail-to-head distance of the combination. In this example, the plane will actually travel in the speed and direction of the resultant vector, which can be shown to be 60 mph at a heading of 0 degrees, but a bearing of approximately 38.66 degrees on the compass (with East being 0 degrees, and North 90 degrees).

Now, the same thing goes for voltages. In this case, we have two 120 volt sources with a phase angle of 120 degrees. This can be represented by 2 vectors of equal length (both are the same voltage), in this case 120 units. See the picture below:











Now the resultant vector in this case would be a new, third vector with its tail at point A and its head at point C. If you were to measure this distance (which can be accurately done using trigonometry), you'll find that the resultant vector is roughly 208 units long.

A wye distribution system will have 3 such relationships (xfmr 1 to xfmr 2, xfmr 2 to xfmr 3, and xfmr 1 to xfmr 3) and therefore will have 3 different resultant vectors, all measuring 208 volts.

You should note that the 208V (line-to-line) vectors are offset from the 120V vectors (phase vectors) by +/- 30 degrees.

FYI, using vector addition, you can easily calculate the "stinger-to-ground" voltage in a center-tapped delta secondary. You can also see how an open-delta configuration works.


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## Tyha

A really great way to really undrstand exactly what is going on is to look at the sine wave graph in relation to a generator that is producing it. This is truly where the EMF (electromotive force - voltage) is being created. you will see the start points for each phase and there coralation between each other is due to the position of the outgoing conductor and the distance between it and the magnet that is spinning around creating an EMF. You remember the science experiment with the magnet and the lamp cord, well thats how most of our electricity is produced. If you put a little thinking into positioning diffferent magnets and some sort of other force to drive it you can create your own 3 phase generator.


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## sparky105

InPhase277 said:


> Yes. What Toronto was saying is that most stoves aren't just 240 V. They are 120/240, and require a neutral. So if you have just a straight 240 V delta, then you couldn't use anything that required a neutral. But if you had a 120/240 V delta with a center tap providing a neutral, then it would be fine.
> 
> Specifically, the single phase 240 could be used for anything that required it. You will usually find a 120/240 V delta in heavy commercial buildings, like machine or cabinet shops. Very often, table saws are 240 V single phase. Many window unit and through-the-wall A/C units are 240 V single phase.


using delta for contant balanced loads eg single phase or 3 phase heaters more accurate element control. like heat trace or barrel heaters on plastic extrution machinery


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## BCSparkyGirl

Please dont get your info from wikipedia. It can be put up there by anyone, and that being said, some douche can put up any info they want nd have people believe it. I never look at that site.


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## Mr. Sparkle

BCSparkyGirl said:


> Please dont get your info from wikipedia. It can be put up there by anyone, and that being said, some douche can put up any info they want nd have people believe it. I never look at that site.


Hahaha my brother was the drummer for The Who for a few days.


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## zgozvrm

InPhase277 said:


> Yes, it does. But it is just convention that when two of the three phases is used it is called single phase. And when using only two of the three phases, the resulting waveform is that of a single sine wave, a composite of the two.


:001_huh: There seems to be some confusion here. I think you're confusing a "leg" with a "phase." Each phase is produced by a single secondary transformer coil and thus, 2 wires per phase. The 3 secondary transformers are then connected in such a way that only 3 wires are connected to a breaker panel (in either a wye or delta configuration). Tapping off any 2 of the 3 wires is essentially the same as tapping off of only 1 of the 3 secondaries, and thus only a single phase. So the single phase sine wave is not a "composite of the two," rather it is simply just one of the 3 sine waves that make up the 3-phase voltage, depending on which wires were tapped from. It's not "just convention."

Remember that a sine wave (a phase) doesn't exist on a single wire (a leg) by itself. For instance, how do you measure a sine wave with an o-scope? You connect the probe to one point and the ground clip to another ... it takes 2 wires!


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## euroah

guitarboyled said:


> The delta configuration with center tap concept seems harder to grasp.
> 
> If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?


So does this mean that with a 120/208v wye system you do not have any more than 208v single phase (A-B, B-C, C-A)? Thanks, Greg


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## zgozvrm

Yes, 208V is the most you can get.

