Edison circuit on single phase systems

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JesseETG

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I always had the understanding that you are able to share a neutral for multiple circuits if they are on different phases, since each phase is 120 degrees out of sync with the last, the neutral is never carrying the loads simultaneously at any given time. Now this begs the question to why it is allowed on single phase systems since the sin wave is the same. Wouldn't the loads compound and overload the neutral. Maybe I have a misunderstanding on all of this. Please help.
 
A shared neutral in a 3-wire circuit powered from a 3-phase wye system actually does carry some of the load from each wire simultaneously. That's why it must considered a current carrying conductor.

If you accidentally put both hot wires of a 3-wire circuit on the same phase, the neutral definitely can be overloaded.
 
HEYDOG said:
In a single phase system they are a 180 degrees out of phase with L1-L2.
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Hence the ever ongoing dispute over whether it is a single phase or two phases.
From a mathematical standpoint, it really is a single phase since the two collinear (but oppoisitely directed) voltages cannot be combined to break out from that single straight line.
If the two phase vectors (phasors) are not collinear, then from only two phasors, multiplying by different coefficients (including negative numbers) you can produce a sum phasor with any arbitrary direction.
 
It is one sine wave! Just happens to have a center tap which we call neutral and connect to ground. If you put a scope across L1 and L2, it is one waveform (being cautious that the scope case is now hot...). People discuss how one sine wave is opposite of the other sine wave, but that is because they are looking at L1 to neutral (center tap) and L2 to neutral. That is because of the limitation of the scope, not because the waveform is difficult to understand.
 
Sin(x+180)=-sin(x).
But the latter describes the behavior of split phase more accurately in the sense that any perturbations in the waveform will show up at the same spot on the scope in both L-N measurements, not 180deg apart.
 
JesseETG said:
I found this video. Is this man accurate?
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Yes. Good video in my opinion. He does a good job of explaining that whether you see waveforms that look 180 deg apart or 0 deg apart depends on where you put your probes, and is essentially arbitrary. (3-phase is not like that.) I agree with how he defines 'in phase' by the winding direction (which also corresponds to the dot convention or 'dot polarity'). That isn't a nailed down consensus of meaning, which he kind of acknowledges by calling it a 'declaration'. But I agree with it because I think it's the most useful definition.
 
I haven't yet watched the entire video but guessing he didn't expand into what I am about to say, but his setup with the batteries and lamps in the first 5-6 minutes also can be used for explaining why an "out of phase" component in the source there would be sort of equivalent to installing a "bucking transformer" coil in series with a 240 volt source to buck the output voltage to say ~208 volts.
 
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