Incorrectly tapped transformer

[sii]

Long story short on how I discovered this situation: outside scale technician found 132 VAC at a receptacle in our plant while troubleshooting a scale indicator.

I have a 125KVA 480/208 3ph transformer feeding three 200 amp panelboards. When the scale tech approached me about this, I immediately thought loose neutral somewhere so I started looking into it. After checking voltages and lug tightness at the panels and then at the disconnects and finding nothing, I moved on to the transformer. I found that the primary is wired to the 432 volt taps, creating secondary voltages of 133 L-N and 231 L-L.

The transformer is only about 100' from the service entrance (I didn't check conductor size) and the primary voltage was 479 normally loaded so it doesn't seem like voltage drop is an issue.

The contractor that did this install in the late '80's no longer exists and the company electrician at the time has passed away. The maintenance manager from that time says he's sure it was a permitted and inspected installation but I can't imagine this would have flown with an inspector.

Thinking about this again just now, is it possible that they did this to boost the L-L voltage up closer to 240 and the L-N voltage was acceptable to the installer? We do have several welder receptacles. Should I just reconnect to the proper taps? If not, what steps should I take first? I'm not so concerned with the "why" as I am with the "what should I do about it".


PS These panels feed nothing but receptacles and lighting. No three-phase loads.

 

[hillbilly1]

Most inspectors are only looking at code compliance, not an actual functional installation, not really their job. Shouldn't be an issue changing the taps to the correct input. It's possible at the time of the installation the POCO was delivering low voltage to the service due to overloaded lines, and have corrected the problem since then.

 

[bob]

First you need to check the primary voltage. It should be 480 volts and the secondary would read 208 and 120 volts. So you either have a high input voltage or you are on the wrong tap. You also need to verify that the load will be able to work at 208 volts.

 

[sii]

First you need to check the primary voltage. It should be 480 volts and the secondary would read 208 and 120 volts. So you either have a high input voltage or you are on the wrong tap. You also need to verify that the load will be able to work at 208 volts.

I found that the primary is wired to the 432 volt taps, creating secondary voltages of 133 L-N and 231 L-L......and the primary voltage was 479 normally loaded.

Much of the lightning is multi-tap high-bays tapped at 208. The only other load (at the moment) is the occasional maintenance welder that gets plugged in to repair machinery. They are all 240 volt welders. I'm not sure how they would work at 208. Again, I'm thinking maybe that's the reason the transformer was incorrectly tapped to begin with. Hadn't thought about maybe the utility was undervoltage at the time of install.

 

[bob]

Ok I missed the fact that the input voltage was 479 volts. With the secondary voltage reading 133/231 volts,
you are applying 231 volts to your 208 volt lighting system which is not good. I believe you are correct regarding
the the changing of the taps to get welder voltage. I do not think the welder would work properly at 208 volts.
Hopefully someone who is more knowledgeable will address that point. Do you have any load that will need to use 120 volts?
If so, then you will have to make some changes. You can change the taps so the the voltage is 120/208 volts and address the welders as a separate issue.

 

[Open Neutral]

I believe you are correct regarding the the changing of the taps to get welder voltage. I do not think the welder would work properly at 208 volts.

Are they inverter-based welders?

 

[broadgage]

Most welders have an adsjustable output in order to cater for different sizes of welding rod or different materials being welded.
They normally work fine on a slightly lower input voltage, provided that the user selects a somwhat higher output setting than would appear suitable for the work being done.
The only problem occurs if the nature of the welding work requires the maximum output of the welding set. In this case a larger welding set, or a 208 volt input welder, or a voltage boosting transformer would be needed.

I would advise against long continued use of the installation configured as at present.
As others post, the 208 volt lights are being significantly over run. Are you getting through a lot of lamps and ballasts ?
And consider what might happen if the utility supply increases to 105% of nominal as it might.

As to why it was done like that in the first place, might the utility supply have been 440, 450,or 460 volt in the past? I believe that such voltages used to be used, before the general adoption of 277/480.
Or somone thought it was a 120/240 system, not 120/208, and altered the transformer taps to get nearly 240 volts hot to hot.
It might even have been a temporary bodge to get through a welding job, and they forgot to put the taps back to normal.

 

[petersonra]

The contractor that did this install in the late '80's no longer exists and the company electrician at the time has passed away. The maintenance manager from that time says he's sure it was a permitted and inspected installation but I can't imagine this would have flown with an inspector.

I doubt it makes any difference at this time who did it or why, or whether it was ever "right".

If something needs doing, it needs doing.

Wasting any time over trying to figure out who to blame seems pointless.

 

[sii]

I doubt it makes any difference at this time who did it or why, or whether it was ever "right".

If something needs doing, it needs doing.

Wasting any time over trying to figure out who to blame seems pointless.

I know, that's not what I meant. I was just pointing out that I didn't have any resources here to find out why it was done. I was really just looking for as much information as possible before I re-tap it.

Our plant operates 24/7 and this transformer feeds the lighting and receptacles for about 40% of our production area so I'm planning to do this work on Labor Day. It also needs a good cleaning so it may take awhile.

On a happier note, my company bought their first new machine in 23 years today. You have no idea how much of a :jawdrop:moment this is.