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Old 09-21-2017, 01:58 AM   #1
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Default Voltage drop during heavy load

Hi,

We are doing a large commercial building with multiple units for mechanic shops. Each unit is the same and around 3,000 sq ft. We have 3 phase 600v going to each unit with a delta wye transformer at each unit stepping it down to 120/208.

Only 1 of the units is giving me trouble and it is the first one. Panel reads 115-120 volts across all 3 phases to neutral and 205-208v between phase to phase. When i add a decent size load under 15 amps to any phase the voltage drops on that particular phase.

I took all the wires off the panel and switched around a door opener with neutral on each phase and the phase i would hook up would drop.

Transformer is grounded to the steel beam above. All the other units are fine would this be a grounding issue with that 1 beam?

Thanks


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Old 09-21-2017, 02:28 AM   #2
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Quote:
Originally Posted by Buzz355 View Post
Hi,

We are doing a large commercial building with multiple units for mechanic shops. Each unit is the same and around 3,000 sq ft. We have 3 phase 600v going to each unit with a delta wye transformer at each unit stepping it down to 120/208.

Only 1 of the units is giving me trouble and it is the first one. Panel reads 115-120 volts across all 3 phases to neutral and 205-208v between phase to phase. When i add a decent size load under 15 amps to any phase the voltage drops on that particular phase.

I took all the wires off the panel and switched around a door opener with neutral on each phase and the phase i would hook up would drop.

Transformer is grounded to the steel beam above. All the other units are fine would this be a grounding issue with that 1 beam?

Thanks


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Size of the transformers ?

Resistance of the load ?

Short to ground within loading device ?
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Old 09-21-2017, 02:48 AM   #3
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Another possibility is a high resistance neutral connection. That will drop the measured line to neutral voltage whichever phase conductor you put the load on. And the line to line voltages will not change at all.

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Old 09-21-2017, 03:15 AM   #4
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Voltage between phases drops as well.

It is a 30kva transformer i disconncted everything and put a 1000w load and it still dropped.

Under no load it is completely fine voltage wise and i meggered and resistance tested the transformer.
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Old 09-21-2017, 03:15 AM   #5
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Neutral to ground shows continuity at all times
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Old 09-21-2017, 03:32 AM   #6
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What happens to the voltages on the other two phases relitive to neutral,
whilst your monitored voltage drops.
You need to put decent 120v loads on all three phases
and monitor what happens to all three ?
You could also have a high resistance connection on the primary
side of your tranny, this is acting as a current limit !
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Old 09-21-2017, 04:15 AM   #7
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The other possiblty you may have bad delta primary connection at the transformer or one of the coils is already weak.

did you do the load test on other two phase yet ? if other two read good and only one bad then it is bad core or connection but if stay bad on other two test then secondary connection inside the transformer may gone bad too.
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Old 09-21-2017, 05:24 AM   #8
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Does the primary voltage change when the secondary has that load put on it?
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Old 09-21-2017, 09:46 AM   #9
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How much does the voltage drop? You said 115 to 120 volts no load. Is that what? A spread of each, one at 115, one at 120? How about you list the voltage for each phase, phase to neutral and phase to phase under both load and no load. If I was troubleshooting, I do that. Tough to tell from the keyboard with such limited information.
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Old 09-21-2017, 11:20 AM   #10
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If the voltage drop is about the same on all phases, both phase to phase and phase to neutral, only on this one unit, I would tend to guess that the primary breaker or a connection on the line side of the transformer( including any connection from xfmr to the service) is loose are damaged. It would be rare, but possible, for there to be a transformer problem.
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Old 09-21-2017, 01:40 PM   #11
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Check if there is a drop on the primary as already suggested. If not, then the likely culprit is that you have grounded the secondary Wye point (X0) of your transformer to the building frame, but it has a poor connection to the ground grid (if any). So if you are measuring ground continuity to the same building frame, of course it reads OK. But what is your resistance to ACTUAL GROUND?
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Old 09-21-2017, 02:58 PM   #12
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BTW, 30 kVA is dinky for 3,000 square feet.

45 kVA ~> 100 Amp panel

I must presume that these puppies are -- essentially -- warehouses.
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Old 09-21-2017, 04:49 PM   #13
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A ground issue would not change the phase to phase voltage
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Old 09-21-2017, 05:51 PM   #14
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If you have done all the checks as stated previously and checked all connections including drop on primary if any, could be one of two things, what is the relationship of transformer to building, possible secondary to small for length of run, or and this has happened to me only one time in 40 years I had a bad transformer, and that was not long ago. But if the others are fine and everything is the same and you rule everything else out, that could very well be the case. Nothing is made like it used to be so before I try finding fault with my work I just do a double check. After that I am on phone with supplier, I have had contactors last 2 days with coils shot and numerous ballasts and HID bulbs bad out of box. I do suspect it may be a bad winding since such a small load will cause a drop. What is the voltage drop by the way. If it is transformer it's still gonna be your fault, every manufacturer says their products are tested and approved and there was nothing wrong with them. Don't believe it, today there is no pride, even in a menial assembly line job. The workers are paid the least amount possible and don't care at all, or it's automated and the operator don't pay attention. Check the label for where it was made. Sounds like the center tap may be fubar, if you are checking the 120 Volts what about the 208 phase to phase.
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Old 09-21-2017, 06:34 PM   #15
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It seems electricians love a good ol electrical mystery !

Voltage drop during heavy load-scully_mulder.jpg



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Old 09-21-2017, 06:51 PM   #16
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feeder distance / wire size to panel?
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