# Is the American 120/240 volt system single phase or two phase?

#### Yakima

Joined Jan 23, 2012
35
This is the question: Is the 120/240 volt system supplying American homes single phase or two phase? I have also heard the term "split phase" to describe it. Basically, a transformer just before your house's electrical service creates two 120 volt ac sources in series, using a center-tapped secondary. The middle leg is grounded at the "telephone" pole and at your house's service entrance. I consider it all single phase due to the following consideration: Yes, the two high legs are 120 degrees out of phase with respect to one another and one might consider this as two phases. But whether you run a motor off 120 volts or 240 volts, it is still a single phase motor. A two phase motor is used for servos.

#### WBahn

Joined Mar 31, 2012
26,451
The two high legs are 180 degrees out of phase with each other, not 120 degrees.

This boils down to semantics.

One might consider the following scenario:

Take two generators built on a single shaft (the same way that three-phase generators are built on a single shaft) so that you get two 120VAC outputs that are 180 degrees out of phase. Would it be reasonable to call this two-phase power? Now run them to a house and tie them together so that you have 120VAC between the common and either of the other lines and 240VAC between the two lines. Would it still be reasonable to call this two-phase power? Could you tell the difference between that situation and the one in which the wires coming to the house are from a center-tapped transformer? If not, then wouldn't it be reasonable to use the same name for either?

Joined Jan 15, 2015
6,704
I agree with WBahn in that it becomes a matter of semantics. Typically they like to refer to it as "Split Phase". This is a read on the subject from within the forum.

Ron

Last edited:

Joined Jul 18, 2013
25,469
N.A. domestic supply is Single phase or if you wish, split phase, it is not 2 phase.
Max.

#### WBahn

Joined Mar 31, 2012
26,451
N.A. domestic supply is Single phase or if you wish, split phase, it is not 2 phase.
Max.
What would the definition of 2 phase be?

Joined Jul 18, 2013
25,469
...Is this a trick question?
Max.

#### WBahn

Joined Mar 31, 2012
26,451
...Is this a trick question?
Max.
No. I'm asking. Is there an official definition (I don't even know who the defining authority would be, IEC? ISO?) the precludes a split-phase system from being described as a two-phase system?

We know that it can't require that there be at least three phases, since a quadrature-related system is what is normally meant by "two phase" power. It could be part of a definition that it can't be called an N-phase system unless there is a non-symmetric ordering of the phases, but that would seem a bit excessive to have that nitpicking minutia there which would obviously be there specifically to prevent a split-phase system from being described as multi-phase.

But barring an official definition that precludes it, it would seem that any generic scientific or mathematical definition of a multiphase system would have to include one in which the two phases are symmetrically spaced.

#### Yakima

Joined Jan 23, 2012
35
The two high legs are 180 degrees out of phase with each other, not 120 degrees.
Yes, sorry, I'm getting old before my time.

This boils down to semantics.

One might consider the following scenario:

Take two generators built on a single shaft (the same way that three-phase generators are built on a single shaft) so that you get two 120VAC outputs that are 180 degrees out of phase. Would it be reasonable to call this two-phase power? Now run them to a house and tie them together so that you have 120VAC between the common and either of the other lines and 240VAC between the two lines. Would it still be reasonable to call this two-phase power? Could you tell the difference between that situation and the one in which the wires coming to the house are from a center-tapped transformer? If not, then wouldn't it be reasonable to use the same name for either?
It does indeed boil down to semantics.

#### Yakima

Joined Jan 23, 2012
35
What would the definition of 2 phase be?
Two phase is where the two phases are 45 degrees apart if my memory serves me well.

#### Yakima

Joined Jan 23, 2012
35
No. I'm asking. Is there an official definition (I don't even know who the defining authority would be, IEC? ISO?) the precludes a split-phase system from being described as a two-phase system?

We know that it can't require that there be at least three phases, since a quadrature-related system is what is normally meant by "two phase" power. It could be part of a definition that it can't be called an N-phase system unless there is a non-symmetric ordering of the phases, but that would seem a bit excessive to have that nitpicking minutia there which would obviously be there specifically to prevent a split-phase system from being described as multi-phase.

But barring an official definition that precludes it, it would seem that any generic scientific or mathematical definition of a multiphase system would have to include one in which the two phases are symmetrically spaced.
Like I said above a two phase system has two signals 45 degrees apart. It is used for servos.

