"Single Phase"" From a WYE

[Joethemechanic]

So if I'm doing my math right if the load is perfectly balanced the neutral current will be about 57.6 % of what the current is on either phase.

So best case neutral is carrying about 58% of the current, and worst case it's at 100%

 

[winnie]

If you have 20A of current on the A leg and 20A on the B leg, you should get 20A on the neutral.

The easiest way to do this calc: imagine you have all 3 legs present, with 20A on each. In this case you get 0 on the neutral. Now drop the C leg current to 0; 20A has to end up on the neutral.

-Jon

 

[Joethemechanic]

I think I'm going to have to do a lab on this. I've only got an open delta here, but down at the laundromat there is 208//120 wye

 

[Jraef]

Just to confuse things, elsewhere in the world they would call it “two phase” because you are using two-out-of-three phases (plus neutral).

The problem with calling it that here in North America is that we already have a system that truly is “two phase” power, still in use in some isolated pockets like the older parts of Philadelphia and around Niagara Falls. So it would be more confusing for us to use that term for two very different systems.

But there is nothing wrong with calling it single phase, because that’s what it is. If you have three phase, you have current that can flow from A to B, B to C or C to A. If you only have current flowing from A to B, you have current flow in only a single phase. The phase to neutral is not a consideration regarding what we call it, that just changes the voltage level.

As to the neutral current, you will not see an imbalance in your single phase panel, because the neutral current from A to N and B to N will will not be additive in that panel. But back at the three phase service point where this is coming from, there likely WILL be an imbalance from all of the single phase panels not being the same current and current flowing on the neutral back to the transformer.

This does a decent job of explaining the neutral current flow.

https://www.electriciansjournal.com/home/calculate-neutral-current.

 

[Joethemechanic]

Just to confuse things, elsewhere in the world they would call it “two phase” because you are using two-out-of-three phases (plus neutral).

The problem with calling it that here in North America is that we already have a system that truly is “two phase” power, still in use in some isolated pockets like the older parts of Philadelphia and around Niagara Falls. So it would be more confusing for us to use that term for two very different systems.

But there is nothing wrong with calling it single phase, because that’s what it is. If you have three phase, you have current that can flow from A to B, B to C or C to A. If you only have current flowing from A to B, you have current flow in only a single phase. The phase to neutral is not a consideration regarding what we call it, that just changes the voltage level.

As to the neutral current, you will not see an imbalance in your single phase panel, because the neutral current from A to N and B to N will will not be additive in that panel. But back at the three phase service point where this is coming from, there likely WILL be an imbalance from all of the single phase panels not being the same current and current flowing on the neutral back to the transformer.

This does a decent job of explaining the neutral current flow.

https://www.electriciansjournal.com/home/calculate-neutral-current.

There has to ba a neutral current with a perfectly balanced load because if the neutral is opened and it becomes a series circuit there will only be 104 volts across each load. If not, we would be violating Kirchoff's Law by 32 volts

 

[Joethemechanic]

So having the neutral is contributing 15% to the voltage

 

[Joethemechanic]

So having the neutral is contributing 15% to the voltage

 

[Joethemechanic]

So perfectly balanced by two resistive loads the neutral current should be 15%

 

[wwhitney]

So perfectly balanced by two resistive loads the neutral current should be 15%

No, it's 100%. See post #42.

Cheers, Wayne

 

[LarryFine]

No, it's 100%. See post #42.

And #2.

 

[jaggedben]

There has to ba a neutral current with a perfectly balanced load because if the neutral is opened and it becomes a series circuit there will only be 104 volts across each load. If not, we would be violating Kirchoff's Law by 32 volts

If it becomes a series circuit, the voltage across each load is proportional to the impedance of each load.

 

[Joethemechanic]

120 degree phase angle at the neutral. point

360/120 = 3

Should be square root of 3 = 1.73

1/1.73 = 0.578

So I'm back at 57.8% on the neutral for a balanced resistive load

 

[jaggedben]

A wye neutral by definition has a sum of voltage waveforms to the line conductors that equals zero. So balanced resistive load currents must also equal zero. I believe you're misapplying theory for calculating line-line currents.

