Is split-phase AC the same as "regular" AC?

[pdenisowski]

There are people, especially electricians, who use the word "phase" more loosely, kind-of replacement for the word "circuit". They are free to do so. With a split phase (3 wires) entering the house, saying "phase A" and "phase B" is unambiguous, and unnecessary pedantism is tiresome. But here, the OP wants to understand what it really is, and for that, the answer is simple, split-phase is regular single-phase AC with a center tap for half the voltage. That's it.

Exactly.  One of the things I had to "learn" when I started dealing with AC power related measurements is that electricians and linemen use the word "phase" in a much ... looser ... way than electrical engineers. 

I'm actually working on a video explaining split phase power distribution, primarily because most of our existing content on power distribution comes out of Europe.  A few preliminary slides from the video are attached

[paulca]

There are people, especially electricians, who use the word "phase" more loosely, kind-of replacement for the word "circuit". They are free to do so. With a split phase (3 wires) entering the house, saying "phase A" and "phase B" is unambiguous, and unnecessary pedantism is tiresome. But here, the OP wants to understand what it really is, and for that, the answer is simple, split-phase is regular single-phase AC with a center tap for half the voltage. That's it.

Exactly.  One of the things I had to "learn" when I started dealing with AC power related measurements is that electricians and linemen use the word "phase" in a much ... looser ... way than electrical engineers. 

I'm actually working on a video explaining split phase power distribution, primarily because most of our existing content on power distribution comes out of Europe.  A few preliminary slides from the video are attached

So that answers my prior question as well.  The 3 phase is double the 120V "end line voltage", so there are 6 120V phases, not 3.

[paul cotter]

You cannot directly get a 180 difference from a regular 3 phase system. To get the 180 difference one needs a centre tapped winding on a transformer, that transformer being supplied by one of the three phases.

[paul cotter]

By multiple taps on a transformer with both wye and delta windings it is possible to have 12 symmetric phases on the output which if rectified can deliver DC with 3%ripple BEFORE any smoothing caps.

[madires]

In a 2-phase system the phases are 90° apart by definition and for pratical reasons (https://en.wikipedia.org/wiki/Two-phase_electric_power).

[Randy222]

What Im curious about is 180 does not divide into 360 by any factor of 3.  So how do you get 180* phases from a 3 phase source?

Lets not forget, A rotating mag field cuts coils at different times, this is how phases are made. If a mag field cuts all the coils of the Y at the same time, then you cannot get 3 phases, you have just 1.

We could take a Y, remove one leg, bend one remaining leg so it's 180 from the other, but we only get 2 phases if a mag field cuts each coil at different times.

[paulca]

I remember watching an animated video trying to explain "Why 3 phases?"  It started explaining how the phases were made and how they could be used in balance to provide an effectively "neutral-less" wiring scheme, saving on copper by carrying 3 current carring phases on just 3 conductors.

But it went on to point out that there are many different sets of phases which do this, 6, 12 and many other multiples .

I never do remember the answer as to why most transmission systems stopped at 3.

[Monkeh]

I remember watching an animated video trying to explain "Why 3 phases?"  It started explaining how the phases were made and how they could be used in balance to provide an effectively "neutral-less" wiring scheme, saving on copper by carrying 3 current carring phases on just 3 conductors.

But it went on to point out that there are many different sets of phases which do this, 6, 12 and many other multiples .

I never do remember the answer as to why most transmission systems stopped at 3.

You answered your own question already.

saving on copper by carrying 3 current carring phases on just 3 conductors.

More phases means more windings, more complicated assemblies, more joints, and more wires to loads.

[madires]

I never do remember the answer as to why most transmission systems stopped at 3.

It's the sweet spot of benefits, complexity and cost. In some industrial applications much more phases are used to run large motors. Usually they run their own power plant with a multi-phase generator, or drive the multi-phase generator with a 3-phase motor.

[bdunham7]

Please explain; what is meant by "rotation"?

Polyphase systems (Tesla's original polyphase was two phases with one leading the other by 90 degrees) allow you to set up a series of coils that will generate a rotating magnetic field that can power an induction motor.  It can also start the motor from a stop.  Single or split-phase systems cannot do this and motors powered by this need an auxiliary starting method. 