#### Yakima

Joined Jan 23, 2012
35
This is the question: Is the 120/240 volt system supplying American homes single phase or two phase? I have also heard the term "split phase" to describe it. Basically, a transformer just before your house's electrical service creates two 120 volt ac sources in series, using a center-tapped secondary. The middle leg is grounded at the "telephone" pole and at your house's service entrance. I consider it all single phase due to the following consideration: Yes, the two high legs are 120 degrees out of phase with respect to one another and one might consider this as two phases. But whether you run a motor off 120 volts or 240 volts, it is still a single phase motor. A two phase motor is used for servos.
It's interesting that I posted this same question one or two years ago and this time I have gotten more intelligent replies. Last time there was a big debate. Have we become more savvy?

#### WBahn

Joined Mar 31, 2012
26,451
Two phase is where the two phases are 45 degrees apart if my memory serves me well.
In the early days of electrical systems there was a two-phase system that was in quadrature, which is 90 degrees out. I believe synchros and resolvers also use quadrature signals, but I never did much with them and what little I did do was nearly thirty years ago.

Joined Jul 18, 2013
25,469
The only two phase system I have come across was 120°, I believe it was used in early induction motor circuits, but it has all but disappeared.
How it was generated or created, I have no idea.
Resolvers used for commutation signals do have two sine/cosine (90°) outputs.
Max.

#### Yakima

Joined Jan 23, 2012
35
In the early days of electrical systems there was a two-phase system that was in quadrature, which is 90 degrees out. I believe synchros and resolvers also use quadrature signals, but I never did much with them and what little I did do was nearly thirty years ago.
I am actually an old man talking out of the side of my mouth. I think you're right, there is a 90 degree angle associated with two phase. Pardon me, but I'm just getting back into all of this. Believe it or not I was once quite good at it.

#### WBahn

Joined Mar 31, 2012
26,451
The only two phase system I have come across was 120°, I believe it was used in early induction motor circuits, but it has all but disappeared.
How it was generated or created, I have no idea.
Resolvers used for commutation signals do have two sine/cosine (90°) outputs.
Max.
I haven't heard of one that was 120° out, but I would imagine it would have been generated the same way that the quadrature one was, namely with collinear windings that are mechanically offset by the desired phase angle.

BTW: I am NOT disputing that calling the 120V/240V either single phase or split phase is the preferred name or even that calling it two-phase should be avoided if, for no other reason, than that it does not conform to the common parlance and jargon of the power industry. I'm just not aware of any technical reason why that description would not be accurate, strictly speaking, short of some official proclamation by an appropriate body precluding it.

Joined Jul 18, 2013
25,469
short of some official proclamation by an appropriate body precluding it.
Hear ye:
We are making it here and now, forthwith it will be known far and wide as Single Phase (with centre tap).
Max.

#### profbuxton

Joined Feb 21, 2014
419
Since this supply is powered by a transformer and from what I understand from what has been written here the secondary of this transformer is center-tapped to provide 120/220 out put to a domestic service, is this correct?
No one has described the primary connection so I will make an assumption that it would be a single primary winding connected across two of the three phase lines running in the street, am I right so far?

So I would put forward the argument that the secondary is a center tapped transformer providing 120v each side of the center(with center hard wired to "earth"and providing the "neutral connection") and 220v across the whole winding. This does NOT constitute a "2 phase" system as I understand it. Would a center tapped transformer feeding two diodes to make a full wave rectifier be regarded as a two phase supply, I think not.

Power generators (in base load station) generate a three phase supply over three wires with the voltage peaks being 120 degrees apart. This is a feature of the way the stators of generators are wound and that is how power is transmitted(via suitable transformers) around the country.

I make no mention of specialised generation methods for special purposes.

#### WBahn

Joined Mar 31, 2012
26,451
So I would put forward the argument that the secondary is a center tapped transformer providing 120v each side of the center(with center hard wired to "earth"and providing the "neutral connection") and 220v across the whole winding. This does NOT constitute a "2 phase" system as I understand it. Would a center tapped transformer feeding two diodes to make a full wave rectifier be regarded as a two phase supply, I think not.
What if, in the country of Xanadu, power was generated by a generator containing N sets of stators that produced N voltage outputs that were 360°/M apart. Then one leg of each winding is tied to a neutral conductor and the other N are the hot lines. Would you call that an N-phase system? If N=6, would you call that a 6-phase system? If N=3, would you call that a 3-phase system? If N=2, would you call that a 2-phase system?

#### crutschow

Joined Mar 14, 2008
30,109
In a 2-pole single-phase generator the pair of poles are 180° apart.
In a 2-pole two-phase generator each pair of poles for the two phases are 90° offset (for symmetry) making the two phase voltages 90° apart in phase.

So a split-phase 120V-240V is quite different from a standard 2-phase circuit and should not be called 2-phase.