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  1. #1
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    Default Can someone explain the difference between phases and poles??

    I am still confused about the meaning of "phase" in the context of typical North American household wiring. Is anyone able to explain the difference to me?

    My understanding is that a typical household is wired with a "Single-phase, 3-wire, common neutral" power supply from the utility company. The power coming from the high voltage line at the power pole is a single phase from the distribution grid. The transformer serving each household steps the power down to 240 volts, which is split into 2 wires carrying 120 volts each, and a center-tap neutral off the transformer. These are the three wires coming into our main distribution panel. Thus the red and the black hot wires are actually different poles of the same phase.

    Electrical discussions sometimes say you can share a common neutral between different phases, since the neutral wire sees the load at different times in the cycle (120 degrees out of phase). My understanding is that this is only true in a 2-phase power supply. However, hooking up a common neutral to circuits fed by a black and red wire in a typical household would be dangerous since in fact the neutral is now carrying 240 volts from the same phase.

    Is my understanding of this correct, or am I confused somewhere? I keep reading stuff, and the terminology does not always seem to be clear.....

  2. #2
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    Default Re: Can someone explain the difference between phases and poles??

    Hey Canuk:

    Thanks for the reply. The discussion you referenced is what got me thinking about this topic...I've read it over several times, trying to figure this out.

    I am still slightly confused...how can the different feed legs be at 90 degrees to each other when they both come from the same phase?? How does that happen? I guess that's where I'm getting hung up.

    Am I correct in that the power produced by the utility at the generating station is typically 3-phase power, with each phase at 120 degrees to each other? By the time it gets to the transformer on the pole outside our house, we are only seeing one phase...the other phases have gone to a different part of the grid. Is this true?

    Also, I assume that household circuits sharing a neutral must therefore be on adjacent breakers, so that each circuit gets power from a different leg. If the breakers were spaced 2 apart, then the neutral would be carrying the full 240 volts. This would be dangerous. Is this correct?

    Any further explainations from the forum would be much appreciated.

  3. #3
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    Default Re: Can someone explain the difference between phases and poles??

    ]3-wire AC 240 volts 60 cycle is still single phase 240 ac just split phase - from the top of a peak to the bottom of a valley the total is 240 but. picture one 120vac side being the top arcs and going flat between for half of the cycle at neutral or zero in the middle and the other 120vac half being the bottom valleys being flat in the middle. it is 120 from the middle to the top of the peak wave and 120 from the middle to the bottom valley of the other half of the spilt wave. 90 degrees would be a real two-phase system we dont have that we have single split phase each split is in-phase so between positive peak and netural is 120 v and negative and netural is 120 v, we're on one phase and that single line is 180 degrees relationship off the netural axis, not two-phase that are 90 degress off peak to peaks (canuk made mistake) or three-phase are three phases each 120 off on the timing cycle of peak).

    so you prolly still have single phase just split to two poles (north/positive south/negative?) the top half or A side and the bottom half or B side. between the two splits rides current on the neutral not voltage. does that make sense?

    not what you'd see on a scope but think of it kind of like this, blue and orange are each split.

    the other way to think of it is kind of like speed walking not running heal to toe like a drunk test on a balance beam or a striped line. as you move forward you transfer your weight from the left foot to the right foot as you make continuous forward motion but until you reach total weight transfer from your left foot to your right foot you do not lift your leftt foot to move forward, and vice versa. unlike running where you spring off each foot and jump to the next foot. if you think of split single phase power like speed walking 120 steps a second (or sixty left foot forward steps and sixty right foot steps) and the weight transfer between steps of your body and its relationship to the ground as neutral that is kind of like how the two splits or legs of the split phase single phase works. if you look at the graphic and think of the center zero as your body and the path you are going in time and turn the graphic sideways and think of the positive half as your left leg and the negative side as your right leg speed walking on a balance beam (or a stripped line) it might make more sense?
    Last edited by Blue RidgeParkway; 09-08-2008 at 11:34 AM. Reason: put in a graphic for explanation.