The term "split-phase" is just a handy way of remembering that in practice it has some attributes of a multiphase system (the voltages add up differently depending on which "phases" you select (L1 to L1 = 0V, L1 to L2 = 240V, L1 to N = 120V, etc) but doesn't have other attributes (rotating field, continuous power transmission)

Remember that you can add taps and so forth to get any number of "pseudophases" (new term I just made up) from a transformer but the key is that their magnitudes will always be in sync.

[IanB]

There are several comments here about two phases "being 180° apart". But how do we know one of them is actually rotated 180° from the other, rather than being reflected or inverted? Reflection and inversion do not involve rotation. It is just an arbitrary choice to say they are phase shifted or inverted. Either is true.

[Randy222]

There are a few ways to create phases. My 3ph drill press has a 1-to-3 VFD.
I just not aware of any big 1ph generator that cuts that up into 3ph for power distribution.
Simple windings separated by 120deg is just easy.

[Analog Kid]

What Im curious about is 180 does not divide into 360 by any factor of 3.  So how do you get 180* phases from a 3 phase source?

You don't of course. Why even ask that question? Why the assumption of 3 phases?
Apples and oranges, man.

[Analog Kid]

Just a quick note to point out that with regard to my other terminology-related thread about "sodder/soLder", I would never walk into an electrical-supply store and tell the person behind the counter that my home had 2-phase electrical service, because they would give me a strange look: "Do what?".

I recognize that this is simply not the terminology used by that particular industry, for historical reasons, and who am I to go against that? I'm just pointing out the absurdity of it from a technical standpoint, but again, I ain't never gonna win that argument. At least not with that person behind that counter.

[TimFox]

“2-phase” is a reasonable term for Tesla’s original system with 90^o difference.
Note that steam locomotives also drive opposite-side wheels in phase quadrature, allowing velocity reversal and starting.  There is a classic O Winston Link photograph pair with simultaneous flashes.
There is a subtle difference between “negative” or “inverted” sinusoids and 180^o phase difference (think about it), but not relevant to this discussion.

[Analog Kid]

There is a subtle difference between “negative” or “inverted” sinusoids and 180^o phase difference (think about it), but not relevant to this discussion.

I thought about it but couldn't come up with any real difference: seems like a distinction without a difference.

I mean:




What's the "subtle difference"? And how could you possibly know whether one signal was inverted with respect to another versus that signal being shifted 180°?

[brucehoult]

What Im curious about is 180 does not divide into 360 by any factor of 3.  So how do you get 180* phases from a 3 phase source?

The kms of poles leading cross-country to the transformer at my road gate (and no further ... I am literally the last house on the line, at the boundary of the NorthPower and Top Energy regions) has only two wires. There is only one 11kV phase. So, for whatever reason, they decided to step that down to 480V not 240V.

My half-hour metering actually records each phase separately, plus another field for the controlled hot water heating.

Here's my data from the electricity company for .. let's do today last year :-) 

NB Ch1 and Ch2 are the two 240V phases. Ch1 is the one my Pecron power station happens to be plugged into a wall socket on. Ch3 is the controlled hot water heating. My "Free Hour of Power" is set to 9-10 PM. I have a number of things in the house that are not powered by the Pecron e.g. water pump, LED ceiling lighting, stove oven, dishwasher, clothes washer. It was a day with plenty of sun for my 2.64kW of panels.

Code: [Select]

ICP          xxxxxxxxxxxxxxx
Serial       xxxxxxxxx
Date         2025-09-28
Flow         X

