3 Phase wye connected load and neutral

[Pitt123]

I know that if I have a 3-Phase wye connected load with only 3 wires and no neutral then as long as each leg is balanced, current will be balanced on all three legs and the neutral point will be 0V. However if the phases become unbalanced then isnt' it true that without a neutral to return unbalanced current, the neutral point will elevate to something higher then 0V to ground?

When this happens and neutral point is elevated above ground, then is it possible that the L-N voltages across each load on each of the phases would be something other than 120V? Would all the L-N voltaegs be the same still or could they all vary? In other words, could I have 3 different voltages that weren't 120V for the L-N connection on this 3 phase wye load?

 

[charlie b]

I know that if I have a 3-Phase wye connected load with only 3 wires and no neutral

If it is wye connected, then it has a neutral. If it has no neutral, then it is not wye connected. So we have to change your description of the connections, or the question cannot make sense. Let us say that you have a main panel, with a 4-wire (plus ground) feeder to a sub panel, and from that subpanel you have a three phase, unbalanced load served by three wires (no neutral wire).

However if the phases become unbalanced then isn’t it true that without a neutral to return unbalanced current, the neutral point will elevate to something higher then 0V to ground?

Back at the subpanel, the unbalanced load that it serves will cause the feeder supplying it to have an unbalanced loading as well. But that feeder has a neutral wire. So (again from the perspective of the subpanel) the load side unbalance causes an unbalance current to flow on its line side. The neutral point of the subpanel (i.e., its neutral bar) will indeed rise to a voltage level that is higher than 0 volts. The amount of voltage will be small, and will equal the unbalance current through the feeder’s neutral conductor times the resistance of that conductor. So you will be looking at a volt or two or three.

When this happens and neutral point is elevated above ground, then is it possible that the L-N voltages across each load on each of the phases would be something other than 120V?

Yes. The voltage phase to neutral on each phase will be slightly different, and it is possible that none of them will be 120 volts anymore. The difference will be small, and I doubt that any 120 volt load will be harmed by the difference, nor will any discernable change take place in the operation of any 120 volt load. The exact amount of the difference in phase to neutral voltages will be hard to calculate. It would require an analytical tool called “symmetrical components.” I haven’t used that tool myself in a few years, so I’ll just leave that part of the discussion to someone else.

 

[Pitt123]

If it is wye connected, then it has a neutral. If it has no neutral, then it is not wye connected. So we have to change your description of the connections, or the question cannot make sense.

What if we were talking about a 3 phase wye connected heater on a 3-Phase 3 wire system?

 

[charlie b]

What if we were talking about a 3 phase wye connected heater on a 3-Phase 3 wire system?

Is the question related to what is happening inside that heater, or to the panel that supplies it? If you are talking about the panel, then see my earlier answer. If you are talking about the internals of the heater, then,

1. Yes, its center point will no longer be at 0 volts, and
2. Yes, the voltage of each heater element (measured from phase to the center point) will not be 120 volts.

 

[kingpb]

What if we were talking about a 3 phase wye connected heater on a 3-Phase 3 wire system?

If it requires the N-connection, then you have a problem, because a 3-ph, 3-wire system does not have a neutral to hook too.

You need to verify if you need 3-ph, & a ground, or do you actually need 3-ph, a neutral, & a ground, i.e 3-ph 3-wire vs. 3-ph, 4-wire.

 

[winnie]

Charlie,

I don't see how an unbalanced line-line loading of a panel can cause neutral current to flow. If the example unbalanced load is only connected to the three ungrounded conductors, then there will only be current flow on those ungrounded conductors.

I believe that it is common to call loads 'wye connected' whenever the circuit diagram of those loads looks like a 'Y', eg. a wye connected motor. Such loads have a neutral, but if balanced often do not have a neutral conductor connected.

As I read the OP, he has some wye connected load that requires a neutral conductor, but for whatever reason the neutral conductor is not connected. For example, consider a three phase subpanel with various line-neutral loads. Such a subpanel requires 4 circuit conductors, 3 line plus neutral. How do you calculate the voltage of the neutral bar in this panel if the neutral conductor fails? How do you calculate the 3 line-neutral voltages in this panel if the neutral conductor to the panel fails?

As a quickie answer to my rephrasing of the OP: an unbalanced wye load has a floating neutral point, then the voltage of the neutral point will drift from zero. As the voltage of the neutral point changes, the various line to 'local neutral' voltages will also change. As these voltage change, the current through the connected loads will also change. The 'local neutral' voltage will drift to the point that the the net current in all three legs of the wye is balanced, since the total current flowing into the 'local neutral' must equal the total current flowing out.

