# Three phase question



## Dennis Alwon (May 9, 2009)

If you are working on a 3 phase system then one neutral can share up to three conductors as long as the conductors are on opposite phases. Also a 3 pole breaker or handle ties must be used on those circuits.


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## 360max (Jun 10, 2011)

...but please note, most plans and specs call for* separate neutrals*.


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## CDN EC (Jul 31, 2011)

3 phase structured wiring? :001_huh:


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## BBQ (Nov 16, 2010)

360max said:


> ...but please note, most plans and specs call for* separate neutrals*.


But please note that is BS, some plans call for separate neutrals and others do not.


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## BBQ (Nov 16, 2010)

CDN mini-EC said:


> 3 phase structured wiring? :001_huh:


I was wondering the same thing.


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## nicosd (Mar 18, 2012)

So I called a buddy of mine and he says 3 phases can share a neutral if it's a 3 phase 220 system, is that correct?


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## nicosd (Mar 18, 2012)

Dennis Alwon said:


> If you are working on a 3 phase system then one neutral can share up to three conductors as long as the conductors are on opposite phases. Also a 3 pole breaker or handle ties must be used on those circuits.


What if circuit 5 is for vending machines, 3 is for plugs and 7 for lights can they share I neutral without a 3 pole Breaker


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## Dennis Alwon (May 9, 2009)

nicosd said:


> So I called a buddy of mine and he says 3 phases can share a neutral if it's a 3 phase 220 system, is that correct?


Yes it must be 3 phase to share the neutral with 3 conductors on opposite phases. The voltage doesn't matter- 240,208, 480, etc.




nicosd said:


> What if circuit 5 is for vending machines, 3 is for plugs and 7 for lights can they share I neutral without a 3 pole Breaker


If they share a neutral they need a common disconnect- 3 pole breaker or 3 sp breakers with a handle tie. 210.4 and 240.15(B)(1)


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## Southeast Power (Jan 18, 2009)

Dennis Alwon said:


> Yes it must be 3 phase to share the neutral with 3 conductors on opposite phases. The voltage doesn't matter- 240,208, 480, etc.
> 
> 
> 
> ...



BTW,
This is something recently required by our beloved NEC so, expect to see hundreds of thousands of installations where three phases share a neutral and have the breakers scattered all over the panel.
A word of caution here. Do not assume that individual three phase loads on a WYE system will cancel each other out on their neutral the same way they will on a Delta system.


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## Dennis Alwon (May 9, 2009)

jrannis said:


> A word of caution here. Do not assume that individual three phase loads on a WYE system will cancel each other out on their neutral the same way they will on a Delta system.


Please clarify. Do you mean because of nonlinear loads I have never heard this except where non linear loads are involved.


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## Fibes (Feb 18, 2010)

jrannis said:


> BTW,
> A word of caution here. Do not assume that individual three phase loads on a WYE system will cancel each other out on their neutral the same way they will on a Delta system.


Of course not, when dealing with a Delta a single winding is center tapped so in essence, it will work the same as any single phase system.


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## Fibes (Feb 18, 2010)

nicosd said:


> So I called a buddy of mine and he says 3 phases can share a neutral if it's a 3 phase 220 system, is that correct?


Are you and your Buddy in the US or somewhere else?


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## Southeast Power (Jan 18, 2009)

Dennis Alwon said:


> Please clarify. Do you mean because of nonlinear loads I have never heard this except where non linear loads are involved.


It seems as though every time I put a meter on a WYE system more current flows through the neutral than I would expect to find.

The neutral current equals the square root of the squared current of A+B+C minus the product of the current between the phases (the current of a x c minus the current of b x c)

So it appears that something interesting has to come back if the load is not perfectly balanced.

It looks like if you had 20 amps on phase A, 20 amps on phase B, nothing on phase C the neutral current should be 20 amps.

BTW,

I have no idea what this means, I jacked it from an article on page 53 of this months Electrical Contractors Magazine.

www.ecmag.com










Link to full article: 

http://www.ecmag.com/index.cfm?fa=article&articleID=13495


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## Spridle (Mar 31, 2012)

nicosd said:


> So I called a buddy of mine and he says 3 phases can share a neutral if it's a 3 phase 220 system, is that correct?


A 3 phase 220 system is a delta configuration. If you install all three legs with a common neutral you should expect the high leg will provide a higher voltage to neutral. In other words don't do it. The neutral is only for the single phase portion of the transformer, 2 hots and 1 neutral. The high leg is never used with a neutral, but only for 3 phase loads.


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