K8MHZ
Senior Member
- Occupation
- Electrician
Books aren't enough. I have had teachers make the same mistake.
Why is residential wiring known as single phase?
- Thread starter TimWA
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Books aren't enough. I have had teachers make the same mistake.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Just because somebody gave them a job teaching, doesn't mean they know anything.
K8MHZ
Senior Member
- Occupation
- Electrician
For teachers casually related to the trade, I can see how it would be an easy error. The one that surprised me was my IBEW teacher. He was very intelligent and knew the subject very well. This came about after I graduated, and I heard it second hand from the EC I was working for. I couldn't believe my ears and made a call to the hall. I got the teacher on the phone and after a while on the phone, he conceded that he was wrong. I was #1 in my class and really liked the teacher, so I was almost honored to catch him on an error. I caught several errors in our text books while I was in class, but I never heard this particular teacher be in error. We got along very well in school so it was just good to talk to him again.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Maybe it's just me. Our original system was 2 phase here in Philly. And there are still remnants of it around. But to me 2 phase is pretty basic stuff
K8MHZ
Senior Member
- Occupation
- Electrician
Practically non-existent here, in fact I have never actually seen a real 2 phase system in operation.
And I'm no spring chicken.
Oh, and since you are from Philly, check your PM. I have a question not related to this thread for you.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Let's break this into bite size steps.
Draw a triangle, label the points A, B and C.
A to B is phase one,
B to C is phase two,
C to A is Phase three.
Three phases. This is what is distributed all over the country.
Now if you connect a transformer that goes from A to B, that is a single phase transformer, right? Because if it was 2 phase, it would have to be connected A-B and B-C, but not C-A.*
So if a transformer connected from A to B is single phase, and you bring the wires of the secondary down to your house, what yo you have at the house? You still have single phase, right? You haven't added any phases have you? You have not made any primary connection to C phase right? So with no connection to C, you can have no B-C phase or C-A phase, right? You only have A-B; one phase out of three.
Now if you take the secondary winding of that transformer and put a center tap in it, then ground that center tap as a neutral, you have "split" that phase into two parts right? In other words half of the voltage from A to N, the other half from B to N. Got that?
So how does putting in the center tap change the fact that the main part of that circuit is still just the one phase you started with, from A to B? Did adding that center tap make a connection to C phase? No.
Now just substitute A for L1 and B for L2. They are not Phase One and Phase Two, they are just opposite ends of the SAME phase. The residential voltage is a SINGLE phase of a three phase system that is center tapped to provide two levels of voltage; from Line to Line (L1 to L2) you have 240V, from either Line to Neutral you have 120V. Still the same SINGLE phase you started with though.
*(For those who know otherwise, let's not get this discussion clogged up by discussing true 2 phase systems yet OK? It just confuses people who have a hard time with this concept)
Draw a triangle, label the points A, B and C.
A to B is phase one,
B to C is phase two,
C to A is Phase three.
Three phases. This is what is distributed all over the country.
Now if you connect a transformer that goes from A to B, that is a single phase transformer, right? Because if it was 2 phase, it would have to be connected A-B and B-C, but not C-A.*
So if a transformer connected from A to B is single phase, and you bring the wires of the secondary down to your house, what yo you have at the house? You still have single phase, right? You haven't added any phases have you? You have not made any primary connection to C phase right? So with no connection to C, you can have no B-C phase or C-A phase, right? You only have A-B; one phase out of three.
Now if you take the secondary winding of that transformer and put a center tap in it, then ground that center tap as a neutral, you have "split" that phase into two parts right? In other words half of the voltage from A to N, the other half from B to N. Got that?
So how does putting in the center tap change the fact that the main part of that circuit is still just the one phase you started with, from A to B? Did adding that center tap make a connection to C phase? No.
