L1 is 180 degrees out of phase with L2, hurr dee durr

Thread Starter

strantor

Joined Oct 3, 2010
5,369
On another forum I got dog-piled by a bunch of folks with the faux-patient condescending tones of educators handling a dolt, insisting that the two legs of 120/240v split phase residential mains are 180 degrees out of phase with each other. This has happened more times than I can count, and I had enough of it. So I made this video. Hopefully it can be of use if the topic ever comes up here. Peer review welcome, I've already caught one error and corrected myself.

 

nsaspook

Joined Aug 27, 2009
7,728
Love the meme, but are you face-palming me or the people saying split phase L1-L2 are 180 degrees out of phase?
It's the split-phase (Phase "the relationship to a specific time reference" vs Polarity "a in or out condition") subject in general, not you or any person.
 
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crutschow

Joined Mar 14, 2008
25,640
This seems to be a matter of semantics.
If you reference each leg to neutral, then there clearly is a 180° phase difference between them, as the sim below shows (V1 and V2 emulate a center-tapped transformer).
Otherwise the voltage sum across the legs would be zero.

But, of course, that is not a 2-phase system.

1598198510279.png
 

ElectricSpidey

Joined Dec 2, 2017
1,328
If the two legs of the service were not out of phase, how would you do load balancing?

It's always been my understanding that when L1 is at maximum current L2 is at minimum current, thus load balancing protects the neutral.

And if current is out of phase, it stands to reason that voltage is also out of phase.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,369
This seems to be a matter of semantics.
If you reference each leg to neutral, then there clearly is a 180° phase difference between them, as the sim below shows (V1 and V2 emulate a center-tapped transformer).
Otherwise the voltage sum across the legs would be zero.
I had a feeling that someone would call it a matter of semantics, which is why I did the transformer experiments with unequal voltages. I should have addressed why I was doing the experiments.
To me, it's not so easy to dismiss as a matter of semantics.
There is an important difference between a wave appearing out phase because of where we've placed our measurement reference (center tap) and actually being out of phase.
If one wave actually is out of phase with another, the sum of the waves is less than the individual constituents.
If one wave only appears out of phase with another, the sum of the waves is more than the individual constituents.

In my opinion, unequivocally stating "L1 is 180 degrees out of phase with L2," is 100% in error, and in a way that transcends semantics. That's the whole reason I made the video.
If L1 were 180 degrees out of phase with L2, then L1-L2 would read 0V.
 

nsaspook

Joined Aug 27, 2009
7,728
It is simply an ordinary single phase AC transformer with a centre tapped secondary winding.
Nothing mysterious!
Max.
Correct! I'm sure it's the misinterpreted (as a plural instead of singular) term phase that makes it mysterious.

Silly semantics example here: If we split a single phase we get 0.5 phase per leg. 0.5 phase per leg? What?
 

crutschow

Joined Mar 14, 2008
25,640
In my opinion, unequivocally stating "L1 is 180 degrees out of phase with L2," is 100% in error, and in a way that transcends semantics.
Who said that?
I didn't unequivocally state L1 is 180 degrees out-of-phase with L2.
I said they were180 degrees out-of-phase with respect to neutral.
And that is 100% true.
 
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Thread Starter

strantor

Joined Oct 3, 2010
5,369
Who said that?
I didn't unequivocally state L1 is 180 degrees out-of-phase with L2.
I said they were180 degrees out-of-phase with respect to neutral.
And that is 100% true.
I didn't mean to imply you said that. I was referring to the "others" who have said such, and sparked this whole thing. It gets said quite often, and by people who should know better.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,369
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