low voltage issue

[hilltop]

Need input on low voltage issue.

Readings at the feeder lugs of a 3 phase 480V distribution panel are
A phase to nuetral = 268V
B phase to nuetral = 246V
C phase to nuetral = 284V

Voltage to ground on each phase is within 1 volt of 264V in the same distribution panel.
This panel is fed by a main distribution load center which is fed by a new 4000A service.

I have not had a chance to open up the load center that feeds this panel, however; the new 4000A service has been tested and commisioned by the contractor who installed it.


What would be the likely cause of this difference in voltage readings?

 

[masterinbama]

what is your phase to phase voltage?

 

[mivey]

What are the currents? Is your neutral floating?

 

[bob]

Need input on low voltage issue.

Readings at the feeder lugs of a 3 phase 480V distribution panel are
A phase to neutral = 268V
B phase to neutral = 246V
C phase to neutral = 284V
What would be the likely cause of this difference in voltage readings?

Check the load to see if it is balanced. If it is close to being balanced,
then the utility has low voltage on two of their phases. You will need to get
the utility to check the phases and balance then to increase the voltage on phases A and B. The voltage should be about 277 volts phase to neutral.

 

[mivey]

Check the load to see if it is balanced. If it is close to being balanced,
then the utility has low voltage on two of their phases. You will need to get
the utility to check the phases and balance then to increase the voltage on phases A and B. The voltage should be about 277 volts phase to neutral.

He said it was 264 to ground +/- 1 volt so I'm assuming balanced.

As for the voltage, that is within acceptable levels considering he is at a distribution panel. The service could normally be as low as 263 and be fine, although marginal.

 

[bob]

He said it was 264 to ground +/- 1 volt so I'm assuming balanced.

As for the voltage, that is within acceptable levels considering he is at a distribution panel. The service could normally be as low as 263 and be fine, although marginal.

263 is not good enough for me. If the load is ballanced then I would have the utility check its system.

 

[brian john]

What are the currents? Is your neutral floating?

1, Check this first.

 

[mivey]

263 is not good enough for me. If the load is ballanced then I would have the utility check its system.

Considering that this is two panels away from the service, it is well within industry standards so would be deemed good enough as far as the utility is concerned.

The acceptable service voltage is 263 to 291 volts. The acceptable utilization voltage can be as low as 254 volts. These ranges are for the sustained voltage levels and does not mean the voltage can't stray outside these levels temporarily.

It is possible the utility could change the taps on the service transformer if it would not adversely effect any other customers. But, the transformers do not always have taps available.

It never hurts to ask the POCO as they may have a some room on their voltage regulators but I would not expect there would be a lot. They may also be willing to install some regulators close to you, but have no obligation to do so since the voltages, while closer to the minimum than the maximum, are within industry standards.

For that matter, the ANSI standard is a suggested level and the utility may not be required to follow them, although I would expect they would follow them, or some similar standard.

 

[hurk27]

the average voltage across all phases is 266 volts, with the high 284 on C and the low 246 on B, while balance can be a problem, and it looks like a weak neutral, it seems the whole service is low, but I would check the neutral a little deeper by loading one phase and see how far it drops.

 

[hurk27]

He said it was 264 to ground +/- 1 volt so I'm assuming balanced.

Errrr what voltage drop would you have on a grounding conductor, if there should be no current on it?:-?

the current is on the neutral. grounding should show the service voltage at the MBJ.

 

[mivey]

Errrr what voltage drop would you have on a grounding conductor, if there should be no current on it?:-?

the current is on the neutral. grounding should show the service voltage at the MBJ.

Errrr what voltage drop on the grounding conductor are you talking about? I said nothing about that. FWIW, with no current on the ground, the voltage drop on the grounding conductor is zero.

Also, measuring the voltage to ground will not show the service voltage unless there is no load on the system or the voltage drop is less than your measured resolution.

 

[hurk27]

Because this statement was about the voltage measurement to the grounding conductor.

He said it was 264 to ground +/- 1 volt so I'm assuming balanced

the phase to the grounding voltage will always be balanced:grin:

the phase to grounding or neutral at the MBJ will or should be the same voltage as the phase to ground at any panel down streem.

