High current on neutral with a balanced load

[ptonsparky]

Since the poco only pulled the fuse, which is on the primary side, how did you test with the poco transformer still connected? They had the neutral floating at the transformer?

The meter was out and I disconnected the neutral on the second set of tests.

 

[kwired]

I suggest shutting off all 2-pole breakers but leaving the main breaker ON. Then try to get the current on both hot legs equal to eachother again (around 15 to 30 amps would be OK) and measure the neutral current.
The reason for doing this is to check whether the house might be powered by two legs of a 3-phase wye configuration that are 120 degrees apart. If that's the case I would expect the neutral current to be about the same as the current on each hot leg.

Simple thing is to check line to line volts. If he has ~124 volts line to neutral then a wye source would read ~ 215 line to line where split single phase would read ~148 line to line.

 

[Little Bill]

Simple thing is to check line to line volts. If he has ~124 volts line to neutral then a wye source would read ~ 215 line to line where split single phase would read ~148 line to line.

I hope you meant 248V L-L

 

[kwired]

I hope you meant 248V L-L

Yes I did.

 

[kwired]

We use Fluke 1507 Insulation Testers. The POCO had the transformer fuse pulled, with meter out when I got there and were waiting for me. The customer owns and is responsible for the lateral. The owner had turned the Main breaker off on his panel. Service was 4/0 4/0 2/0 direct bury Al. First check was from meter can load lugs to pole ground. Reading was .010 meg with a voltage maintained at 14v. Test range was 500 volts. I removed all the load wires at the meter can, removed the neutral in the service panel and confirmed the main was off. Readings were the same. Faults were where whoever buried a private propane line years ago and had cut through the lateral. Two splices on each side for all three conductors.

Readings after repair were >550 M @ 526 VDC.

The buried maker tape didn’t appear to have prevented any damage

Marker tape is only good for telling you what you hit after you actually see the tape.

 

[Another C10]

I'd consider the mystery amperage of the neutral is derived from the dwelling when energized because if the Mains phases are disconnected at the panel, the neutral is still connected directly to the utility return, so I'd tend to think back feed from utility is not the scenario here, if using an amp probe at the mains neutral. Although it was stated the unknown amperage goes away when the dwellings voltage is open. I'm pretty sure its an issue within the home. I'm still a little puzzled on this but interested.

 

[subtech_2]

What typically is not tested in situations like this the difference in power factor (read phase angle)
between what is essentially two (in phase - 0 degrees displaced) 120 volt circuits. It sounds like that
up to this point, everybody has been measuring only the magnitude of current flow in the two circuits
(which share a common return conductor) and then assuming that because there is
little/no difference between them there should be little/no current flow in the neutral. What's missing
is the fact that there are two things that can cause difference current to be measured in the neutral conductor.
First is just a difference in magnitude of the currents. You don't have this. If I read correctly you've
measured current in both hot legs with several devices in the home turned on and the currents measured
within about 1 ampere of each other. The other thing that can cause a difference/neutral current to flow
is if there's a difference in the power factor (read phase angle) between the two circuits. Say that hot leg
circuit 1 has mostly resistive devices connected. The current flowing in this circuit would be nearly in phase
with the voltage. But let us say that the same is not true with hot leg circuit 2. Most of the devices connected
to that circuit are either inductive (most likely) or capacitive (less likely, but possible) in nature and this causes
the current in hot leg circuit 2 to either lag or lead its voltage to some degree. In that case, the currents (there are two)
flowing in the neutral/return conductor are no longer are 180 degrees separated (and cancelling each other)
but something more or less than 180 degrees. Hence, the measured neutral current. It's a commonly accepted
"fact" that with equal currents flowing in the two hot legs that there will always be 0 current in the neutral,
and many electricians and linemen have accepted it as hard fact, but it just isn't true. In order for you to know for sure,
turn on the same devices you had on earlier. Take measurements to ensure you've got near equal current flows in
each of the two circuits, (20 to 30 amps in each is plenty) then measure and compare the power factor of the two.
You'll likely find a moderate difference between them. If so, the currents returning in the neutral conductor are not
180 degrees separated and there will be the reason for your neutral current. Whatever you decide to do, make sure the
correct instruments are being used, and you are conducting the tests properly and safely. If you are not sure how to conduct
these tests, find a competent person to help you.
I hope this helps.