  4. #4
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by rickpantel View Post
    If the breakers were spaced 2 apart, then the neutral would be carrying the full 240 volts. This would be dangerous. Is this correct?

    Any further explainations from the forum would be much appreciated.
    well for one thing it wouldn't be regular split single phase and the wires especially the netural would have to be much bigger. i don't think this is allowed with less than 1 awg.

  5. #5
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    Default Re: Can someone explain the difference between phases and poles??

    Transformer
    Primary side Secondary side.
    The poles are the three legs of the secondary side of the trans former. The top leg is line 1 (hot) the bottom is line 2 (hot) the center is the common. All legs have voltage and current flow but the center is tied to ground, that references it to ground so you see no voltage difference between it and ground. As the current is flowing out of the top leg it is flowing in the bottom leg so the current flow is in opposite directions on the two legs then reverses direction.This happens 60 times per second. Measuring between the top leg and the bottom leg you would read 240 volts RMS. By using the center tap (common) and the top leg for some of the loads you only measure the current flow and voltage for the top leg in this case 120 volts RMS. The current flow goes out the top leg and in the common then reverses.The voltage goes from 0 to 120 volts RMS to 0 then to -120 volts RMS. That is as referenced to ground.

    Any single leg of the secondary could be connected to ground and you would get the close to the same results. If the top leg was tied to ground instead of the center you would read 240 volts RMS from the top leg to the bottom leg and 120 volts RMS from the center to either of the other two legs. The only difference would be reading to ground (the reference point). In this case you would read o volts from the top leg to ground, 120 volts RMS from the center tap to ground, the difference would be reading from the bottom pole to ground which in this case would be 240 volts RMS.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

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    Default Re: Can someone explain the difference between phases and poles??

    Thanks for the GREAT answers!!!...lots of information here. It will take me a while to understand this stuff. Not easy for a layperson! Thanks again to the forum!

  7. #7
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    Default Re: Can someone explain the difference between phases and poles??

    Yep .... I knew this was going to be a confusing one.
    Unfortunately i added to your confusion with my previous post.

    When leaving out of town on business I brought along a couple of my old text books for some light reading

    So ... I'll take a cue from another member and delete my original post and re - submit the revised version.



    Quote Originally Posted by rickpantel View Post
    I am still confused about the meaning of "phase" in the context of typical North American household wiring. Is anyone able to explain the difference to me?

    My understanding is that a typical household is wired with a "Single-phase, 3-wire, common neutral" power supply from the utility company. The power coming from the high voltage line at the power pole is a single phase from the distribution grid. The transformer serving each household steps the power down to 240 volts, which is split into 2 wires carrying 120 volts each, and a center-tap neutral off the transformer. These are the three wires coming into our main distribution panel. Thus the red and the black hot wires are actually different poles of the same phase.


    Am I correct in that the power produced by the utility at the generating station is typically 3-phase power, with each phase at 120 degrees to each other? By the time it gets to the transformer on the pole outside our house, we are only seeing one phase...the other phases have gone to a different part of the grid. Is this true?
    I think what's causing the confusion is the term " phase" and makes it harder to follow what's going on.

    Without getting too involved with what's happening with power generation , transmission and distribution .... we''ll talk about the residential feed.

    At the point of power generation the electricity is produced from the generators with 3 lines of electricity ..... which alternate their cycles 120 degrees phase shifted apart from each other ...... and sent down the transmission lines. There are 4 wires along the transmission lines ..... with a line for each phase #1 , #2 , #3 and the fourth is a grounded line called the " neutral " . These go to a distribution center and in cases these lines will be split up routed down your street to feed your house.

    For example .... it may be the utility decided to take the line that is phased leg #2 and use it for feeding your street.

    This " single phase " line which is a very high voltage along with a neutral line will go to a transformer to be reduced down to the 240 volts to feed your house.

    As JLM described ..... there is a primary side and a center tapped secondary side to the transformer.