-Jon

 

[kingpb]

Charlie,

I don't see how an unbalanced line-line loading of a panel can cause neutral current to flow. If the example unbalanced load is only connected to the three ungrounded conductors, then there will only be current flow on those ungrounded conductors.

I believe that it is common to call loads 'wye connected' whenever the circuit diagram of those loads looks like a 'Y', eg. a wye connected motor. Such loads have a neutral, but if balanced often do not have a neutral conductor connected.

As I read the OP, he has some wye connected load that requires a neutral conductor, but for whatever reason the neutral conductor is not connected. For example, consider a three phase subpanel with various line-neutral loads. Such a subpanel requires 4 circuit conductors, 3 line plus neutral. How do you calculate the voltage of the neutral bar in this panel if the neutral conductor fails? How do you calculate the 3 line-neutral voltages in this panel if the neutral conductor to the panel fails?

As a quickie answer to my rephrasing of the OP: an unbalanced wye load has a floating neutral point, then the voltage of the neutral point will drift from zero. As the voltage of the neutral point changes, the various line to 'local neutral' voltages will also change. As these voltage change, the current through the connected loads will also change. The 'local neutral' voltage will drift to the point that the the net current in all three legs of the wye is balanced, since the total current flowing into the 'local neutral' must equal the total current flowing out.

-Jon

Well stated.

 

[charlie b]

I don't see how an unbalanced line-line loading of a panel can cause neutral current to flow. If the example unbalanced load is only connected to the three ungrounded conductors, then there will only be current flow on those ungrounded conductors.

Start with a three phase, 120/208 volt branch panel. Connect a three phase load that has the ?floating neutral point? you used to describe the OP?s load. Consider it first as being a balanced load, with the same resistance (let it be pure resistance, for simplicity) on all three legs. Let the current in each leg serving this load from the branch panel be 5 amps. Let this be the only load presently in operation from that branch panel. Thus, the currents in the feeder to that panel (A, B, C, and N) are 5, 5, 5, and 0.

Now let?s say the resistance of each leg of the load can be adjusted with a control knob. Let?s turn the three knobs in such a way as to cause the three legs of the load to have an unbalanced current. Adjust them in order to cause the three currents to the load (A, B, and C) to be 10 amps, 10 amps, and 5 amps. Inside the load, the voltages and currents will behave as you described (and I think my description was similar). But what about the feeder to the branch panel? Since this is the only running load, the three phase currents in that feeder will be 10, 10, and 5. Using the formula In^2 = Ia^2 + Ib^2 + Ic^2 ? IaIb ? IaIc ? IbIc, the resulting neutral current in the feeder is 5 amps. That was my point.

 

[kingpb]

Start with a three phase, 120/208 volt branch panel.

A 120/208Y V system is a single phase 3-wire system derived from a 208Y/120V system. Please review ANSI C84.1, they provide a nice chart on preferred voltages used in the US. I can post one if you don't have one handy.:grin:

 

[Smart $]

Start with a three phase, 120/208 volt branch panel. Connect a three phase load that has the ?floating neutral point? you used to describe the OP?s load. Consider it first as being a balanced load, with the same resistance (let it be pure resistance, for simplicity) on all three legs. Let the current in each leg serving this load from the branch panel be 5 amps. Let this be the only load presently in operation from that branch panel. Thus, the currents in the feeder to that panel (A, B, C, and N) are 5, 5, 5, and 0.

Now let?s say the resistance of each leg of the load can be adjusted with a control knob. Let?s turn the three knobs in such a way as to cause the three legs of the load to have an unbalanced current. Adjust them in order to cause the three currents to the load (A, B, and C) to be 10 amps, 10 amps, and 5 amps. Inside the load, the voltages and currents will behave as you described (and I think my description was similar). But what about the feeder to the branch panel? Since this is the only running load, the three phase currents in that feeder will be 10, 10, and 5. Using the formula In^2 = Ia^2 + Ib^2 + Ic^2 ? IaIb ? IaIc ? IbIc, the resulting neutral current in the feeder is 5 amps. That was my point.

Your example is fine until you got to the highlighted part about the neutral. The "floating neutral" load would have no effect on the feeder neutral, which would be zero no matter how you adjusted the knobs.

 

[charlie b]

A 120/208Y V system is a single phase 3-wire system derived from a 208Y/120V system. Please review ANSI C84.1

Thanks, but I decline to be precise in my use of 120/208 or 208/120. I am not alone in this. But I see that we are having this same conversation in another thread, so I'll say no more here.

