# Strange transformer voltage.



## Forge Boyz (Nov 7, 2014)

Today I went to an old feed mill on a service call. They had had some computers toasted and wanted the electrical system checked out. The service to the mill is a 240 closed delta that was originally ungrounded, but several years ago we had done some repairs and added a ground. 
There is a single phase 15kva 240 x 120/240 transformer that provides power for lights and receptacles, and from that a 30A feed is run to a subpanel in the office. 

When I checked the voltage on the load side of the transformer I had 115v to nuetral from each leg, but had almost 0v leg to leg. The markings on the leads are very difficult to read and I wasn't able to shut off the transformer while I was there, so I didn't get to dig into it very deeply, but is it possible that the windings could be wired to produce this? Or is the transformer just failing. 
I took amperage readings on the feed to the subpanel, and from the nuetral current it did seem that the two legs are cancelling each other out. I'm confused.

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## just the cowboy (Sep 4, 2013)

Sounds like same phase. If I had to guess one side og the xformer failed and someone disconnected that bad phase and connected that side to the working phase, "to get it running". 


With no 240 volt load it would not be seen.


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## Forge Boyz (Nov 7, 2014)

The two legs are definitely coming off of separate leads in the transformer. I had the cover off to check.

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## Navyguy (Mar 15, 2010)

Or, the one leg has failed (open fuse, blown winding, etc) and the 115 volts is feeding backwards through the loads.

Check both primary and secondary fuses on the tranny, main panel breaker and subpanel breaker. If you are getting the same at the tranny, the problem is related to the tranny. If you are getting it only at the subpanel, then it is the main panel breaker or subpanel breaker.

You see this a lot in 3 phase, you measure voltage across the line and across the load, but forget to measure across the fuse or breaker.

Cheers
John


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## Southeast Power (Jan 18, 2009)

I would be more interested in the line side coming into that transformer.


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## Forge Boyz (Nov 7, 2014)

Fuses are good on the line side of tranny, about 233v. I did not check on the actual leads since I would have had to cut the tape on split bolts and couldn't do a shut down at the moment. The load goes directly to a lug panel and the odd voltage is there too. 

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## MikeFL (Apr 16, 2016)

Can you get to the point where the tranny is fed from and check it there?


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## Forge Boyz (Nov 7, 2014)

MikeFL said:


> Can you get to the point where the tranny is fed from and check it there?


This is where it gets interesting. The feed is a 10-3 Romex with insulated ground. Black and red are paralleled for one leg and green and white for the other. These come out of a fused disconnect on 60A fuses. There is no bonding whatsoever. Most of the electrical system in the building is floating.


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## MikeFL (Apr 16, 2016)

And what do you get across those paralleled legs?


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## Forge Boyz (Nov 7, 2014)

About 233v

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## MikeFL (Apr 16, 2016)

You say load side of the tranny goes straight to a MLO panel with no OCPD at all? So there's no way at all to take the load off and see what you measure on the load side of the tranny with no load?


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## Forge Boyz (Nov 7, 2014)

MikeFL said:


> You say load side of the tranny goes straight to a MLO panel with no OCPD at all? So there's no way at all to take the load off and see what you measure on the load side of the tranny with no load?


I can. I should check that. I just have to be there outside of business hours. My main question is: can the transformer windings be wired to produce this, or is it simply the tranny going bad?

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## just the cowboy (Sep 4, 2013)

*?*



Forge Boyz said:


> I can. I should check that. I just have to be there outside of business hours. My main question is: can the transformer windings be wired to produce this, or is it simply the tranny going bad?
> 
> Sent from my Moto Z (2) using Tapatalk



If the lead are coming off out both ends and center tap is correct, if should not be a hooked up wrong issue.
Are both side of panel are working correct, not just reading voltage?
Did you hang a load on each side?


If both sides are working and it is hooked up correct, it has to be a bad xformer. But if it went bad it went in an odd way, one end of winding opened and load shorted to other end. Don't make sense.


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## tmessner (Apr 1, 2013)

Was this setup working "properly" at one time or has this problem been around and just not noticed.


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## Navyguy (Mar 15, 2010)

If the winding is shorted on the tranny then it should blow the primary fuses if sized correctly. So I suspect if the winding is blown (more like a fuse or connection), it is open which is allowing one side of the panel / phase to be energized, feed the voltage through the loads and back to the tranny on the other phase.