 

[mivey]

the phase to the grounding voltage will always be balanced:grin:

No it won't.

the phase to grounding or neutral at the MBJ will or should be the same voltage as the phase to ground at any panel down streem.

No it won't.

 

[quogueelectric]

I would suspect that the service is in a fringe area of the utility with a poorly balanced load on the utility side. I have run across this problem before where the electronic telemechaniqe OL nits trip on a volt differential of more than 10% causing havoc at the facility.

 

[mivey]

I would suspect that the service is in a fringe area of the utility with a poorly balanced load on the utility side. I have run across this problem before where the electronic telemechaniqe OL nits trip on a volt differential of more than 10% causing havoc at the facility.

These readings were at a 7.5% unbalance, but I'm still not convinced it is the utility until we get more info from the OP.

The utility should try to keep it at less than 3% at the service and I like to see below 2%.

 

[broadgage]

The voltages to ground are well balanced, though a bit low, this suggests that the transformer, the HV supply into the transformer, and the three hot conductors of the service drop, are all in order.

The problem would appear to be an undersized or poorly connected neutral conductor, any voltage drop in the neutral would reduce the phase to neutral voltage on the most loaded phase, and increase the voltage from neutral to the least loaded phase.
This could give the readings repported.

The O/P does not state what the phase to voltages are, but I would expect these to be well balanced, though a bit low, probably about 460 volts.

Although better balancing of the single phase loads between phases should help, the correct approach would be to locate and fix the high resistance neutral.
If the neutral is correctly connected but very undersized, then it is probably getting hot, which is undesirable.
If the neutral has a poor connection, the many KW may be disipated at the point of poor connection, with consequent risk of fire or equipment damage.

 

[hurk27]

No it won't.

No it won't.

I guess I wasn't thinking too straight last nite:-?

I was thinking that measurements to the grounding conductor will reflect the same imbalance that you would have back at the service, which in this case would seem the service is balanced, its true the voltages to ground two panels away from the service, will also reflect the voltage drop of each phase also, but this would be limited.

 

[mivey]

I guess I wasn't thinking too straight last nite

You can't be held accountable for what you say after bedtime.:grin:

I was thinking that measurements to the grounding conductor will reflect the same imbalance that you would have back at the service...

With added balanced loads in the feeder you will see the same type imbalance, but the voltage levels will be different. With no loads in the feeders, you should see the same voltage as at the service, unless there is a stray current present.

...which in this case would seem the service is balanced...

That's what I'm thinking

...its true the voltages to ground two panels away from the service, will also reflect the voltage drop of each phase also...

and that seems to be the key here. Something about the neutral is odd. That's why my first post was asking about the currents and if there was a floating neutral. It appears the neutral has shifted from what would normally be the grounded neutral point and moved towards the ungrounded equilibrium point.

...but this would be limited.

Not any more than with any normal conductor.

 

[Rick Christopherson]

Something about the neutral is odd. That's why my first post was asking about the currents and if there was a floating neutral.

I agree. As I look at this, the "assumed" voltage drops at this location do not add up to a normal condition.

Because the phase-to-ground voltages (and assumed voltage drops) are nearly balanced, then it can be somewhat safely assumed that the phase currents are fairly well balanced. That doesn't mean that the neutral currents have to be as neatly balanced, but it does if the neutral currents are approaching the same level as the phase currents.

In other words, if the voltages between phase and neutral were closer to phase to ground voltage (low voltage drop on the neutral) then some imbalance would be expected. However, because the voltage drop on the neutral, as referenced from the grounding conductor, is approximately 22 volts at 90deg (A-phase ref), this is too large of an (assumed) voltage drop for there to be a high imbalance in the phase currents.

For the phase currents to be nearly balanced and the neutral current to be significantly unbalanced, the individual components of the neutral current must be a small percentage of the phase currents--meaning that the total voltage drop on the neutral should be pretty low, but it is not.

While it is just speculation without more data, I think that these voltages strongly point to a floating or high impedance neutral.

 

[augie47]

Does this not sound like a undersized neutral or poor connection ? If I understand the OP, this is a subpanel off a 4000 amp distribution panel.
If his voltages to ground are decent and only the voltages to neutral are in question, would that not indicate a poor neutral on this feeder ?
(assuming we don't have a similar problem on other panels)