High current on neutral with a balanced load

jhardy13

Member
Location
Joplin Missouri
Occupation
Industrial Engineering student
One way to go about this is to first consider what assumptions are being made in order for the current in the neutral conductor to be equal to the difference between currents on line conductors L1 and L2. And then determine and list possible ways these assumptions could be violated and cause a different result, such as the excess neutral current you're observing.
Here are some thoughts:

Assumptions
1. The vector sum of the sinusoidal currents through L1, L2, and N is zero.
This means that if you put a clamp meter around the L1, L2, and N you should see zero current.

2.The L1-N and L2-N currents are 180° apart.


Ways the assumptions can be invalid and resulting consequences

Assumption #1 would be invalid if there are any currents that do not flow between the L1, L2, and N conductors on the load side of the clamp, and therefore represent unwanted leakage or “common-mode” current on another path that’s not going through your clamp.
Things that could make this happen are:
a. Paths which allow current to bypass around the clamp meter.
b. External sources of current applied on the load side of the clamp meter.

If assumption #2 is not true then you can get a current on the neutral conductor even though L1 and L2 have equal RMS currents and assumption #1 is true.
But you’ve already verified that the L1-N and L2-N voltages are at 180°. So the only way that their currents could depart significantly from 180° is if the power factors on L1 and L2 were very different. In your situation I think this is very unlikely to be causing the amount of neutral current observed.
I appreciate your input. Why do you say you don't think it could be power factor? Also, if current was finding an alternate path back to the transformer wouldn't that mean I would see less than the difference between the two phases on the neutral? The excess neutral current is always 3amps or more. In the last picture I uploaded it is 8.2amps excess. That seems pretty substantial.
 

jhardy13

Member
Location
Joplin Missouri
Occupation
Industrial Engineering student
I wonder if symptoms like the OP has could exist in the following hypothetical scenario:

The main panel has two neutral bars NB1 and NB2 which don't have a jumper between them.
The grounded service conductor (neutral) is connected to NB1, but NB1 doesn't have a bonding screw or jumper to the the panel. However, NB1 is indirectly connected to the panel because the grounded service conductor is bonded to the meter housing and there's a metallic conduit between the meter and panel.
Also assume that NB2 is bonded to the panel with a screw or jumper.
And for the sake of argument NB1 and NB2 are connected to branch circuits fed by lines L1 and L2 respectively.

In the scenario above the currents from L1-N and L2-N loads do not go through a jumper between NB1 and NB2 (like they should). Instead these currents must go through a roundabout path that includes the section of service neutral conductor between it's bond to the meter housing and NB1.
So in this case if the clamp meter is put on this section of neutral conductor it could show significant current even if the L1-N and L2-N loads are balanced.
I can't tell from the pictures inside of the meter housing whether the clamp on the neutral was placed on the service or load side of a bond point to the housing. If the clamp was on the service side then it would not be on the roundabout path mentioned above, and so it would not show excess current.

If the Op doesn't have two neutral bars then none of the above is applicable to his situation.
I do have two neutral bars but I don't know if they have a jumper. I actually had this same theory. I will look into this and update.
 

jhardy13

Member
Location
Joplin Missouri
Occupation
Industrial Engineering student
Do you have any unusual electrical equipment in the house? Particularly transformers but more likely how about UPS battery backup systems for computers or electronics. Cutting power kicks a UPS into operation that is supplying the neutral or is causing the unbalanced neutral during normal power.
No unusual electrical equipment that I am aware of. We do not have a UPS system. We do have a 24 volt transformer in our panel but I don't know what it is powering. I will check the current coming from it.
 

robertd

Member
Location
Maryland
Occupation
electrical contractor
No unusual electrical equipment that I am aware of. We do not have a UPS system. We do have a 24 volt transformer in our panel but I don't know what it is powering. I will check the current coming from it.
>We do have a 24 volt transformer
Doorbell, most likely.
 

techmavin

Member
Location
New Jersey
What is the voltage between neutral and ground ?
Also, what is the votage between the 2 legs?
What is the current in the 2 live legs?
Is any of the neutral bars connected to ground?
 
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synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Why do you say you don't think it could be power factor?
In your first picture the neutral current exceeded each of the L1 and L2 line currents. If these line currents were equal then this would require them to be at phase angles less than 120° apart from each other. Or in other words a more than 60° departure from the normal 180° between the L1 and L2 currents if there were purely resistive loads.
One scenario to make this happen would be resistive loads on L1 (i.e., power factor PF = 1.0), and a 120V motor but no other loads on L2 that might increase the PF of the total L2 current. Then the motor PF would have to be less than cos(60°) = 0.5 which implies the motor is running at less than 1/2 of its full load amperage. At half load a 5HP motor would typically draw a little over 20A at 120V.
Adding any L-L loads of course increases the line currents but not the neutral current, and so the difference in PF's between the L1-N and L2-N loads would have to be even more to keep the neutral current higher than the line currents.
The bottom line is that it's unlikely that PF is causing your excess measured neutral current, unless you have a situation like I described above.