I have seen this a lot over the years, but almost exclusively it is the secondary fuse that has blown on inrush or some large motor starting that put some "start-up" load on an undersized xmfr.

While the CEC does allow for primary protection only, almost every installation I have worked on has secondary fusing.

Cheers
John


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## Forge Boyz (Nov 7, 2014)

As far as I know everything is working. They haven't said that things aren't. There are no 240v loads in the panels so is it possible to be back feeding? This is an an old install that is still working, but no one has looked at it very closely.

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## just the cowboy (Sep 4, 2013)

Still think someone jumped something somewhere. At panel or a j box, a leg went bad and they just jumped a to b phase in panel or elseware.


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## Southeast Power (Jan 18, 2009)

Its very possible someone wire it as two 120 volt and not 120/240.


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## glen1971 (Oct 10, 2012)

Forge Boyz said:


> Fuses are good on the line side of tranny, about 233v. I did not check on the actual leads since I would have had to cut the tape on split bolts and couldn't do a shut down at the moment. The load goes directly to a lug panel and the odd voltage is there too.
> 
> Sent from my Moto Z (2) using Tapatalk


Did you measure upstream of the fuses? Down stream of the fuses? Across the fuses?


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## Forge Boyz (Nov 7, 2014)

glen1971 said:


> Did you measure upstream of the fuses? Down stream of the fuses? Across the fuses?


Not sure if I did across. There is another disconnect in the electric room that it is feed from and I checked it there as well.

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## Forge Boyz (Nov 7, 2014)

Southeast Power said:


> Its very possible someone wire it as two 120 volt and not 120/240.


It is wired as though it should be 120/240, and one of the two leads that is tied to the nuetral is marked 2. I can't read the markings on the other leads very well.

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## gpop (May 14, 2018)

If you test both L1 and L2 with a amp clamp while the power is on that should tell you if the panel is one leg being backfeed or 2 legs of the same voltage. 

If it was a back feed i would expect at least 2v between L1 and L2 this is why its important not to round numbers when you diagnosis a problem. (not saying you did but im not sure if you didnt)


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## Forge Boyz (Nov 7, 2014)

gpop said:


> If you test both L1 and L2 with a amp clamp while the power is on that should tell you if the panel is one leg being backfeed or 2 legs of the same voltage.
> 
> If it was a back feed i would expect at least 2v between L1 and L2 this is why its important not to round numbers when you diagnosis a problem. (not saying you did but im not sure if you didnt)


The amperages are different. At the subpanel they did seems to cancel out as far as nuetral current. The voltage between legs IIRC was 0.2v

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## gpop (May 14, 2018)

Forge Boyz said:


> The amperages are different. At the subpanel they did seems to cancel out as far as nuetral current. The voltage between legs IIRC was 0.2v
> 
> Sent from my Moto Z (2) using Tapatalk


So its the same 110v before the panel not a back feed from the panel. Now amp clamp the wires at the transformer if one is close to zero and the other is high then you have a place in the field where someone has splices to make one leg into 2 before the panel. If both have amps then i would think that the transformer was wired 120v only.


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## B-Nabs (Jun 4, 2014)

Forge Boyz said:


> It is wired as though it should be 120/240, and one of the two leads that is tied to the nuetral is marked 2. I can't read the markings on the other leads very well.
> 
> Sent from my Moto Z (2) using Tapatalk


If the polarity of one of the secondary windings is connected backwards, it would behave exactly as you describe. For example, referencing the above connection diagram, if X2 were connected to X4 instead of X3 and used as the "neutral", you would measure 120v to each of the other legs (X1 & X3), but 0v between X1 & X3. 

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## Forge Boyz (Nov 7, 2014)

B-Nabs said:


> If the polarity of one of the secondary windings is connected backwards, it would behave exactly as you describe. For example, referencing the above connection diagram, if X2 were connected to X4 instead of X3 and used as the "neutral", you would measure 120v to each of the other legs (X1 & X3), but 0v between X1 & X3.
> 
> Sent from my SM-G920W8 using Tapatalk


Thanks. This is what I was looking for. I was wondering if that can happen.

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