208Y/120V Wye diagram


Old 04-17-17, 07:39 PM
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208Y/120V Wye diagram

Greetings, I have been attempting to graph three-phase voltage. I understand that the RMS between any two hot legs is supposed to be approximately 208V. My graphing solution is showing that the actual RMS varies between 179.994 and 207.839 at different points of a 360-degree cycle. Is this correct, or am I forgetting something?

This is an animation of the plot with the degree offset step set to a small integer (like 1). The peak voltage is represented by the taller purple bar, and the corresponding RMS is the shorter one.

However If the step is set to 60 or 120 degrees, then the RMS is always 207.839 and it never changes. I think people are used to seeing these graphs frozen in time with offsets of only 0, 120, 240, and 360 degrees. But if you look at the graph at any odd degree that is not a multiple of 60 then the RMS will fluctuate (at least that's what my graph is showing).

Link to graph:

Last edited by electric_dummy; 04-17-17 at 09:26 PM.
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Old 04-19-17, 10:14 AM
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Your forgetting that rms voltage and real volatage are two different things. Rms = root mean square. Peak voltage will be higher then the rms voltage.
Old 04-19-17, 10:59 AM
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I think what you're seeing with the fluctuating RMS voltage may just be an artifact of how you are graphing or modeling it. Over a bounded range, the RMS voltage certainly could fluctuate depending on what start and end points you happen to have chosen for the computation interval. It is also possible that how ever you are discretely representing the continuous analog waveform may be contributing to the bounce. In an actual 3 phase circuit however the RMS does not vary like that. There is enough "smoothness" (for lack of a better word) in the analog power system to cover up any potentially odd behavior in the RMS value.
Old 04-19-17, 12:20 PM
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Correct. 3 phase is a mess if you don't divide the power line cycle in equal parts.
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