# Current though Neutral in 3phase circuit



## McCready00 (Jan 4, 2010)

How do you calcul the current returning through the neutral in a 3phases circuit.. when only using fictures on 208volts...

Give exemple if you have any!


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## David Ashton (Jan 4, 2010)

for constant loads try
Paul Pelletier - LD Calculator
for dimmed loads, it is too complex, just measure it.


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## mstaylor (Jan 4, 2010)

Are you having a problem or trying to size a neutral for your system. If the latter then use the calculator to size it for a fully loaded dimmer.


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## McCready00 (Jan 4, 2010)

Well.. kind of.. Never really had an issue concerning it but I was just questionnning how do we calcul it on paper... 

Thanks for the info, but I already have the Paul's LD Calculator.. Works fine even though it's sad that it never had been updated.

I got a few books explaining the question, but it was never good enough to give me a clear answer..

If someone could give me an exemple of how it's calculated.. it would help alot..

thanks.


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## derekleffew (Jan 4, 2010)

This thread moved to *Question of the Day* department. Yes, STEVETERRY may answer.


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## STEVETERRY (Jan 4, 2010)

McCready00 said:


> How do you calcul the current returning through the neutral in a 3phases circuit.. when only using fictures on 208volts...
> 
> Give exemple if you have any!



There is no current flowing through the neutral on a 3-phase circuit when only delta-connected 208V loads are used. Neutral current only occurs with 120V wye-connected loads.

ST


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## McCready00 (Jan 4, 2010)

Well but what is the phases are not well balanced?


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## headcrab (Jan 4, 2010)

It doesn't matter if the phases aren't balanced. There is no physical or otherwise connection to the neutral. Thus there can be no neutral current.


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## STEVETERRY (Jan 4, 2010)

McCready00 said:


> Well but what is the phases are not well balanced?



OK, let's clarify what you are looking for:

A. Neutral current on wye-connected 208Y/120V systems that are unbalanced?

or

B. Line currents on 3-phase 208V delta-connected systems that are unbalanced?

ST


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## JD (Jan 4, 2010)

McCready00 said:


> How do you calcul the current returning through the neutral in a 3phases circuit.. when only using fictures on 208volts...
> 
> Give exemple if you have any!



If it is a 120/208 system and the fixtures are operating across the 208 legs, than there is no current traveling on the neutral that is load related. You still need the neutral there, and there will be some stray current there depending on the dimmer filter wiring and how the actual control circuit transformers are wired. Somewhere, there is a nice picture from Derek of a room full of dimmers wired with 208 loads, and I believe he measured neutral current at two amps, but that was mainly due to 120v work lights. (It was last year, so I may be off a bit.)


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## derekleffew (Jan 4, 2010)

JD said:


> ...of a room full of dimmers wired with 208 loads, and I believe he measured neutral current at two amps, but that was mainly due to 120v work lights. ...


JD is likely referring to this post, containing amp meter readings. Note that the dimmers powered only 120V incandescent loads. The 208V distros are for the moving lights.
The neutral current on the ML service can be explained as the FOH console and the optos were plugged into the 120V convenience outlets of the ML PDs.

Here are the pictures--I can't remember where I posted them before.


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## epimetheus (Jan 4, 2010)

Some clarification might be in order here. I'm seeing what strikes me as some confusion when we're talking about delta- and wye-connected systems here. When I here of a 208V, 3 phase system, I assume that the source is wye-connected, as that is easily the most common configuration that produces 208V. In this wye-connected system, each phase is 120V with respect to neutral, and 208V with respect to the other phases. I've not run across a 208V delta-connected source. If you did run across such a source, you would only have 208V available, no 120V.

A 208Y, 3 phase, wye-connected system, denoted as 208Y/120V, can have delta- and wye-connected loads. A 208Y, 3 phase, delta-connected system can only support delta-connected loads. The distinction between how the source is connected and how the loads are connected is an important one.

If I've made any mistakes, someone please correct me.


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## STEVETERRY (Jan 5, 2010)

epimetheus said:


> Some clarification might be in order here. I'm seeing what strikes me as some confusion when we're talking about delta- and wye-connected systems here. When I here of a 208V, 3 phase system, I assume that the source is wye-connected, as that is easily the most common configuration that produces 208V. In this wye-connected system, each phase is 120V with respect to neutral, and 208V with respect to the other phases. I've not run across a 208V delta-connected source. If you did run across such a source, you would only have 208V available, no 120V.
> 
> A 208Y, 3 phase, wye-connected system, denoted as 208Y/120V, can have delta- and wye-connected loads. A 208Y, 3 phase, delta-connected system can only support delta-connected loads. The distinction between how the source is connected and how the loads are connected is an important one.
> 
> If I've made any mistakes, someone please correct me.



Almost correct--one can make a delta connection to a 208Y/120 source in two ways:

A. Connect 3-phase loads that use no neutral, such as motors.
B. Connect multiple single-phase 208V loads between phases, with no neutral connection. This is typical for large moving lights in North America.

It would be rare to find a 208V delta source with no neutral. More likely, a delta source would be 240V or 480V in North America. To complicate matters, that 240V delta source might be a high-leg delta with the neutral as a center-tap of one of the phases, in order to provide a small amount of 120V in a facility that uses mostly motor loads, such as a factory.


ST


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## epimetheus (Jan 5, 2010)

STEVETERRY said:


> Almost correct--one can make a delta connection to a 208Y/120 source in two ways:
> 
> A. Connect 3-phase loads that use no neutral, such as motors.
> B. Connect multiple single-phase 208V loads between phases, with no neutral connection. This is typical for large moving lights in North America.
> ...



I've more commonly seen the 240V high-leg delta used to provide a small amount of 3 phase power to a facility that has mostly 120V loads. The center-tapped transformer is generally the larger of the 3, say a 100kVA center-tapped 240/120V transformer and (2) 50kVA 240V transformers. This is a common commercial and industrial setup that I've ran across multiple times. Though I can't say I like it too much. It makes it difficult to upgrade the 120V portion or the service. I've got a substation project in Utah that I'm working through this problem on right now.


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## STEVETERRY (Jan 5, 2010)

epimetheus said:


> I've more commonly seen the 240V high-leg delta used to provide a small amount of 3 phase power to a facility that has mostly 120V loads. The center-tapped transformer is generally the larger of the 3, say a 100kVA center-tapped 240/120V transformer and (2) 50kVA 240V transformers. This is a common commercial and industrial setup that I've ran across multiple times. Though I can't say I like it too much. It makes it difficult to upgrade the 120V portion or the service. I've got a substation project in Utah that I'm working through this problem on right now.



Interesting! I have never run into that type of configuration before. Thanks!

ST


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## JD (Jan 5, 2010)

I too have seen what epimetheus is talking about, mainly in older installations. It's a standard tapped Delta, just a fatter transformer for the tapped leg. Often, the 240 delta is used by the HVAC and the 120-0-120 for lights, outlets, etc. Can't say I have seen that on any newer construction. (< 40 years!) 

Regarding Wye and Delta, one of the reasons that Wye is popular is that you can load it either way. (Wye or Delta, or both.)


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