    The high voltage distribution line and the grounded neutral are attached to the primary side and by means of electromagnetic properties the voltage is transfered to the secondary windings to be stepped down ( reduced ) to the 240 volts.

    There are certain induction properties that result with transformers.


    With this type of center tapped secondary there are basically 2 separate windings in series. The mid point between these 2 is the center tap which is a grounded neutral.

    What you see will be 3 wires coming from the secondary which I'll label as A, B, C.

    Referring to Jack's diagram we'll say that A is the bottom .... B is the center tap .... C is the top.

    So ... when you measure the voltage between A & C there is 240 volts.

    When A is referenced to B we see only half .... 120 volts .... with the same between B & C.

    Also .... the sine wave of B & A are 180 degrees opposite ( mirror image ) compared to the sine wave between C & B .

    This is where the term phase is being used and can be the confusing part. So instead I'll use the term " leg ".

    From the transformer there will be 3 wires going to the house ... leg 1 ..... grounded neutral ..... leg 2.

    If you have utility poles you will see this.

    This results in the 3 wire single phase ( more correctly -- split-phase ) used in the average home.

    Originally Posted by rickpantel

    Electrical discussions sometimes say you can share a common neutral between different phases, since the neutral wire sees the load at different times in the cycle (120 degrees out of phase). My understanding is that this is only true in a 2-phase power supply. However, hooking up a common neutral to circuits fed by a black and red wire in a typical household would be dangerous since in fact the neutral is now carrying 240 volts from the same phase.




    Also, I assume that household circuits sharing a neutral must therefore be on adjacent breakers, so that each circuit gets power from a different leg. If the breakers were spaced 2 apart, then the neutral would be carrying the full 240 volts. This would be dangerous. Is this correct?

    Any further explainations from the forum would be much appreciated.
    Now the 3 wire single phase wires come from the utility to the main panel of your home.

    As mentioned earlier these are leg 1 .... grounded neutral .... leg 2 attached to the main panel.

    Also previously mentioned leg 1 and leg 2 are 180 degrees opposite to each other with respect to the neutral.

    This means that one leg will be at plus 120 volts while the other leg will be at minus 120 volts ( referenced to the neutral ) at a given time.

    Inside the main panel the breakers are physically attached to a bus that corresponds to each feed leg. and will be laid out in a staggered fashion to balance the loads.

    For example ....

    Leg 1 may have circuit breaker positions ( 1 , 2 , 5 , 6, etc. ) attached to this bus.

    Leg 2 may have circuit breaker positions ( 3, 4, 7, 8, etc. ) attached to it's bus.

    Using the above example ...

    Now you want to wire a 120 volt light circuit .... you run a black wire from a breaker in position #1 to the black wire of the light with the white neutral returned to the panel ... simple.

    Then you want to wire a 240 volt water heater .... a double pole breaker ( basically 2 separate breakers with the trip mechanism tied together ) is installed so one pole of the breaker is in position #2 and the other pole is in position #4.

    By attaching a red wire to position #2 and the back wire at position #4 there is 240 volts supplied to the water heater .... again simple.

    You noticed the neutral wire was omitted here ... we'll get to that.


    To the topic of shared neutral multi wired circuits.

    Lets say you want to run 2 separate 120 volt 15 amp circuits to power a light and a receptacle.
    By using a 14/3 cable has the benefit of being a little easier running one cable instead of two separate cables.

    The 14/3 cable has a red , black , white and a ground wire.

    So ... with a double pole breaker ...... the red wire ( for the lights ) is attached to one pole of the breaker in position #3 .... the black wire is attached to the other pole of the breaker in position #5 .... the neutral is attached to the panel on one end and is shared or " T " for the the lights and receptacle on the other end.

    You end up with 120 volts on the red wire ( leg 2 ) and 120 volts on the black wire ( leg 1 ) 180 degrees opposite from each other.... referenced to the shared neutral wire.

    This is where it gets tricky....

    ...... lets say the red wire has a load of 6 amps from the lights .... the black wire has a load of 8 amps from the receptacle.

