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SmokinJoeR (Electrical) (OP)
17 Aug 07 8:09
There seems to be some controversy on the internet wherever I check on how 240 volts is brought into your house. Some sources say that the electrical output of the transformer on the power pole is a 3 wire single phase, or others say it is 2 single phase 120V that have 180 deg of phase difference. I know the transformer has a secondary that is center-tapped, but I could never get the clear answer whether or not its 240V/single phase and (2) 120V/single phase output or (2) 120V with 180 deg of difference between them.

Anyone know for sure?

Marmite (Electrical)
17 Aug 07 9:16
It's all a question of terminology.In the UK the two phase centre tap arrangement is called split phase. When I went to work in Bermuda, which applies North American standards I found it confusing when contractors would ask for 200A single phase because to me that meant 200A 120V. What they wanted was 2 phase 200A. Your three descriptions are all the same. You have a centre tapped transformer. The centre tap is  neutral. Between either of the phases and neutral you get 120V. Between the two phases you get 240V. This is commonly referred to as single phase, but in my view is actually 2 phase.
davidbeach (Electrical)
17 Aug 07 10:25
Marmite, sorry; it is not 2 phase.  The 240 is a single phase.  There is also a center tap that provides two 120V single phase connections phase-to-phase.

2 phase is something else all together and now quite rare.  2 phase could use as few as three conductors, two phases and a neutral but in that case there is a 90° phase difference between the two phase conductors, often referred to as a quadrature connection.

Think of it this way - a single phase circuit can not create a rotating magnetic field in a motor with out use of axillary connections such as capacitors or shaded poles while a 2 phase or 3 phase circuit can create a rotating magnetic field without the need for other connections.  The 240V of a 120/240V circuit cannot create a rotating magnetic field any more than either of the 120V.  Therefore it is single phase.
Marmite (Electrical)
17 Aug 07 10:50
I'm interested in how you would correctly distinguish in terminology between a 240-N single phase supply and a 120-0-120V single phase supply. ie if you ask for a 240V single phase supply how do you know which one you will get? In the UK single phase is single phase, the centre tapped arrangement is called split phase or split single phase, and three phase is obviously three phase.
davidbeach (Electrical)
17 Aug 07 11:02
Nothing that will run on 240V will care or know which version of 240V, because there is no means of distinguishing them.  If you throw in ground (earth) as a reference, then you could make a distinction, but what ever is using the 240V won't care.
Marmite (Electrical)
17 Aug 07 11:06
I went looking for some defintions and came up with this website, which explains the differences very well. It also has information on the historic 2 phase system.
waross (Electrical)
17 Aug 07 14:40
In North America, A 120/240 volt supply is single phase. To call it two phase is fun with words and vectors. If you want to refer to the center tap as the origin, then consider one winding with the polarity reversed rather than rotated 180 degrees.
If you must call a center tapped supply two phase then I will feel justified in calling two 12 volt batteries in series with the center tap brought out a two phase DC system.
Please don't use the term "180 degree phase shift" to describe a reversal of polarity. You have not "shifted the phase", you have just changed your reference point.
jghrist (Electrical)
17 Aug 07 16:09
A google search of "electric '120/240' phase" finds many utility websites referring to single-phase service, but none calling it any other kind of phase:

PGE - "single-phase 120/240 V"
City of Concord - "Single Phase 120/240 volt"
City of Dowagiac - "single-phase, three wire, 60 Hertz, at approximately 120/240 volts"
Nipsco - "120/240 volt, 1 phase, 3 wire"
Magic Valley Electric Cooperative - "120/240 volts, 3 wire 1ø"
Cuero Development Corporation - "single phase, 120/240 volts"
City of Riverside - "single-phase service voltages are 120, 120/240, 120/208, 240 and 480 volts."

If you want to be understood by your electric utility, refer to it as single phase.
stevenal (Electrical)
17 Aug 07 16:10
To add to the confusion: Some single phase transformers have dual secondaries (four wires). The two windings can be connected in series to provide 120/240, or in parallel to provide full kVA capacity to a 120 V circuit.
waross (Electrical)
17 Aug 07 16:20
Hi stevenal;
Virtually all single phase distribution transformers have dual secondaries. A very few have 4 secondary bushings. The majority have the series or parallel connections made internally. It is common for standard distribution transformers to be opened and the normal series connection changed to parallel for 120/208 volt installations.
An exception would be 277 volt transformers for 277/480 service. These transformers have only one winding.
jraef (Electrical)
17 Aug 07 18:15
I know of no place in the US where there is a 240-N 1 phase distribution. That doesn't mean it doesn't exist, but that I believe it would be the exception. 1 phase is, as far as I know, always referring to 120/240 (nominal) 3 wire distribution system; 2 hots and a neutral. You have 120V to N on either side and 240V Line to Line. The 240V is the same phase, just opposite ends of the winding.
itsmoked (Electrical)
17 Aug 07 18:23
Jeff you left out 120V service.  We have a lot around here.  Just 120V period.

