# 3 Pot transformer bank



## JesseETG (19 d ago)

I'm currently studying up on transformer banks. From my understanding, if you are wiring a wye configuration secondary you can either connect bushings 2,4,and 6 together and use 1,3, and 5 as phase connections, or vice versa 1,3, 5 together and 2,4,6 as phases. Is it true that both can be done to achieve same output? If both can be done is their any difference at all ?


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## SWDweller (Dec 9, 2020)

Depends on delta or wye configurations. I used to see lots of 3 pot services (all the same size)
and a few with 3 pots (2 larger and one smaller). The 3 same size were either 208 or 480.
The ones that were different sized were 240v wild leg services.
Depending on where you live and work you may not see either of these any more as a lot of POCO's have gone to pad mounted.
First thing I learned about the trade was assuming anything was dangerous to your health.


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## Almost Retired (Sep 14, 2021)

JesseETG said:


> I'm currently studying up on transformer banks. From my understanding, if you are wiring a wye configuration secondary you can either connect bushings 2,4,and 6 together and use 1,3, and 5 as phase connections, or vice versa 1,3, 5 together and 2,4,6 as phases. Is it true that both can be done to achieve same output? If both can be done is their any difference at all ?


surprisingly a simple search
transformer banks
yielded this result


transformer banks - Google Search



you can memorize how to do this for each individual situation
how ever to understand what you are accomplishing and the why and how of it
you will need to learn the fundamentals of electricity and how transformers manipulate it

the choice is yours
to be a robot doing things because that is what you were told
or
to do things because you understand the cause and effect of your actions
when you learn this , you can now do things that you werent taught how to do, but rather because you already understand the concept

in other words you can go home and use electricity in ways that they didnt give you the instructions for
to help yourself or to help someone else accomplish a task that no one else around knows how to do

and im not saying that being a robot is less than good
i have done robotic type work all my life, it is what i get paid for
but i can also use what i know about electricity to learn things that dont involve work and can give me pleasure or more money


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## kb1jb1 (Nov 11, 2017)

Find a transformer catalog because they usually have all the configurations and diagrams that you will ever need. I think your connection numers are wrong. Assuming single voltage transformer and not 208/120 - 480/277
4,5,6 are tied together and 1,2,3 are the terminals. But you raised a good thinking point that I now have to look up. How are the numbers set up if you have three separate single coil transformers? Are they connected 4-4-4, 2-3 2-3 2-3 and the 1 1 1 are the phase legs?


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## paulengr (Oct 8, 2017)

Do not memorize bushing numbers.

ALWAYS read the labels. They are there for a reason. The diagram will show you the wiring scheme, bushing numbers, everything.

There are a LOT of transformer schemes out there. Based on your limited knowledge what happens if you tie bushings together like you described on a delta-delta? How about a 120/240 high leg delta? Ever seen a 2 pot high leg? Have you seen buck-boost? What happens with your scheme on an autotransformer?

And excuse me but why would you take 3 three phase wye-eyes, operate them as single phase, wired up as wye-wye? Just buy the proper transformer! You have 900% more bushings, all waiting to fail, plus all the unnecessary wiring. This is the kind of stuff that NERC gets upset over. Don’t try to tell me about replacement. Are you going to attempt to inventory matched sets to get the %Z right? Transformers last 30-50 years if not abused. By the time 1 of the 3 fails it stresses the other 2, never mind the fact that all 3 will probably be in bad or worse condition, and you’ll be doing a lot more than swapping just one.

In addition there is one and only one reason to do this. The idea of a wired wye is for emergencies when all you have to work with are single phase pots or smaller 3 phase transformers. Then you don’t need to match %Z for instance because it’s all short term. Until you can get the transformer you need. Because unless you get into the used market or smaller sizes it takes months to get one. They are made to order. But some utility managers are so short sighted they attempt to reduce yard storage claiming they are saving money. Which is BS. No accountant has EVER managed to show a profit from reducing inventory but it easily drives your outage times through the roof!


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## kb1jb1 (Nov 11, 2017)

Only picture I could find but the numbers are shown. Just picture three of them in the Wye pattern. For the higher voltage, all the X4 are tied together and is the midpoint and all the X1 are the terminals. X2X3 in each bank is tied together respectively. 
For the lower voltage all the X4 and X2 are tie together as the midpoint and X1 & X3 are tied together and are the phase legs.