I have to admit I actually hadn't fully studied this before, so I was a bit dissatisfied with posts 2 and 42. I couldn't visualize in my head how the sum of two equal L-N waveforms must have the same amplitude as either waveform by itself. So I graphed sin(x), sin(x+120) and their sum, just to visualize. (It's insanely easy to do this on the web nowadays. Felt spoiled not having to dig out my old TI-81.)

After staring at it for a minute, I realized that the sum must necessarily have the same amplitude, because in order for the third waveform to add to zero, it must be the inverse of that wave form.

sin(x)+sin(x+120)=-sin(x+240)

 

[LarryFine]

After staring at it for a minute, I realized that the sum must necessarily have the same amplitude, because in order for the third waveform to add to zero, it must be the inverse of that wave form.

sin(x)+sin(x+120)=-sin(x+240)

Does that mathematically confirm what we described, i.e., remove one line from a balanced system?

 

[electrofelon]

Another way to think about it is:

If the two lines were the same (0 degrees offset) then the neutral current would be the sum of the two/double. If the two lines are 180 degrees apart, then the neutral current would be 0. So there must be a point of offset where the neutral current will be the same as either line current. Of course this reasoning doesn't provide a specific solution to what that offset is, just a quick sanity check.

 

[wwhitney]

Does that mathematically confirm what we described, i.e., remove one line from a balanced system?

Yes. The statement that sin(x) + sin(x+120deg) + sin(x+240deg) = 0 is the same as the statement that if you add up 3 equal length phase vectors, 120 degrees apart, you get 0. The phase vector (or phasor) is just a convenient representation of the sinewave, when you are dealing with only one frequency.

The idea of looking at the fully balanced case, and adding to that the negative of the case of a single load from line to neutral (i.e. removing that load), is an example of the superposition principle. Electrofelon's last post is an example of the Intermediate Value Theorem.

Cheers, Wayne

 

[Frank DuVal]

Calling these sub panels single phase kind of leads to confusion. People hear single phase and think balanced loads equals 0 neutral current.

What people?

In large apartment buildings/commercial buildings/industrial complexes one should think 3 phase source, even when staring at a single phase panelboard. Only in other residential setting does one see "balanced neutral" single phase.

 

[Joethemechanic]

What people?

Most of the people I work with out in the field. Hell they get confused changing the voltage on a 9 lead motor, god forbid you give them a 12 lead

 

[electrofelon]

Most of the people I work with out in the field. Hell they get confused changing the voltage on a 9 lead motor, god forbid you give them a 12 lead

Why should WE change? It appears they are the ones with the lack of knowledge

 

[s_colligan]

@Joethemechanic: 120 degree phase angle at the neutral

I'd argue your angle is wrong. The 120v/208v service voltages are offset by 30 degrees, not 120 degrees... so for for purely resistive (linear) loads, the 'neutral' currents will be offset by 30 degrees as well.

Remember, all of the 208v loads are connected line-to-line, so the return current (or 'neutral current' as you may call it), travels 'back' on a phase conductor. All of the 120 volt loads are connected line-to-neutral, so all of the return current travels on the neutral conductor.

I think you just need to teach them basic three-phase-power theory in order to fully grasp this concept. As the post states, you need to think three-phase to understand this.

Think of single-phase as how you connect to the three-phase conductors. You can connect these three ways: A-B, B-C, or C-A. Single-phase wye just means that it is a wye connected system that is supplying one single phase (remember it can always be transformed from wye to delta or vice-versa). I suppose a more exact answer is that it is a three-phase wye-connected load center supplying a single line-to-line voltage source and a line-to-neutral voltage source. Better yet, we need to educate our work force to understand what they are working on, so they can fully understand it and probably make it better for us in the future.

You may be confusing your mathematics on a 120v/240v service compared to a 120v/208v service. They are not the same.

Many have correctly answered your question, and as @Jraef pointed out, we apparently have to call things differently and we still call it 'single-phase' in the power industry (even though you could argue that it is sourced by two phases). Youngsters (myslef not included in your bracket) just need to learn how it works.