Keith Cress
Flamin Systems, Inc.-

rbulsara (Electrical)
17 Aug 07 19:24
My two cents to affirm why a center tapped 240V is still a single phase system.

1. The two 120V measured from two lines to the center are in phase with each other and not so 180 out of phase. The vectors are ----->-------> and not <------.------->.

2. Also the such trasformer's primary are fed by single phase supply, or only two wires the very definition of a single phase circuit.

3. Speaking of the definition, 240V center-tapped supply can only feed loads with two wires either 240V or 120V. The third or neutral wire is only for 120V loads. There is no such thing as two phase 240V load.

4. To add to confusion (or explanation), many residences in USA are fed by two phases of a 208Y/120V 3 phase system and the neutral. Even there, the service at each house still called (and are) single phase 120/208V system, because they can only feed two wire circuits, (again the definition of single phase circuit), either 208V or 120V. There is no two phase 208V load or appliance.

rbulsara (Electrical)
17 Aug 07 19:27
To emphasize point 1 in my last post..the net voltage of two 120V, 180 deg out of phase voltages will be zero, which is not the case in a 240V/120V center-tapped system.

[I will stop my rant]
sslobodan (Electrical)
20 Aug 07 4:39
Well you are running out of the point
Its very simple we all have 3 phase system because it is the smallest phase number system that will allow motor rotation (copper savings in wires) it can be done with 4,5,6... phases cant be done with 1 or 2, but most equipment does not need three phase system and they are on voltage rating phase to neutral.

So you have 2 kinds of voltage ratings
phase to neutral and phase to phase. does not mater if your base rating(phase to neutral) 120V/60Hz (US) or 230V/50Hz (Europe) or some other voltage level principles are the same.
Difference between phase to neutral and phase to phase system is in the angle that two phases have between them selves If you take book of basics of electrotehnics you will find very nicely explained why the difference is exactly sqrt(3) between these two voltages.

So you calculate voltage between two phase and have 3 phase (3wires)or 3 phase and neutral system (4wires) or 3 phase neutral  with grounding (5wires)
4 wire system is usually used in distribution systems to end consumers because of save of copper (4 instead of 5 wires) and you have your own grounding system that will make you 5 wire system in house. Distribution system that transmit power between two transformers is 3 wire because load is balanced (98% times at least)and you don't need 4th wire between two transformers.  
jghrist (Electrical)
20 Aug 07 8:58


Its very simple we all have 3 phase system because it is the smallest phase number system that will allow motor rotation (copper savings in wires) it can be done with 4,5,6... phases cant be done with 1 or 2, but most equipment does not need three phase system and they are on voltage rating phase to neutral.
Great! Now we find out that there is no such thing as a single-phase motor.
sslobodan (Electrical)
20 Aug 07 10:04
Of course there is... don't be sarcastic :)but you know why people use 3 phase motor.... I was implying rotary field as a breakthrough in electro energetic, and I would love to see how would you make that with 1 phase system....
davidbeach (Electrical)
20 Aug 07 11:52
A two phase system, with the phases in quadrature, can produce a rotating magnetic field.  Three phases are not the minimum required for a motor that does not require capacitors or other aux devices (shading poles, etc.).  Three phases are, however, the minimum required such that the instantaneous power - and torque - is a constant rather than pulsating with each half cycle.
rbulsara (Electrical)
20 Aug 07 15:49

I am trying to be patient here. Take a deeeep breath! The key is to understand that single phase circuit means that it uses only two wires, one supply and one return. The return does not have to be a neutral!

A circuit fed from two phases of a three phase system is still a single phase circuit. For example, a sigle phase pole mounted trannsfomer more street power, fed from two line wires of a 3 phase system on primary side.

Other way of looking at it, if you draw a phasor of voltage between the two points of a single phase source, it will be a single arrow or sum of two or more arrow in the same direction and angle.

A true two phase system will have "three" wires, but phasors of the two voltages will have some phase angle between them, as in davidbeach's example in his last post.

The "single phase" motors are called single phase as they require only two wire connections and it is a blackbox to the source. The motor gets its starting torque by splitting the single phase using a capacitor. Now that is a split phase case in point, and not a center-tapped transformer.

sslobodan (Electrical)
20 Aug 07 15:52
I know you are right (I have just read lecture of DR Gerhard Henneberger few days ago concerning that subject) But we strayed from subject, and such discussion is the reason why I haven't mentioned 2 phased and single phased engines. tread is about difference in interpreting system marking on internet. ("There seems to be some controversy on the internet wherever I check on how 240 volts is brought into your house....")
And making deep theory about engine types and their construction is a little bit out of subject... Don't you agree?
davidbeach (Electrical)
20 Aug 07 16:02
There has been no prior reference to engines in this thread, so I don't know how they matter (engine and motor are not the same thing at all).  Engines might be two stroke or four stroke, but none have phases that I know of.