Also, if current was finding an alternate path back to the transformer wouldn't that mean I would see less than the difference between the two phases on the neutral? The excess neutral current is always 3amps or more. In the last picture I uploaded it is 8.2amps excess. That seems pretty substantial.
There could be an alternate path that you are not including in your measurements but it doesn't have to be all the way back to the transformer. In the example I posted with two neutral bars, the alternate path started in the watt-meter at the point the grounded conductor (neutral) was bonded to the meter housing. The alternate path was the metallic conduit between the meter and panel.

Also the current isn't necessarily reduced because there is an alternate path. It would only be reduced if the currents on the main and alternate paths are flowing in the same direction and then split. In the example I gave with two neutral bars the AC current flows back and forth through a section of the main path (neutral conductor) and the alternate path (conduit), from one neutral bar to the other. This is because the current on one neutral bar is 180° out of phase with the other because the line voltage on their loads is 180° from each other.

In the ideal case the branch circuit neutrals are tied to one point and balanced L1-N and L2-N currents flow from one leg to another, with no net current on the neutral service conductor. In the scenario I gave with two neutral bars the point where the L1-N and L2-N currents are tied has been brought upstream to the neutral / meter housing bond connection. And so the neutral current upstream of this bond should be small with equal L1-N and L2-N loading.
 

oldsparky52

Senior Member
I wonder if symptoms like the OP has could exist in the following hypothetical scenario:

The main panel has two neutral bars NB1 and NB2 which don't have a jumper between them.
The grounded service conductor (neutral) is connected to NB1, but NB1 doesn't have a bonding screw or jumper to the the panel. However, NB1 is indirectly connected to the panel because the grounded service conductor is bonded to the meter housing and there's a metallic conduit between the meter and panel.
Also assume that NB2 is bonded to the panel with a screw or jumper.
And for the sake of argument NB1 and NB2 are connected to branch circuits fed by lines L1 and L2 respectively.

In the scenario above the currents from L1-N and L2-N loads do not go through a jumper between NB1 and NB2 (like they should). Instead these currents must go through a roundabout path that includes the section of service neutral conductor between it's bond to the meter housing and NB1.
So in this case if the clamp meter is put on this section of neutral conductor it could show significant current even if the L1-N and L2-N loads are balanced.
I can't tell from the pictures inside of the meter housing whether the clamp on the neutral was placed on the service or load side of a bond point to the housing. If the clamp was on the service side then it would not be on the roundabout path mentioned above, and so it would not show excess current.

If the Op doesn't have two neutral bars then none of the above is applicable to his situation.
I'm not sure I'm following you on this, but from what I think you are saying I'm not sure it explains the neutral current being higher than the difference in L1 and L2 (on this presumed single phase service).
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I'm not sure I'm following you on this, but from what I think you are saying I'm not sure it explains the neutral current being higher than the difference in L1 and L2 (on this presumed single phase service).
In the scenario I brought up, the portion of the neutral conductor that is being measured is connected to neutral bar #1 (NB1) which has no bonding screw to the panel. Also there is no jumper between NB1 and NB2. Therefore the current being measured would be the current from NB1 only, not the difference of the currents from NB1 and NB2. But you're right that the measured current would still be the difference of currents on the L1-N and L2-N circuits tied to that bar.
So shutting off all the single pole breakers on one side might be a good test to see how that effects the measured current.
 
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synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I'm not sure I'm following you on this, but from what I think you are saying I'm not sure it explains the neutral current being higher than the difference in L1 and L2 (on this presumed single phase service).
So the simplest example would be having only two 1-pole branch breakers in the panel:

One breaker is on L1 and its branch circuit neutral conductor is connected to neutral bar #1 (NB1). A 12A load is placed on this circuit. NB1 has the neutral service conductor connected to it, but it has no bonding screw to the panel.
The second breaker is on L2 and its circuit neutral connects to neutral bar #2 (NB2) which doesn't have a bonding screw. An 8A load is placed on this circuit.
There's no jumper between the neutral bars.

The current on the section of neutral service conductor going between the meter bond connection and NB1 (where I think the OP may be putting the clamp meter) is only the current through NB1 or 12A, because the neutral bars are not jumpered to combine the neutral currents. The current on the metallic conduit between the meter and panel is 8A because that is what flows from NB2 through its bonding screw to the panel.
The neutral currents from NB1 and NB2 sum together (which is a difference because of the 180° phase relationship) at the point where the neutral service conductor is bonded to the meter housing, and not within the panel like they should. So upstream of this bond the neutral conductor current would be 12A - 8A = 4A, as would normally be expected. But I think the OP may be measuring the neutral current downstream of this bond connection.
 

oldsparky52

Senior Member
So the simplest example would be having only two 1-pole branch breakers in the panel:

One breaker is on L1 and its branch circuit neutral conductor is connected to neutral bar #1 (NB1). A 12A load is placed on this circuit. NB1 has the neutral service conductor connected to it, but it has no bonding screw to the panel.
The second breaker is on L2 and its circuit neutral connects to neutral bar #2 (NB2) which doesn't have a bonding screw. An 8A load is placed on this circuit.
There's no jumper between the neutral bars.

