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Old 08-24-2011, 08:00 PM   #1
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3-phase and neutral wire.


Hey guys, can somebody please explain, or preferably give examples of why a 3-phase set up might have a neutral wire. I thought that the loads are simply connected between phases.

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Old 08-24-2011, 08:08 PM   #2
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3-phase and neutral wire.


'Cause the single phase line to neutral loads need it if there are any.

All services require a neutral, but not all panels.

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Old 08-24-2011, 08:08 PM   #3
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3-phase and neutral wire.


3 phase commercial oven with 120V controls.
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Old 08-24-2011, 11:01 PM   #4
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3-phase and neutral wire.


Example of need: a typical 120v receptacle.
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Old 08-24-2011, 11:38 PM   #5
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3-phase and neutral wire.


277V lighting circuits. 480V equipment with 277V controls. Lots of things. Any single-phase component of a 3-phase load could be connected either across two phases or from one phase to neutral.
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Old 08-25-2011, 03:10 AM   #6
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3-phase and neutral wire.


To compound the mix in here with triphase supply they will come in either WYE or DELTA system.

Most Wye system is useally found most place where you have phase to netural load connected the voltage will varies a bit depending on the POCO supply and location.

I will give you a quick USA side voltage on WYE connection
208Y120
480Y277
600Y347 { not too often in USA but pretty common in Canada }

In France we do have 415Y240 volt system as well { the Delta service is pretty rare in France you will see Delta part in a moment }

Those connection I mention above that is 4 wire set up { 5 with grounding conductor included } but it can use in 3 wire format as well

The 208Y120 is pretty common found in med to large apartment complex and office building and few case large homes as well { if the POCO do allow it with very large service } however 480Y277 is more common used in commercal location like larger office and industrial useage. The 600Y347 is the same as 480Y277 again used in large office and commercal and industrail { most used in Canada but few spots in USA do use them as well }

Now for DELTA conneciton they also can come in either three or four wire format however this part can get tricky due you will get involded with wild leg Typically either 208 or 416 volts depending on which system it used

The typical Delta voltage system will be :

240D120 4W
240D 3W
480D 3W { few did use 4W }

but for Delta service that is slowly dropping out of the POCO system for safety reason { there were some issue with wild leg connection and connection at the meter et breaker box }

For the colour code in Stateside there is no specfic part in NEC code however the white et grey and green are off limit colour due the white or grey is genrally used with netural ( grey is in iffy spot depending on which code cycle it involded ) and Green always for grounding { earthing }

And one colour they used on delta for wild leg will be marked or have orange colour conductor { just be aware with old delta system will have red instead of orange marking } and genrally the wild leg will be on B phase but old system it will be either B or C phase.

For common colour code on triphase this will cover most states unless noted otherwise.

240Y120 or 208Y120

Phase A black
Phase B ( Wye Red / Delta Orange )
Phase C bleu

480Y277 and 480 Delta

Phase A Brown
Phase B Orange { few will required Purple }
Phase C Yellow

There were few case all the phase conductors can be all black on phase part { I have see it from time to time }

There is other part of Delta system but it called Corner grounded delta I will discuss that other time. { it kinda compaicted part }

Just Beware that with triphase supply the colours are not always the the correct sequenice and the best answer with this part is have a electrician assist you on this one there is too many thing can go wrong real quick and I have see aftermath what some people try to repair on triphase system.

If have more question just ask one of the electricians in this forum we will give you more details with it.

Merci,
Marc
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Old 08-25-2011, 08:21 AM   #7
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3-phase and neutral wire.


Standard 3 phase systems in the U.S. with 120 volts use a neutral for all 120 volt loads and use phase to phase connection for other equipment (240, 208 volt as the case may be).
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Old 08-26-2011, 07:24 PM   #8
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3-phase and neutral wire.


Why is it 208 volts?
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Old 08-26-2011, 10:35 PM   #9
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3-phase and neutral wire.


sorry about "butting in" but this made me think: coming into your panel are 2 hot wires, a ground and a neutral. are the 2 hot wires "in phase" or are they 120 deg. out of phase with each other?

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Old 08-26-2011, 10:56 PM   #10
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3-phase and neutral wire.


Quote:
Originally Posted by jlmran View Post
Why is it 208 volts?
The nature of 3 phase power is such that if hot to neutral is 120 volts for each of the 3 phases then hot to hot (different phases) is 208 volts.

If hot to neutral is 120 volts and hot to hot is 240 volts then the aforementioned hot to neutral will be 120 volts for two of the three phases and will be 208 volts for the third phase.
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Originally Posted by analogmusicman View Post
sorry about "butting in" but this made me think: coming into your panel are 2 hot wires, a ground and a neutral. are the 2 hot wires "in phase" or are they 120 deg. out of phase with each other?,
If you have a 120/240 volt feed then the two hot to neutral combinations are 180 degrees out of phase with each other. (This is not called a 2 phase system)

Sometimes the two hots coming into a panel are two phases of a 120/208 volt 3 phase system in which the two hot to neutral combinations are 120 degrees out of phase. If you took the two hots and did not use the neutral then there is no way of distinguishing that 208 volt current from the current coming from a single phase source for example a 120 to 208 volt single phase transformer or a 240 to 208 volt single phase transformer.

Also, any two hot to hot combinations on a 3 phase system are 120 degrees out of phase.
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Last edited by AllanJ; 08-26-2011 at 11:09 PM.
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Old 08-26-2011, 11:00 PM   #11
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3-phase and neutral wire.


Quote:
Originally Posted by jlmran View Post
Why is it 208 volts?
When you measure across phases on a three phase distribution, one of the legs will be 120 deg lagging or leading the other phase. If you could stop the cycles and take an instantaneous reading when one phase is at peak you would read 120 volts to common on that phase but the other would read 88 volts. Technically it is more complicated than this but this should give you a basic understanding. 120 + 88 is 208
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Old 08-27-2011, 12:46 PM   #12
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3-phase and neutral wire.


A common method of explaining 3 phase and other similar topics is vector diagrams.

Take an ordinary sheet of paper and put a dot in the middle. THis will be "neutral". For one phase draw a line from "neutral" horizontally to the right, make the line 120 millimeters long. (Let's call 1 mm equal to one volt). Put an arrow head at the far end. Let's label the far end "A".

For the second phase, draw a line from neutral up to the left 120 degrees above horizontal and label the far end "B". For the third phase, draw a line from neutral down to the left 120 degrees and label the far end "C". At this time ask yourself the question: "Why does the term "wye" apply to this system?"

Measure the distance between A and B. Ths srepresents the voltage between legs A and B. It will be about 208 mm.

I'll leave it as an exercise to do the 120/240 volt delta 3 phase system vector diagram. Hint: Start with a horizontal out from neutral to the right to be the A phase and a horizontal out to the left to be the C phase with arrow heads at the far ends. Note that A to B should also be 240 mm. Did you verify that B to neutral should be 208 volts?

A delta 3 phase 120/240 volt system has one (asymmetric) neutral, not three. Some 3 phase systems have no neutral and have just one nominal voltage such as 240 only. The 240 volt delta systems are intended for major 240 volt loads while the 120/208 volt system is intended for mostly 120 volt loads.

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Last edited by AllanJ; 08-27-2011 at 01:20 PM.
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