 

[robertd]

There should be a ground wire or two. If the house has metal water pipes there should be a ground connection to the water pipe a few feet from where it enters the house. There may also be a ground wire going to an outside ground rod. Turn off all of the breakers and measure the current on each of them. Assuming you have a clamp on amp meter, try to measure the current on the cable that goes from the electric meter to the circuit breaker box. Also check the current on the cable TV wire.

 

[mikeames]

What typically is not tested in situations like this the difference in power factor (read phase angle)
between what is essentially two (in phase - 0 degrees displaced) 120 volt circuits. It sounds like that
up to this point, everybody has been measuring only the magnitude of current flow in the two circuits
(which share a common return conductor) and then assuming that because there is
little/no difference between them there should be little/no current flow in the neutral. What's missing
is the fact that there are two things that can cause difference current to be measured in the neutral conductor.
First is just a difference in magnitude of the currents. You don't have this. If I read correctly you've
measured current in both hot legs with several devices in the home turned on and the currents measured
within about 1 ampere of each other. The other thing that can cause a difference/neutral current to flow
is if there's a difference in the power factor (read phase angle) between the two circuits. Say that hot leg
circuit 1 has mostly resistive devices connected. The current flowing in this circuit would be nearly in phase
with the voltage. But let us say that the same is not true with hot leg circuit 2. Most of the devices connected
to that circuit are either inductive (most likely) or capacitive (less likely, but possible) in nature and this causes
the current in hot leg circuit 2 to either lag or lead its voltage to some degree. In that case, the currents (there are two)
flowing in the neutral/return conductor are no longer are 180 degrees separated (and cancelling each other)
but something more or less than 180 degrees. Hence, the measured neutral current. It's a commonly accepted
"fact" that with equal currents flowing in the two hot legs that there will always be 0 current in the neutral,
and many electricians and linemen have accepted it as hard fact, but it just isn't true. In order for you to know for sure,
turn on the same devices you had on earlier. Take measurements to ensure you've got near equal current flows in
each of the two circuits, (20 to 30 amps in each is plenty) then measure and compare the power factor of the two.
You'll likely find a moderate difference between them. If so, the currents returning in the neutral conductor are not
180 degrees separated and there will be the reason for your neutral current. Whatever you decide to do, make sure the
correct instruments are being used, and you are conducting the tests properly and safely. If you are not sure how to conduct
these tests, find a competent person to help you.
I hope this helps.

A power factor in a residential home that creates a 7 amp difference on 20 amp loads is not likely and maybe not even possible.

 

[ptonsparky]

A power factor in a residential home that creates a 7 amp difference on 20 amp loads is not likely and maybe not even possible.

I wondered, but I haven't seen everything and my education is lacking.

 

[kwired]

A power factor in a residential home that creates a 7 amp difference on 20 amp loads is not likely and maybe not even possible.

Most homes probably is not likely, especially on just two 20 amp branch cicuits or a single 20 amp MWBC . But if you have large enough variety of loads maybe can have enough cumulative effect on the service neutral.

Distortion power factor is likely to be more of a contributor than displacement power factor. Things like air conditioners tend to have fairly high power factor level and aren't major problem either. Small motorized kitchen appliances or power tools just don't have long enough run time to factor into this much regardless what their power factor is.

 

[oldsparky52]

Do we know the leg to leg voltage yet?

 

[synchro]

... The other thing that can cause a difference/neutral current to flow
is if there's a difference in the power factor (read phase angle) between the two circuits. ...

I agree that even when the currents through L1 and L2 are equal in magnitude, if they are not 180° from each other then there will be a non-zero current through the neutral conductor.

Just to put this issue in a more familiar perspective, suppose we have two legs of a 120/208V wye that have equal resistive loads and with each drawing a current IL. The load currents will be 120° apart, where one is effectively leading by θ=30° and one lagging by 30° relative to the 180° phase relationship needed for complete current cancellation. So each leg contributes sin(30°) x IL = 0.5 IL and the total neutral current is IL (i.e, equal to the current in each phase).

Now using this line of reasoning we can determine the angle between two currents of IL=20A in L1 and L2 that is necessary to get 7A in the neutral.
As above, we want sin(θ) x 20A = 7A /2 = 3.5A because each leg contributes half of the unbalanced current to the neutral, since the magnitudes of their currents are equal. Dividing both sides of this equation by 20A and taking the arcsin (inverse function of sin) we get θ = arcsin (3.5/20) = 10.08°. Therefore the currents in the two legs form a vector triangle with two 10.08° angles and with the legs 180° - 2 x 10.08 =159.8° apart. So 159.8° is the required angle between L1 and L2 currents of 20A to get 7A in the neutral.