 

[charlie b]

Your example is fine until you got to the highlighted part about the neutral. The "floating neutral" load would have no effect on the feeder neutral, which would be zero no matter how you adjusted the knobs.

The currents in the four wires of the branch panel's feeder will have to add to zero. If the three phase currents are 10, 10, and 5, then how would you reconcile the situation with Kirchhoff's Law, and still keep the neutral current at zero?

 

[Smart $]

The currents in the four wires of the branch panel's feeder will have to add to zero. If the three phase currents are 10, 10, and 5, then how would you reconcile the situation with Kirchhoff's Law, and still keep the neutral current at zero?

The feeder neutral is not connected to the load's floating neutral. The balance would be maintained by phase-shifted current on the line conductors.

 

[Pitt123]

Great discussion guys thanks for all the good responses.

What if I had a wye connected load as described but now I had a neutral. All phases were balanced so there is no current flow in the neutral. What if now I remove one of the phases or drive one of the phases voltage to zero somehow. Now with only two phases in operation will there be any current on the connected neutral, or will all current (higher in magnitude) only flow between the two remaining phases?

 

[Smart $]

Great discussion guys thanks for all the good responses.

What if I had a wye connected load as described but now I had a neutral. All phases were balanced so there is no current flow in the neutral. What if now I remove one of the phases or drive one of the phases voltage to zero somehow. Now with only two phases in operation will there be any current on the connected neutral, or will all current (higher in magnitude) only flow between the two remaining phases?

You could "drive" the third phase voltage to zero by either opening the conductor path or shorting it to Neutral. The former is the better option :roll:

The loads connected to the other functioning lines would maintain operation as before, only now there will be a neutral current equal to that of each line conductor. For example, with loads of 10A on each of the three lines, it will be tha same as turning off one load. The neutral will then have 10A of current.

 

[jghrist]

The currents in the four wires of the branch panel's feeder will have to add to zero. If the three phase currents are 10, 10, and 5, then how would you reconcile the situation with Kirchhoff's Law, and still keep the neutral current at zero?

The same way that you reconcile the unbalanced currents in the load which add to zero because there is no neutral return. The phase relationships of the currents adjust by virtue of the floating neutral point.

The current from the panel is the same (magnitude and phase) as the load current.

 

[LarryFine]

For example, with loads of 10A on each of the three lines, it will be tha same as turning off one load. The neutral will then have 10A of current.

Only if the load-end's neutral point is connected to the source's neutral point. Without it, you'd simply have two loads in series across 208v, with equal currents no matter what.

A neutral of a MWBC (which includes single, multi-pole loads for this discussion) carries current to (attempt to) keep the load-end's neutral at the same potential as the source.

But, if the MWBC's neutral opens, the load neutral point can vary wildly, depending on each phase's impedance, but the intact service neutral won't carry current because of that.

If you have 10a, 10a, and 5a loads, you'll have a 5a neutral current. If you open the shared neutral, its load-end's voltage will rise up from zero, away from the 5a-loaded phase.

That phase will now fdrop more than 120v, but, again, the service neutral won't carry any current. This is how we feed a Y primary: as a Delta load, with a floating neutral point.

 

[LarryFine]

What if I had a wye connected load as described but now I had a neutral. All phases were balanced so there is no current flow in the neutral. What if now I remove one of the phases or drive one of the phases voltage to zero somehow. Now with only two phases in operation will there be any current on the connected neutral, or will all current (higher in magnitude) only flow between the two remaining phases?

If any of the remaining loads are connected line-to-neutral, there will be neutral current, which is what keeps the load end maintained at (or near) the supply neutral's voltage, which is hopefully real close to zero.


If you have a balanced MWBC, and switch off one phase's load, the neutral current will become what the removed current was. For each amp you remove from a phase, the neutral current increases by that same amount

 

[Smart $]

Only if the load-end's neutral point is connected to the source's neutral point. Without it, you'd simply have two loads in series across 208v, with equal currents no matter what.

...

He said, "What if I had a wye connected load as described but now I had a neutral." I took that as meaning a neutral conductor connected between source and load. :roll:

 

[LarryFine]

He said, "What if I had a wye connected load as described but now I had a neutral." I took that as meaning a neutral conductor connected between source and load. :roll:

Agreed. I only went into that detail because of the OP's original premise: a load with a neutral connected to a source with a nuetral, but the source neutral was not run to the load.