The current on the section of neutral service conductor going between the meter bond connection and NB1 (where I think the OP may be putting the clamp meter) is only the current through NB1 or 12A, because the neutral bars are not jumpered to combine the neutral currents. The current on the metallic conduit between the meter and panel is 8A because that is what flows from NB2 through its bonding screw to the panel.
The neutral currents from NB1 and NB2 sum together (which is a difference because of the 180° phase relationship) at the point where the neutral service conductor is bonded to the meter housing, and not within the panel like they should. So upstream of this bond the neutral conductor current would be 12A - 8A = 4A, as would normally be expected. But I think the OP may be measuring the neutral current downstream of this bond connection.
In your example the neutral current would not exceed the highest L current, the OP claims a higher neutral current than either of the L currents. Right?
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
In your example the neutral current would not exceed the highest L current, the OP claims a higher neutral current than either of the L currents. Right?
Yes his first picture showed the neutral current being about 1 amp higher than either line current.
He also said there's some lower amount of current that remains on the neutral when the main breaker is shut off. So that would have to be due to some other mechanism.
Perhaps that residual current is adding to the total neutral current and allowing it to exceed the highest line current.
The GEC system might be conducting this residual current from some external source. It should be measured with a clamp meter to see if it's contributing any noticeable current.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I just noticed that in my post #150 I said:
"The second breaker is on L2 and its circuit neutral connects to neutral bar #2 (NB2) which doesn't have a bonding screw."
It should be: does have a bonding screw.
 

jhardy13

Member
Location
Joplin Missouri
Occupation
Industrial Engineering student
Could the high neutral current be caused by an illegal neutral to ground bond somewhere other than in the main panel? Such as at an appliance or something? I personally haven't seen any low or high voltages while measuring with my meter but we do have flickering lights and damaged electrical equipment likely due to brownouts or high voltage. Things that we have replaced in the last 5 years: our hot water heater, our AC unit, our fridge, our oven, and three ceiling fans. Also, with the breaker turned off, I still get 1.5 volts at some of our outlets neutral to ground. My electrician said its probably just inductance from other live circuits running near the wiring. Do you agree with him or is it something I should look into? He also says it could be caused by a bad appliance. How would it be possible to get excessive neutral current from an appliance?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I think you need to back up and look at one problem at a time.
Bad power from the POCO? Get recording equipment and prove it. High voltage and brownouts are not a normal circumstance but your neighbors would have been affected as well.

Applying a volt meter doesn’t tell you a lot unless you have experience in how to use them.
 

kwired

Electron manager
Location
NE Nebraska
Could the high neutral current be caused by an illegal neutral to ground bond somewhere other than in the main panel? Such as at an appliance or something? I personally haven't seen any low or high voltages while measuring with my meter but we do have flickering lights and damaged electrical equipment likely due to brownouts or high voltage. Things that we have replaced in the last 5 years: our hot water heater, our AC unit, our fridge, our oven, and three ceiling fans. Also, with the breaker turned off, I still get 1.5 volts at some of our outlets neutral to ground. My electrician said its probably just inductance from other live circuits running near the wiring. Do you agree with him or is it something I should look into? He also says it could be caused by a bad appliance. How would it be possible to get excessive neutral current from an appliance?
Unless your POCO has a voltage regulation problem ( IE tap changer not working) and has that problem for extended time your overvoltages are probably transient in nature (only a few cycles or even lightning transients) and you should maybe consider surge protection at the main panel/service equipment.

I once was working on piece of equipment for a client in my shop and happened to notice my supply volts that should have been 240 nominal (usually between 243 and 247 at my place) was running near 270. Called POCO and they discovered their regulation equipment was malfunctioning. Don't know how long this went on in that incident but definitely is pushing it for some equipment, other equipment may not matter much at all. My neighbors that are all on that section of the distribution system should have had pretty much the same amount of overvoltage, so something like that is not limited to one customer unless maybe that one customer is a large industrial plant that is the only customer served by the regulation equipment that malfunctioned.

Most the appliances you mentioned can take voltage swings pretty well, some electronic controls associated with them maybe not, but heating elements and motor windings can take pretty fair amount (at least 10% and probably even up to 25%) of voltage rise or sag especially if for short periods of time without any catastrophic failure.
 
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