    You would think the shared neutral would have a total of 8 + 6 = 14 amps running through it.

    Actually it would have only 2 amps.

    The reason being the neutral wire only carries the unbalanced ( difference ) current between the 2 legs.

    How this works is by cancellation .

    Remember .... Leg 1 and Leg 2 are 180 degrees opposite .... when one leg is plus 120 volts the other is minus 120 volts and the same applies to the current ( Amps ) component .

    So .... basic math .... (+ 8) amps added to ( - 6 ) amps = 2 amps.

    ( there is a more official method to explain this cancellation ... however ... I chose this simple method for illustrating )



    BTW .... the main service to the home is also shared neutral with respect to the outside transformer.

    Yes there are complications that can arise if the neutral becomes disconnected in a shared configuration.

    I'm spent from all this techno speak so it will have to be discussed later.





    I hope this makes sense and helps.
    Last edited by canuk; 09-12-2008 at 07:44 PM. Reason: consolidate 2 posts
    "" an ounce of perception -- a pound of obscure "
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  8. #8
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    Default Re: Can someone explain the difference between phases and poles??

    the poles are like magnet poles i dont think rick is asking about utility poles.

    the A half or left tap is your A half of your 240 split single phase half being 120 volts to neutral (zero). the B half or right tap is your B half of your 240 split single phase. the two halfs share the same neutral zero on the graph.

    my road has like 7000 volts AC split single phase 60 cycles then to transformer on the pole steps it down to 240 volts split single phase which goes to our meter.

  9. #9
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by Blue RidgeParkway View Post
    ]3-wire AC 240 volts 60 cycle is still single phase 240 ac just split phase - from the top of a peak to the bottom of a valley the total is 240 but. picture one 120vac side being the top arcs and going flat between for half of the cycle at neutral or zero in the middle and the other 120vac half being the bottom valleys being flat in the middle. it is 120 from the middle to the top of the peak wave and 120 from the middle to the bottom valley of the other half of the spilt wave. 90 degrees would be a real two-phase system we dont have that we have single split phase each split is in-phase so between positive peak and netural is 120 v and negative and netural is 120 v, we're on one phase and that single line is 180 degrees relationship off the netural axis, not two-phase that are 90 degress off peak to peaks (canuk made mistake) or three-phase are three phases each 120 off on the timing cycle of peak).

    so you prolly still have single phase just split to two poles (north/positive south/negative?) the top half or A side and the bottom half or B side. between the two splits rides current on the neutral not voltage. does that make sense?

    not what you'd see on a scope but think of it kind of like this, blue and orange are each split.

    the other way to think of it is kind of like speed walking not running heal to toe like a drunk test on a balance beam or a striped line. as you move forward you transfer your weight from the left foot to the right foot as you make continuous forward motion but until you reach total weight transfer from your left foot to your right foot you do not lift your leftt foot to move forward, and vice versa. unlike running where you spring off each foot and jump to the next foot. if you think of split single phase power like speed walking 120 steps a second (or sixty left foot forward steps and sixty right foot steps) and the weight transfer between steps of your body and its relationship to the ground as neutral that is kind of like how the two splits or legs of the split phase single phase works. if you look at the graphic and think of the center zero as your body and the path you are going in time and turn the graphic sideways and think of the positive half as your left leg and the negative side as your right leg speed walking on a balance beam (or a stripped line) it might make more sense?
    Great way to explain it !!!!!you be supprised how many electricians do not know AC theroy
    Harry

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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by Blue RidgeParkway View Post
    the poles are like magnet poles i dont think rick is asking about utility poles.

    the A half or left tap is your A half of your 240 split single phase half being 120 volts to neutral (zero). the B half or right tap is your B half of your 240 split single phase. the two halfs share the same neutral zero on the graph.

    my road has like 7000 volts AC split single phase 60 cycles then to transformer on the pole steps it down to 240 volts split single phase which goes to our meter.
    I realize that ... like I mentioned ... not getting involved with details on the generation side of things.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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