At least this is how I remember it.


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## kb1jb1 (Nov 11, 2017)

paulengr said:


> Do not memorize bushing numbers.
> 
> ALWAYS read the labels. They are there for a reason. The diagram will show you the wiring scheme, bushing numbers, everything.
> 
> ...


I think the OP was just talking theory. We all do it the get an understanding of how things work. We ask the "what ifs". Of course it is always better to buy the right transformer but sometimes that cannot happen. I once had to wire up several 277 volt heaters that were special order. They were to be fed from a marked 480/277 volt panel. Because of the nature of the facility it is extremely complicated to get a panel shut down. We installed the heaters and finally received the shut down approval. After taking the panel cover off we saw that it was not 480/277 but only 480 volts. We could not get a transformer to generate a neutral for six weeks but we could get three 1kva single phase and we wired them as a wye.


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## ScooterMcGavin (Jan 24, 2011)

JesseETG said:


> I'm currently studying up on transformer banks. From my understanding, if you are wiring a wye configuration secondary you can either connect bushings 2,4,and 6 together and use 1,3, and 5 as phase connections, or vice versa 1,3, 5 together and 2,4,6 as phases. Is it true that both can be done to achieve same output? If both can be done is their any difference at all ?



Yes there is a difference. The difference is the secondary will be 180 deg out of phase with each other. This only matters if you’re paralleling transformers. For 99% of installations it won’t matter.


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## paulengr (Oct 8, 2017)

kb1jb1 said:


> I think the OP was just talking theory. We all do it the get an understanding of how things work. We ask the "what ifs". Of course it is always better to buy the right transformer but sometimes that cannot happen. I once had to wire up several 277 volt heaters that were special order. They were to be fed from a marked 480/277 volt panel. Because of the nature of the facility it is extremely complicated to get a panel shut down. We installed the heaters and finally received the shut down approval. After taking the panel cover off we saw that it was not 480/277 but only 480 volts. We could not get a transformer to generate a neutral for six weeks but we could get three 1kva single phase and we wired them as a wye.


This is becoming more common, with everybody. Tried ordering PTs lately? I ended up using this instead.









Also have you tried ordering VFDs? We have inventory but nobody else does.










Transformers have months in normal times except cast coil types. So often you have to get creative.


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## power (Feb 27, 2012)

@ JesseETG - Answer - Yes, connect 2, 4, 6 together. Ensure you ground this connection (star point). 1, 3, and 5 are your 3 phase conductors. Yes, you can connect visa versa....there's no difference in efficiency/output.

AlmostRetired is correct in saying "...understand what you are accomplishing and the why...". Read up more on xfmr's. 

BTW - Electrically speaking, using three individual xfmr's (pots) is nothing alarming.....utilities do it all the time.


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## kb1jb1 (Nov 11, 2017)

2,4,6 tied together and the midpoint? Is that right? 
Not 4,5,6 tied together and that is the midpoint?


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## CMP (Oct 30, 2019)




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## power (Feb 27, 2012)

I was basing my earlier comment (post #10) on the following diagram.


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## kb1jb1 (Nov 11, 2017)

power said:


> I was basing my earlier comment (post #10) on the following diagram.
> View attachment 172008


Now I understand. You used your own numbering system. As long as the concept is correct. We should standardize the numbering. Single coils will be labeled 1,2,3,4 as diagramed above. When I started out we had access to many transformer catalogs which were free and they had all the configurations with numbering. Now the internet is full of non standard diagrams and then throw in European methods and a zig zag transformer.


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## micromind (Aug 11, 2007)

JesseETG said:


> I'm currently studying up on transformer banks. From my understanding, if you are wiring a wye configuration secondary you can either connect bushings 2,4,and 6 together and use 1,3, and 5 as phase connections, or vice versa 1,3, 5 together and 2,4,6 as phases. Is it true that both can be done to achieve same output? If both can be done is their any difference at all ?


 Either connection will work just fine. The only difference is that one of the connections will give clockwise phase rotation, the other will be counter-clockwise.


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