# reverse feed transformer



## NolaTigaBait (Oct 19, 2008)

Can you feed a transformer with 240 3 phase on the secondary and get 480/277 out of the primary?...In other words use it as a step up transformer, even though it's a step-down transformer...


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## BBQ (Nov 16, 2010)

NolaTigaBait said:


> Can you feed a transformer with 240 3 phase on the secondary and get 480/277 out of the primary?...In other words use it as a step up transformer, even though it's a step-down transformer...


Unlikely you can get 277 out of it but yes on the 480.

The high voltage side of it will likely be delta and not have a 'HO' terminal for the neutral.


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## Hippie (May 12, 2011)

Yeah. Not sure about code though


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## NolaTigaBait (Oct 19, 2008)

BBQ said:


> Unlikely you can get 277 out of it but yes on the 480.
> 
> The high voltage side of it will likely be delta and not have a 'HO' terminal for the neutral.


Ahhh thats right...THanks Bob.


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## user4818 (Jan 15, 2009)

They do make transformers for this application.


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## NolaTigaBait (Oct 19, 2008)

Peter D said:


> They do make transformers for this application.


I assumed that, it was a curiosity question. I saw this at a job and was curious why they would do it.


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## CraigV (May 12, 2011)

Generally it's a bad idea.

1. No neutral in most cases, unless the primary happened to have a center tap.

2. Inrush current can be excessive, and cause OCP trips at startup.

3. In the event of a catastrophic failure, very large fault currents can be generated.

4. Wrong (low) voltage. All transformers will have some voltage drop caused by winding resistance at full load.. The engineers design the transformer to produce the desired voltage by compensating. Backfeeding will cause the compensated winding voltage drop, plus additional voltage drop. So your output could be 10% below what you expect at full load.


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## brian john (Mar 11, 2007)

CraigV said:


> Generally it's a bad idea.
> 
> 4. Wrong (low) voltage. All transformers will have some voltage drop caused by winding resistance at full load.. The engineers design the transformer to produce the desired voltage by compensating. Back feeding will cause the compensated winding voltage drop, plus additional voltage drop. So your output could be 10% below what you expect at full load.


 
Typically we see that issue on small transformers (though I cannot give a precise size range). With larger transformers all the other issues you mentioned, plus it messes up the the thought process for about 97% of the electricians that run into a transformer with a grounded delta or ungrounded delta.


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## BBQ (Nov 16, 2010)

CraigV said:


> Generally it's a bad idea.
> 
> 1. No neutral in most cases, unless the primary happened to have a center tap.
> 
> ...





brian john said:


> Typically we see that issue on small transformers (though I cannot give a precise size range). With larger transformers all the other issues you mentioned, plus it messes up the the thought process for about 97% of the electricians that run into a transformer with a grounded delta or ungrounded delta.


OTH I have done it myself, have seen it done by others in the compaines I have worked for and come across it done by strangers and in each case it was working fine.:thumbsup:

Square Ds position is it is not recommended (they would rather sell another trans) but not prohibited.


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## 220/221 (Sep 25, 2007)

NolaTigaBait said:


> Can you feed a transformer with 240 3 phase on the secondary and get 480/277 out of the primary?...In other words use it as a step up transformer, even though it's a step-down transformer...


 
I did it years ago (not my idea/design) to feed a huge Ac unit the size of a trailer. They ordered 480 and only had 208 on the property. Their solution was a transformer running 24/7 even though the church was only occupied generally on Sundays.

The company the built the transformer told me it works either way but, If I wanted a *label *stating 208 to 480, it would cost an extra $300 or so.

I paid for the label and the transformer only lasted a year. They sent a replacement....with the 480 to 208 label :jester:. They swore to me that it was the same exact unit.


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## NolaTigaBait (Oct 19, 2008)

220/221 said:


> I did it years ago (not my idea/design) to feed a huge Ac unit the size of a trailer. They ordered 480 and only had 208 on the property. Their solution was a transformer running 24/7 even though the church was only occupied generally on Sundays.
> 
> The company the built the transformer told me it works either way but, If I wanted a *label *stating 208 to 480, it would cost an extra $300 or so.
> 
> I paid for the label and the transformer only lasted a year. They sent a replacement....with the 480 to 208 label :jester:. They swore to me that it was the same exact unit.


Lol...It all about the dollars...


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## azsly1 (Nov 12, 2008)

i've done that for some domestic water pumps in a high rise. they got new pump skids with 480 motors and only had a 240 service. worked well.


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## Smoke (Feb 25, 2011)

NolaTigaBait said:


> Can you feed a transformer with 240 3 phase on the secondary and get 480/277 out of the primary?...In other words use it as a step up transformer, even though it's a step-down transformer...


i got a call a few years ago from a coworker on a job where they tried to do it as temp for something (i forget what). they failed to realize the input/output amps would be reversed and toasted the xfmr :laughing:


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## Jlarson (Jun 28, 2009)

BBQ said:


> OTH I have done it myself, have seen it done by others in the compaines I have worked for and come across it done by strangers and in each case it was working fine.:thumbsup:


It's very common for me too.

Sent from my Droid


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## Chris Kennedy (Nov 19, 2007)

Jlarson said:


> It's very common for me too.


Used to be, now Siemens and SqD I know both have 45 and 75 kva 208Delta 480y/277 tranny at the same price as the 480Delta 208y/120's.


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