I wouldn't worry about what you might read on the internet, present site excepted, when it comes to how 240V might be brought into a house.
sslobodan (Electrical)
20 Aug 07 16:06
Sorry for misspelling I am not native English speaker, and It just slipped my mind.... Thanks for correction (read motor not engine, I am overwhelmed with my own Engine projects... ) :)
sreid (Electrical)
20 Aug 07 17:03

I have to disagree about your point 1 but it could be semantics.  The usual convention would be; take a two channel scope with the ground connected to neutral and the channels connected to Hot line 1 and Hot Line 2.  Then when Hot Line 1 is at peak positive amplitude, is not Hot Line 2 at maximum negative amplitude?  And is this not the basic definition of 180 degrees out of phase?
rbulsara (Electrical)
20 Aug 07 17:32

The basic answer is NO. They are not two 180 degree out of phase 120V phasors. If they were, their sum would be zero. Is it? The sum of two 120V, in this case, is 240V so they are in phase with each other.

You have just shifted the reference points, as waross had mentioned earlier.

As for the scope, hook it up, check it out and post your results here.

stevenal (Electrical)
20 Aug 07 17:57
BJC (Electrical)
20 Aug 07 17:58

"There is no two phase 208V load or appliance"
Loads are treated differently on 208/120 systems even if the two phases serving a single residnece are "single phase".
Neutral currents on 240/120 volt systems add algebraically, Neutral currents in a 208/120 systems have to be added vectorially.
rbulsara (Electrical)
20 Aug 07 20:00

How? except for 208 vs. 240V rating?
BJC (Electrical)
20 Aug 07 20:13
Draw the vecotrs,  Line-1  to neutral, Line - 2 to neutral.
10 amps in each load.  in a 120/240 system I-neutral = 0.
in two wires on a 120/208 system I-neutral = 10 amps.
If you wire three panels A-B,B-C and A-c with the same load the total current is 0, but not at each panel.

There is a section of the nec that talks about this, Id on't have my copy here.
stevenal (Electrical)
20 Aug 07 20:16
The 120/208 three wire service brought up by rbulsara does not meet the IEEE definition of single phase, although I have heard it called that. ANSI C12.1 calls it an open wye service.
rbulsara (Electrical)
20 Aug 07 22:57
What else is that other than a single phase serice? It is called single phase service because it can only feed single phase loads.

Do not confuse the type of circuits to type its source.

A 3 phase source still can provide single phase circuits.


Vector calculation has has nothing to do with how a circuit is treated. Two line wires of a 208/120 service can only feed a single phase 208V load, same as two lines of a 240/120V service can only feed single phase 240V load. Other single phase loads they can feed are 120V.

Single phase load does not care where its voltage is coming from. All it looks for its rated voltage on its "two" terminals. A 120V bulb does not behave differently, just because its 120V comes from a 208/120V service or a 240/120V service or just a 120V control transformer.

rbulsara (Electrical)
20 Aug 07 23:36
The following is the excerpts from my local utility (northeast utilities):

"Normally, one of the following will be supplied:
Nominal Voltage    Phase     Wires    Comments    
120/240      1           3    a,b,c,d    
120/208            1        3    c,e    
208Y/120    3        4    f,g,h    
480Y/277    3        4    f,g,h    

e. Three-phase supply is not normally available for single
family housing. For large residential complexes, which
may require a three-phase service to the building,
individual residential customers will be served only with
single-phase 120/208v."
BJC (Electrical)
21 Aug 07 1:11
 So an electric range with a 3 wire  208/120 v 3 wire connection is single phase?  Ditto a clothes dryer?
Why are the receptacles different?
rbulsara (Electrical)
21 Aug 07 9:06

Yes. The range has mutiple (usually four) heating elements. They are individual heating elements and individual circuits inside. The neutral is provided only to feed 120V heaters. There no singel heating element it the range that requires three wires. Each heating element is a "two" wire circuit. It is possible that larger heating element could be a 208 or 240V single phase.

There are no 3 wire single phase loads or circuits, the 3 wire single phase services are intended and do only feed two wire circuits. Those two wire can be any two of the three wires available. But no single phase load requires more than two wires!!
rbulsara (Electrical)
21 Aug 07 9:55
Ditto for clothes dryer.
BJC (Electrical)
21 Aug 07 10:20
"But no single phase load requires more than two wires!!"

Don't disagree, don't think I ever said that.
Go to your 120/240 volt panel, measure from L1 to N and L2 to N, you get two readings of 120 volts. Measure from L1 to L2 you get 240 volts.  120 +120 = 240.