Interval            Ch 1     Ch 2     Ch 3    Total
---------------  -------  -------  -------  -------
12:01am-12:30am    0.003    0.005    0.000    0.008
12:31am-1:00am     0.002    0.002    0.000    0.004
1:01am-1:30am      0.002    0.002    0.000    0.004
1:31am-2:00am      0.002    0.003    0.000    0.005
2:01am-2:30am      0.000    0.000    0.000    0.000
2:31am-3:00am      0.000    0.000    0.000    0.000
3:01am-3:30am      0.002    0.005    0.000    0.007
3:31am-4:00am      0.002    0.002    0.000    0.004
4:01am-4:30am      0.002    0.003    0.000    0.005
4:31am-5:00am      0.002    0.002    0.000    0.004
5:01am-5:30am      0.002    0.008    0.000    0.010
5:31am-6:00am      0.002    0.001    0.000    0.003
6:01am-6:30am      0.002    0.003    0.000    0.005
6:31am-7:00am      0.017    0.014    0.000    0.031
7:01am-7:30am      0.011    0.007    0.000    0.018
7:31am-8:00am      0.001    0.003    0.000    0.004
8:01am-8:30am      0.001    0.005    0.000    0.006
8:31am-9:00am      0.001    0.005    0.000    0.006
9:01am-9:30am      0.002    0.003    0.000    0.005
9:31am-10:00am     0.001    0.024    0.000    0.025
10:01am-10:30am    0.001    0.009    0.000    0.010
10:31am-11:00am    0.001    0.004    0.000    0.005
11:01am-11:30am    0.001    0.003    0.000    0.004
11:31am-12:00pm    0.001    0.004    0.000    0.005
12:01pm-12:30pm    0.003    0.007    0.000    0.010
12:31pm-1:00pm     0.001    0.003    0.000    0.004
1:01pm-1:30pm      0.002    0.002    0.000    0.004
1:31pm-2:00pm      0.001    0.003    0.000    0.004
2:01pm-2:30pm      0.001    0.002    0.000    0.003
2:31pm-3:00pm      0.001    0.018    0.000    0.019
3:01pm-3:30pm      0.001    0.002    0.000    0.003
3:31pm-4:00pm      0.001    0.002    0.000    0.003
4:01pm-4:30pm      0.001    0.022    0.000    0.023
4:31pm-5:00pm      0.001    0.002    0.000    0.003
5:01pm-5:30pm      0.001    0.005    0.000    0.006
5:31pm-6:00pm      0.002    0.002    0.000    0.004
6:01pm-6:30pm      0.001    0.006    0.000    0.007
6:31pm-7:00pm      0.001    0.002    0.000    0.003
7:01pm-7:30pm      0.001    0.002    0.000    0.003
7:31pm-8:00pm      0.001    0.005    0.000    0.006
8:01pm-8:30pm      0.001    0.002    0.000    0.003
8:31pm-9:00pm      0.002    0.006    0.000    0.008
9:01pm-9:30pm      1.168    0.007    1.596    2.771
9:31pm-10:00pm     1.081    0.004    1.597    2.682
10:01pm-10:30pm    0.003    0.002    0.010    0.015
10:31pm-11:00pm    0.003    0.002    0.000    0.005
11:01pm-11:31pm    0.002    0.002    0.000    0.004
11:31pm-12:00am    0.002    0.002    0.000    0.004
---------------  -------  -------  -------  -------
Day total          2.343    0.229    3.203    5.775


[bdunham7]

There is only one 11kV phase. So, for whatever reason, they decided to step that down to 480V not 240V.

So your 240VAC service is a 480VAC transformer center tapped for neutral (is that grounded?)  and your two "phases" are 240VAC, both relative to the center tap/neutral and 180 degrees apart?  Not 2 of 3 phases from a 240V 3-PH transformer?

If so, you could have a 480VAC single-phase outlet?  I assume this is a smaller installation way out on the end of a long branch, but I'm wondering what the available fault current might be at 480VAC.  It sounds sparkely!

[TimFox]

There is a subtle difference between “negative” or “inverted” sinusoids and 180^o phase difference (think about it), but not relevant to this discussion.

I thought about it but couldn't come up with any real difference: seems like a distinction without a difference.

I mean:

(Attachment Link)
(Attachment Link)

What's the "subtle difference"? And how could you possibly know whether one signal was inverted with respect to another versus that signal being shifted 180°?

Subtlety:  change the frequency and see what happens.  As I said, not relevant to this discussion.

[Geoff-AU]

Split phase is just a single phase bipolar or normal/inverted polarity phase.  Think of it like unbalanced vs balanced audio.  It's the same signal.  As others have said it's just a centre-tapped 240V system.  rather than having 1 x 240V signal and 1 x Neutral signal you have 2 x 120V signals out of phase. 