Phase to phase voltages will all be 208V
Phase to neutral voltages will all be 120V


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## zgozvrm

zgozvrm said:


> if the plane is heading, say due East at 50 mph (it's a slow plane), we would draw a line segment 50 units long going from left to right with an arrowhead on the right end. Suppose the wind was blowing due north at 40 mph, this would be represented by another vector; this one 40 units long pointing upward.
> 
> ...
> 
> In this example, the plane will actually travel in the speed and direction of the resultant vector, which can be shown to be 60 mph at a heading of 0 degrees, but a bearing of approximately 38.66 degrees on the compass (with East being 0 degrees, and North 90 degrees).



In reading some other threads, I was re-directed to this one and re-read this post of mine. I see that I made an error in my example but, since I can no longer edit my post (it's too old), I'm addressing the issue in a new post...


The resultant vector would be calculated by the following formula:

Square root of (40 squared + 50 squared) = square root of 4100 = 64.03 mph (approx.)

The angle would then be calculated by:

angle = arctan(40/50) = arctan(0.8) = 38.660 degrees (approx.)


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## qmavam

guitarboyled said:


> The delta configuration with center tap concept seems harder to grasp.
> 
> If figure A represents a 120v/208v wye configuration supplying 3 phases at 208v does figure B represent a 3 phase delta configuration with center tap supplying 3 phases at 240v?


 I know this is an old thread but I'm going to try and revive it to make corrections to many errors that have accumulated. I'll probably stir up a wasps nest, but here goes.
First a formula (Vrms x 1.414 = Vpeak). When we discuss 120 Vac it is short for 120 Vrms. RMS stands for root means square, you can look that up if up if you want to know more. Basically the rms of any waveform is a "mathematical method of defining the *effective* voltage or current of an AC wave" Note the 120V is not always 120 during the cycle, so to get an effective 120V, the peak must be higher. In fact the peak is about 170 volts. So to use the formula: 
120 Vrms x 1.414 = 169.68 Vpeak
Now your graph needs to be corrected so the peak voltage is 169.68V.








The next thing is how do you get 208Vrms from 3 phase. Well we know it's from line to line. I have a graph that I got from another group that I think does a beautiful job of describing where and why 208 V.








First note the red, blue and black lines are 3 phase 120Vrms and they peak at 169.71V.
Second, the graphs vertical lines are 30 degrees apart. If you look at the red and blue_ *dotted*_ line, at 90 degrees it peaks at 293.94V, that is our 208Vrms. Notice the red line is at +146.97 and the blue line is -146.97 this equals 293.94V. Back to our formula, this time we know peak so we divide.
293.94Vpeak / 1.414 = 207.88Vrms. If you add the voltages of the red and blue lines at different point along the cycle you will see it will equal the red and blue dotted line.
I hope the attachment works, if not here is a link.
http://i145.photobucket.com/albums/r204/Smart_S/3waveforms.gif
I hope this is helpful, if you have questions let me know.
The graph I inserted seems to have messed with line length, I don't know how to fix it, I need to go for now.
Mike (qmavam)


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## qmavam

Anybody know how to get hold of guitarboyled looks like he's not set up
to get private messages.
Any feedback about the info I posted above?
Mike (qmavam)


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## Electric_Light

raider1 said:


> Peak value is not used in most applications, it is more a theroretical principal than a usable measurement.
> 
> The utility will supply you with a nominal 120/240 volt system for most residential systems. This voltage is expressed as the effective (RMS) voltage and not the peak.
> 
> Chris


Actually, peak value is the only value that matters for almost everything that uses a switching power supply. This covers CFLs and IT equipment with the exception of laser printers, wall warts using coil & core transformer and active PFC controlled power supplies.

IT equipment "grabs" power only at the tips of the wave. Transformers and generators don't like it. It creates a voltage drop only at the tips and distorts the waveform. AC rotating machines that share the power don't like it. 

Distorted waves can't be corrected and PoCo's hate it.

http://web.eecs.utk.edu/~tolbert/publications/ias96_1.pdf


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## qmavam

Electric_Light said:


> Actually, peak value is the only value that matters for almost everything that uses a switching power supply. This covers CFLs and IT equipment with the exception of laser printers, wall warts using coil & core transformer and active PFC controlled power supplies.
> 
> IT equipment "grabs" power only at the tips of the wave. Transformers and generators don't like it. It creates a voltage drop only at the tips and distorts the waveform. AC rotating machines that share the power don't like it.
> 
> Distorted waves can't be corrected and PoCo's hate it.
> 
> http://web.eecs.utk.edu/~tolbert/publications/ias96_1.pdf


 Even simple linear supplies grab there current at the peaks.
This page shows current pulses and a distorted voltage waveform,
although this is more severe than you would normally see from a wall socket. Note this is halfwave, but still representative of the effect.

http://www.electro-tech-online.com/...d1243090102-diode-ripple-source-rectifier.jpg
Mike


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## JohnR

The OP probably isn't around anymore, but guy, your instructors did you a major disservice if they didn't teach you about phasing and how single/three phase power works as you stated. In your trade you will find that without that info you will be unable to fix some AHU units that you are called to repair. 
As for a book that you can find out info on basic electrical prinipals, I would suggest Henry Mileaf 1-7. It has a very basic explanation of electricity (NOT CODES) and is fairly easy to read.