Now using the example in post #47, if the load on one leg is resistive and the other either leads or lags by 2 x 10.08° = 20.16° then the currents will be at a 159.8° angle and the neutral current will be 7A. The power factor would be cos (20.16°) = 0.939. A 2HP motor might draw 20A at 120V at a reasonable load, and you would be lucky to get a power factor this good. But in this situation a better configuration would certainly be feeding the motor with 240V L1-L2 and using no neutral current.
So as was said before, the likelihood of the 7A being due to only P.F. is quite low in a residential environment but still possible.

 

[jhardy13]

Sorry for the long delay in posting. It took awhile, but I got an electrician out so we could try to track down the source of this problem but we didn't have much luck. Line to line voltage is 240 volts on the dot. Here is a picture of the amperages while we were testing things. The neutral wire is the bottom right clamp. You can clearly see that the current is higher on the neutral than it is on either leg. What in the world could be causing this?

 

[kwired]

Sorry for the long delay in posting. It took awhile, but I got an electrician out so we could try to track down the source of this problem but we didn't have much luck. Line to line voltage is 240 volts on the dot. Here is a picture of the amperages while we were testing things. The neutral wire is the bottom right clamp. You can clearly see that the current is higher on the neutral than it is on either leg. What in the world could be causing this?

Kind of forgot all that was mentioned earlier and didn't go back through to see it but I'd say disconnect grounding electrode conductors (especially if metal water pipe that possibly also connects to neighbors) as it can be carrying current from elsewhere.

Those meters say they are true RMS meters, if you have significant non linear loads they might read different than a non true RMS meter does. Harmonics may not be as serious of a problem on neutral of split single phase as on a wye system but I can see they won't necessarily balance like a linear load does either.

You maybe need something that can display the current waveform and then the problem might be more obvious.

 

[jhardy13]

Kind of forgot all that was mentioned earlier and didn't go back through to see it but I'd say disconnect grounding electrode conductors (especially if metal water pipe that possibly also connects to neighbors) as it can be carrying current from elsewhere.

Those meters say they are true RMS meters, if you have significant non linear loads they might read different than a non true RMS meter does. Harmonics may not be as serious of a problem on neutral of split single phase as on a wye system but I can see they won't necessarily balance like a linear load does either.

You maybe need something that can display the current waveform and then the problem might be more obvious.

I have viewed the waveform with my oscilloscope and spectrum analyzer and my readings are normal. I posted screenshots of my data in one of my earlier posts on this thread if you are curious. Waveform distortion doesn't seem to be the problem here.

 

[Hv&Lv]

Kind of forgot all that was mentioned earlier and didn't go back through to see it but I'd say disconnect grounding electrode conductors (especially if metal water pipe that possibly also connects to neighbors) as it can be carrying current from elsewhere.

Those meters say they are true RMS meters, if you have significant non linear loads they might read different than a non true RMS meter does. Harmonics may not be as serious of a problem on neutral of split single phase as on a wye system but I can see they won't necessarily balance like a linear load does either.

You maybe need something that can display the current waveform and then the problem might be more obvious.

Harmonically distorted signals must be measured with a true-RMS meter. I suspect if it is harmonics, an average detecting meter, which “assumes” a sinusoidal wave, would possibly read as much as 50% less.

 

[LarryFine]

Sorry for the long delay in posting. It took awhile, but I got an electrician out so we could try to track down the source of this problem but we didn't have much luck. Line to line voltage is 240 volts on the dot. Here is a picture of the amperages while we were testing things. The neutral wire is the bottom right clamp. You can clearly see that the current is higher on the neutral than it is on either leg. What in the world could be causing this?

Now, shut off your main and see if any neutral current remains. If so, it must be coming from elsewhere.

 

[Hv&Lv]

Now, shut off your main and see if any neutral current remains. If so, it must be coming from elsewhere.

I’m just wondering if it’s neighbors currents

 

[jhardy13]

I’m just wondering if it’s neighbors currents

I think it is likely. What would cause a neighbor's current to reach my home though? We share a transformer. Also, shouldn't the current coming from a neighbor bleed off to ground? Whatever is causing this is damaging my electronic equipment, so it is getting past the ground rods and into our panel. I have lost several computer power supplys now.