[brucehoult]

There is only one 11kV phase. So, for whatever reason, they decided to step that down to 480V not 240V.

So your 240VAC service is a 480VAC transformer center tapped for neutral (is that grounded?)  and your two "phases" are 240VAC, both relative to the center tap/neutral and 180 degrees apart?  Not 2 of 3 phases from a 240V 3-PH transformer?

yup

If so, you could have a 480VAC single-phase outlet?

Theoretically possible for sure.

I assume this is a smaller installation way out on the end of a long branch, but I'm wondering what the available fault current might be at 480VAC.  It sounds sparkely!

Literally the last two houses on the line, on the boundary of the two northernmost of NZs 29 "electricity distribution networks", which between the two cover the top 255 km of NZ, from the Auckland / Northland province boundary up.

[paul cotter]

Brucehoult, just one minor correction to what you said. An 11kV supply is normally 11kV across any two phases with ~6.5kV measured from any phase to ground(earth)/neutral. This is the industry standard and I can't imagine things are different in the antipodes. So your transformer would be fed by two phases, 11kV with 120degrees difference.

[paulca]

What Im curious about is 180 does not divide into 360 by any factor of 3.  So how do you get 180* phases from a 3 phase source?

You don't of course. Why even ask that question? Why the assumption of 3 phases?
Apples and oranges, man.

So.  Transmission lines in the US are not 3 phase?  Or do those little dustbin sized transformers you all seem to put on poles converting out two 180* phases from the 3?

In the UK the 11kV line is dropped down by minivan sized transformer like 2m cubed.  hidden in slack land where houses don't fit. That produces spurs of 3 phase which winds it's way under the roads.  One 240V phase brought out and drops a feed into a sequence of houses, another phase does a different sequence of houses and at the end of the cable run there is roughly the same number of house on each phase.

[Analog Kid]

What Im curious about is 180 does not divide into 360 by any factor of 3.  So how do you get 180* phases from a 3 phase source?

You don't of course. Why even ask that question? Why the assumption of 3 phases?
Apples and oranges, man.

So.  Transmission lines in the US are not 3 phase?  Or do those little dustbin sized transformers you all seem to put on poles converting out two 180* phases from the 3?

I'm no expert in power transmission, so I may be mistaken about part of this.
Some transmission lines here are 3 phase, others are not (just split-phase).
There are only 2 high-voltage lines feeding the transformer on a pole behind where I live, so I'm assuming there must be only 2 "phases" (split phase).

Of course, industrial feeds are all 3-phase. Not sure where the division is between the two; seems like all residential lines are split-phase here.

The utility company must have two completely separate power-distribution networks, one split-phase, the other 3-phase.

I don't see how you could derive 2 180°-apart phases from a 3-phase line (0°-120°-240°), which I think is what you were getting at.

[brucehoult]

Brucehoult, just one minor correction to what you said. An 11kV supply is normally 11kV across any two phases with ~6.5kV measured from any phase to ground(earth)/neutral. This is the industry standard and I can't imagine things are different in the antipodes. So your transformer would be fed by two phases, 11kV with 120degrees difference.

There are three conductors further along the road, until the last milking shed. But the poles to my transformer carry only two wires. Yes, I expect they are two phases with 11kV difference and 6.5kV to earth but that's not felt by anything on the secondary side of the transformer which is inherently floating until you earth something.

What I know because I measured it within a few seconds during the fault:

- neutral on all sockets was at 100V wrt the GND pin

- some sockets had 140V phase to neutral

- some sockets had 340V phase to neutral

So I've never measured phase to phase between two different sockets (which would probably need a very long extension cord to a completely different part of the house, not just different sockets in the same room) let alone put both on an oscilloscope, but I'm pretty confident they are 180º apart, and it's not two 120º phases with the transformer using ground-to-phase on the primary side(s).

I do have a very long extension cord I normally use with weed trimmer or pressure washer, and multimeter and 4 chan oscilloscope so I guess I could make the test ... but of course neutral pin is solidly the same as earth pin at all sockets NOW.