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Phase to Phase vs Phase to Neutral

  1. Jul 7, 2012 #1


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    Is there any difference? I have been thinking about this to the point of confusion. Basically I have a delta 120/208 3 phase service at the shop and we work at a place with 277/480 a lot. I have a test box to connect motors and such and it's crude at best having just a breaker and motor starter. The problem is that the coil for the motor starter is 120. At the shop I just hook one wire to one phase and the other wire to the neutral, being 120/208, it gives it 120.

    At the other place, I'm obviously screwed. The plan is to use a transformer to make it work.

    Now the other thing is that the shop has a wild leg on that delta, so I have to be careful when I am aiming for 120.. I guess its 208 on the wild leg and 240 between each phase. Since each phase is the same voltage be it at the shop or at the other place (all are 240 P-P or 480 P-P), why can't I just run 2 phases into an autotransformer and ditch the neutral issue altogether... 2:1 setting for the shop and 4:1 at the other place.

    But is it that simple? I keep wondering if the 120* phase angle between phases will mess with it since phase to neutral is 180*. Am I wrong here? Am I just thinking too much? Thanks guys.
  2. jcsd
  3. Jul 8, 2012 #2


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    I doubt it. You have 120/208 WYE, or 120/240 DELTA. A WYE connected system doesn't have a wild leg that I have ever heard of. The wild leg voltage on a DELTA system is always lower than the leg to leg voltage but higher than the center tap-to-leg voltage (120 in alot of cases). The term 'wild' refers to a voltage between ground (center tap on one phase) and the leg that is NOT connected the center tapped transformer. Not sure I want to give you any more advice here (yet) since I'm not sure you fully understand what's going on. A bit more discussion will reveal whether this is so.
    Edit: You should probably get used to calling things by their proper names. What you call phase to phase is more properly called leg to leg. What is known as a phase is a pair of conductors. Sounds nitpicky, I know, but it does avoid confusion especially in places like pysics forums. And yes, I am guilty of doing the same thing from time to time.
    Last edited: Jul 8, 2012
  4. Jul 8, 2012 #3


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    I just looked up the stuff on the net, I didn't remember the actual voltages but there is definitely a wild leg at the shop.. I understand all the rest you are saying, just a lack of remembering what the voltages were on my part, the I found a diagram on the net must be wrong too because it showed 240 leg to leg with the high leg to ground being 208 so who knows what they were smoking.
  5. Jul 12, 2012 #4


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    Sooo nobody? All I wanna know is if I can hook the coil of a motor starter between legs if I use a transformer to reduce it to the voltage I need..
  6. Jul 13, 2012 #5

    jim hardy

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    No takers yet ?

    If i understand -

    don't use an autotransformer. Autotransformers dont isolate between primary and secondary
    so none of your wires will be a neutral voltage . That might not be important if the box is insulated for 480 but it'd worry me.

    But you are quite right here -

    if there's only two wires with 240 volts between them it's single phase 240,
    if there's only two wires with 480 volts between them it's single phase 480,
    it takes more than two conductors to make more than one phase. Two points determine a line, takes 3 to make a triangle.

    so what you propose will work but for safety's sake get a control transformer with isolation between HV and LV windings.
    Make DOUBLE SURE both those wires are insulated from the test box and the insulation is in good shape. You don't want to pick up a handful of 480.
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