Now just substitute A for L1 and B for L2. They are not Phase One and Phase Two, they are just opposite ends of the SAME phase. The residential voltage is a SINGLE phase of a three phase system that is center tapped to provide two levels of voltage; from Line to Line (L1 to L2) you have 240V, from either Line to Neutral you have 120V. Still the same SINGLE phase you started with though.
*(For those who know otherwise, let's not get this discussion clogged up by discussing true 2 phase systems yet OK? It just confuses people who have a hard time with this concept)
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
A big part of the problem is our use of the undefined word 'phase' as if it had a single meaning. Some of the most common meanings include a time shift between multiple values.
It takes two conductors to make one phase voltage (of course the conductors can be so short they are really nothing more than points).
Common names for these two phase conductors include 'line' and 'hot', but typically we just shorten the technical description and call them phases.
The result is confusion: how many phases does it take to make phases?
It takes two conductors to make one phase voltage (of course the conductors can be so short they are really nothing more than points).
Common names for these two phase conductors include 'line' and 'hot', but typically we just shorten the technical description and call them phases.
The result is confusion: how many phases does it take to make phases?
Ok great, that is actually the most lucid explanation I have seen for what ends up in a residential panel. What seems to be always missed though is this. You are saying that what originates as single phase is never going to somehow magically access the unconnected points at the transformer for additional phases. That the two legs in a house panel arise from a center tapping of the single phase of transformer winding and so are single phase, period.
But if the two legs in the house panel, as a result of this center tapping, exhibit AC voltages of opposing amplitudes in time, what is the difference? Isn't that what "phase" refers to? The propogation of a sine wave compared to an identical wave time-shifted so the amplitudes don't match?
NO, it isn't:
"In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply VARY IN UNISON. In contrast, in a three-phase system, the currents in each conductor reach their peak instantaneous values SEQUENTIALLY, NOT SIMULTANEOUSLY; in each cycle of the power frequency, first one, then the second, then the third current reaches its maximum value. The waveforms of the three supply conductors are offset from one another in time (delayed in phase) by one-third of their period."
So to clear all this up, the two AC voltages found in a house panel, though they be of opposing amplitudes, are referred to as single phase because their amplitudes VARY in unison. This will always be the case when voltages arise from the single transformer phase, regardles of how it is tapped. They will vary in unison.
Multiple phases only arise when amplitudes vary NOT in unison, but sequentially.
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Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Wait a minute, aren't you the OP?
So now you are agreeing with us?
Most people take longer than that to come around to right minded thinking, what gives here? :?
- Location
- Wisconsin
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- PE (Retired) - Power Systems
And maybe not. All it proves is that Van=-Vbn.
kwired
Electron manager
- Location
- NE Nebraska
Good job, maybe about the best explanation I have seen that could be understood even by someone without much experience in the trade. And it is a confusing topic for beginners, as well as people that have been around it for a long time.
Now lets pick it apart and go well over 100 posts with details that most don't really need to know or will even have a hard time following:happysad:
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
We can add another 200 posts by asking why we call a switch that is located in 2 places a 3 way switch. Let's not go there...
It shows that Van and Vbn are mutually phase displaced by 180deg.
I know you like to say it's single phase but 180deg apart doesn't look so for me.
Have you come across hexaphase rectifiers.
Transformer primary is three phase. The secondary has six phases (hence hexaphase), one every 60degE.
Here's one I made earlier:
I know your stance on the centre-tapped 120-0-120V from the previous thread on this and that you are unlikely to move from it.
I see it differently.
We can politely agree to disagree.
K8MHZ
Senior Member
- Occupation
- Electrician
But what we are discussing is not 180 degrees apart.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
We can add another 200 posts by asking why we call a switch that is located in 2 places a 3 way switch. Let's not go there...
Ya know, that one has been bothering me for at least 35 years. And if there are 3, it's called a 4 way. Somtin ain't right there. What gives?
What are we discussing then?
T
T.M.Haja Sahib
Guest
If it is 180 degree apart,current in the neutral would be double.
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
LOL, I think we were set up! :slaphead:
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