Make the corresponding measurments on a panel fed from 2 phases of a 120/208 v source.
You get 120, 120 and 208. 120 + 120 does not add up to 208. You can't explain it without resorting to vectors in one way or the other.
rbulsara (Electrical)
21 Aug 07 11:01
Both are single phase 3 wire "services", one originates from a 3 phase, 4W source and one from a single phase 3 wire source.

Type of source is not the same as type circuits or loads being served.

I think I have said enough. The single phase 3 wire "services" are intended only to serve two wire loads as I said before.

I am saying your vector math is wrong, it is still true but there are no 2 phase loads to serve.

I do agree that in theory, if you have a motor with two 120V windings (2-phase), it can be connected to two lines and the neutral of a 208/120 service and it will work, but then it will no longer be a single phase load and will require 3 wires, but no such equipment exist for common use.

stevenal (Electrical)
21 Aug 07 11:11

Bolding your statements does not make them any truer. I cited sources to back up my claims; where are your's? By your argument, a three phase source feeding single phase loads cannot be described as a three phase service, and a six phase source feeding three phase loads cannot be described as three phase. And if any of them happened to be disconnected from all load they would be called zero phase?

Utilities may call the open wye service single phase for conveniance when the rate structure and service wire type is indistiguishable from 120/240 single phase. Remember that most utilities are run by the accountants.
rbulsara (Electrical)
21 Aug 07 11:27

I have no idea what you are talking about or how you reached your conclusions. Among all I thought you would be the one who would not misconstrude my statements. I am not up to playing semantics game.

stevenal (Electrical)
21 Aug 07 11:34

You need to understand that rbulsara is defining "load" at the component level. At this level, I believe the only true three phase load would be a three phase motor, the rest being 2 wire single phase. Most of the rest of us define "load" as everything downstream of some convenient interconnection point such as the receptacle, weatherhead, transformer, distribution tap fuse, feeder breaker, etc.
sreid (Electrical)
21 Aug 07 11:42

You are correct in saying that signals 180 degrees out of phase add to zero.  But differential voltage is calculated by subtracting one voltage from the other voltage.  So at peak voltage, 160V - (-)160V = 320V.
stevenal (Electrical)
21 Aug 07 14:53

I cannot see that I've misconstrued anything you've said, because you have stated it over and over above, rewording it each time. Here is what I heard, please correct me if my hearing is bad: A 120/208 three wire circuit derived from a 120/208 4 wire three phase circuit is properly called single phase because it feeds no loads at the component level that are not 2 wire single phase in nature.

I disagree with the above for the reasons cited in previous posts. I agree we are discussing semantics. I disagree that it is a game, since speaking and understanding the jargon is important in this field.
rbulsara (Electrical)
21 Aug 07 15:17

The jargon is used by electrical utility companies in the US and posted on nameplates of UL listed panelboards, so it is widely accepted and it means what it says, it is intended for single phase loads. You may not agree with all that, but that is what it is.

For the likes of you, the definition of a single phase source would be a source with single sinusoidal waveform of a given frequency. The translation of which is a two wire circuit.

BJC (Electrical)
21 Aug 07 16:48
Back to the first post-  It says  residential power not  appliance, coffee pot, garage door opener etc.
They are except for dryers and ranges single phase two wire loads. The residential power can be two phase.
stevenal (Electrical)
21 Aug 07 20:38

I'm not aware of any two phase services used residentially. Two phase is an archaic industrial service. See Davidbeache's description above and the FAQ I linked.

Single phase panel boards are commonly used on open wye systems. I expect their listing allows this. 208 V rather than 240 so insulation is not an issue. The only difference is in the meter base and meter which have five lugs rather than four.

An open wye open delta transformer can be used to convert an open wye service to full three phase, so it is not true that only single phase loads can be fed.
BJC (Electrical)
21 Aug 07 21:32
The attached link has about 150 residental units with 2 phase power. Put a s scope on the two hots in any panel and you'll see two sine waves 120 degrees apart.;listingid=7060651

I know lots more.
rbulsara (Electrical)
22 Aug 07 8:44

No one denied that.
jghrist (Electrical)
22 Aug 07 9:13


The attached link has about 150 residental units with 2 phase power. Put a s scope on the two hots in any panel and you'll see two sine waves 120 degrees apart.
The building is probably served by a 120/208 3Ø service, with two phases going to each apartment service.  By convention of most utilities, the individual services are called single-phase 120/208 volt.  This avoids confusion because the apartment service panelboards are single-phase panelboards and all loads are either 120 volt 1Ø loads or 208 volt 1Ø loads.  It might be more properly referred to as two phases of a three-phase system because the service does not meet the IEEE definition of either a single-phase circuit or a two-phase circuit (see FAQ link in Stevenal's Aug 20 post).

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