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## ibew292guy

running dummy said:


> phase to phase voltage is found by taking the single phase voltage and multiplying it by 1.73.
> 
> 120V phase A-120v Phase B= 120 X 1.73= 207.6
> 
> If you really want to know where 1.73 came from I can find it, I really just remembered the "magic" number.
> 
> EDIT: this would obviously be for a wye connection


 
1.73 is the square root of 3 think 3 phases


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## fraydo

running dummy said:


> phase to phase voltage is found by taking the single phase voltage and multiplying it by 1.73.
> 
> 120V phase A-120v Phase B= 120 X 1.73= 207.6
> 
> If you really want to know where 1.73 came from I can find it, I really just remembered the "magic" number.
> 
> EDIT: this would obviously be for a wye connection


 
1.73 is the tangent value for 60 degrees. There is an algebraic relationship that ends in this. If anyone is interested i can look for it in my notes from school and post it


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## brian john

fraydo said:


> 1.73 is the tangent value for 60 degrees. There is an algebraic relationship that ends in this. If anyone is interested i can look for it in my notes from school and post it


You are right except it is the tangent of 120 degrees (tangent for 60 is same as 120 degree angle)

Take a circle divide it in three you have three lines centered with three 120 degree angles.

From the center of this Y (WYE) each line is 120 inches, 120 feet or 120 volts long or hell go metric if you like.

Now draw a line between the ends of any of the two lines, take the tangent of the 120 degree angle multiply this by length of either 120" line 1.732x120=208.

It ain't electricity it is basic math.


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## JohnR

1.732 is the square root of 3 

You have 3 phases and the voltage is 120 to ground, so 120 X sq rt 3 = 208v and that is the voltage between phases
it works with any 3 phase for the abc points excepting the high leg. Motors or transformers both.


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## kwired

brian john said:


> You are right except it is the tangent of 120 degrees (tangent for 60 is same as 120 degree angle)
> 
> Take a circle divide it in three you have three lines centered with three 120 degree angles.
> 
> From the center of this Y (WYE) each line is 120 inches, 120 feet or 120 volts long or hell go metric if you like.
> 
> Now draw a line between the ends of any of the two lines, take the tangent of the 120 degree angle multiply this by length of either 120" line 1.732x120=208.
> 
> It ain't electricity it is basic math.





JohnR said:


> 1.732 is the square root of 3
> 
> You have 3 phases and the voltage is 120 to ground, so 120 X sq rt 3 = 208v and that is the voltage between phases
> it works with any 3 phase for the abc points excepting the high leg. Motors or transformers both.


Do same thing Brian John did with a delta - draw a triangle with equal angles (all 60 deg) make the sides all 240 inches, feet, miles or whatever unit you prefer for length. place a tap in the center of one side - this equates to the tap for the neutral on a high leg system

Each side is 240 units long equivelant to 240 volts phase to phase.

The length from the center tap to either adjacent corner is 120 units.

The length from the center tap to the opposite corner is 208 units.


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## brian john

kwired said:


> Do same thing Brian John did with a delta - draw a triangle with equal angles (all 60 deg) make the sides all 240 inches, feet, miles or whatever unit you prefer for length. place a tap in the center of one side - this equates to the tap for the neutral on a high leg system
> 
> Each side is 240 units long equivelant to 240 volts phase to phase.
> 
> The length from the center tap to either adjacent corner is 120 units.
> 
> The length from the center tap to the opposite corner is 208 units.


Exactly, this is not magic, it is math.


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## Zaped

Please advise...is there a thread or subject on this forum that encompasses the topic of BOOKS (i.e., books on electrician work topics) ? I seek any information as to a decent book (or booklet, or any published material) on commercial electrical panels, installing, cutting in, materials, equipment, techniques, tips, etc. Just trying to learn myself some more. Thanks for any information.


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