# Installation of transformer



## Robbie Rob (May 14, 2017)

Hello, I am very thankful to have this page to check myself being a fairly new business owner. I have had very little experience with Transformers. Mostly just the installation but never the sizing. I am looking to re-feed a section of a building with 300A 120/208 V my primary voltage is 480V three phase. My supply house told me I needed a 112.5 kVA transformer so my question is if I have the right breaker and wire size for my primary and secondary. The code book is very confusing. Primary would be fussed at 175A and ran in 2/0 wire. Secondary would be fused at 400A and ran in 600 kcm or 300 kcm Parallels. The Bilding only needs 300A to feed it, am I allowed to downsize the secondary fuses to 300 amp. Also to make it more confusing the customer has a unused transformer that is much bigger than 112.5 kVA is it possible to use it and use the same wiring just to avoid the cost of a new transformer. If I’m missing any information let me know. Thank you in advance for all your thoughts and help.


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## Navyguy (Mar 15, 2010)

Disclaimer up front, this is from the CEC so I am not sure it applies to the NEC or your situation.

You do not have to provide a secondary disconnect / fuse protection if the primary fuse will cover the secondary in a overload / short-circuit situation. Generally what that means is sizing the secondary to the max size of the transformer.

I would think (again I don't not know the NEC) that if you oversized the transformer you would have to provide secondary protection. The second part to all of this is the “in-rush” current; if you size the feeder / fuses too small for the in-rush current regardless of the actual load, you will continually blow fuses.

Experience tells me to match sizing of all the components to get a smooth working system.

Without doing the calculation I would say the 112KVA is about right.

Cheers
John


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## B-Nabs (Jun 4, 2014)

Navyguy said:


> Disclaimer up front, this is from the CEC so I am not sure it applies to the NEC or your situation.
> 
> You do not have to provide a secondary disconnect / fuse protection if the primary fuse will cover the secondary in a overload / short-circuit situation. Generally what that means is sizing the secondary to the max size of the transformer.
> 
> ...


I am also in Canada so I may not be correct, but as far as I've learned from reading here, in the US they do require secondary protection.

So, if you use the larger transformer you have there, you would need to size the primary wiring and OCPD for that transformer to allow for inrush as Navyguy stated, but you could size the secondary smaller with appropriate protection if that's all you need.

Someone who knows your local regulations should be along soon enough to comment further. 

Sent from my SM-G920W8 using Tapatalk


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## wildleg (Apr 12, 2009)

Robbie Rob said:


> Hello, I am very thankful to have this page to check myself being a fairly new business owner. I have had very little experience with Transformers. Mostly just the installation but never the sizing. I am looking to re-feed a section of a building with 300A 120/208 V my primary voltage is 480V three phase. My supply house told me I needed a 112.5 kVA transformer so my question is if I have the right breaker and wire size for my primary and secondary. The code book is very confusing. Primary would be fussed at 175A and ran in 2/0 wire. Secondary would be fused at 400A and ran in 600 kcm or 300 kcm Parallels. The Bilding only needs 300A to feed it, am I allowed to downsize the secondary fuses to 300 amp. Also to make it more confusing the customer has a unused transformer that is much bigger than 112.5 kVA is it possible to use it and use the same wiring just to avoid the cost of a new transformer. If I’m missing any information let me know. Thank you in advance for all your thoughts and help.


what is the load calc for what you are feeding
what is the size of the transformer
is this delta/wye or some other config 
?


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## Southeast Power (Jan 18, 2009)

I would buy a new transformer but, we've all pressed old transformers back into service. 
It's just deciding who is going to energize that primary.
Also,
I would use 500s on that 400 amp secondary or some parallel Aluminum.


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## brian john (Mar 11, 2007)

Navyguy said:


> Disclaimer up front, this is from the CEC so I am not sure it applies to the NEC or
> 
> I would think (again I don't not know the NEC) that if you oversized the transformer you would have to provide secondary protection. The second part to all of this is the “in-rush” current; if you size the feeder / fuses too small for the in-rush current regardless of the actual load, you will continually blow fuses.
> 
> ]




You MAY blow the OCP upon startup if you get that transformer online then until the first outage you are OK.

If you go with a CB with adjustable instantaneous you may be able to overcome the inrush issue.

To the OP if the old transformer has been offline for any period of time you should megger the transformer. Transformers that sit can absorb moisture, if the readings are low you can dry out the transformer.


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## raider1 (Jan 22, 2007)

Southeast Power said:


> I would use 500s on that 400 amp secondary or some parallel Aluminum.


If you protect the secondary conductors at 400 amps you could not use 500s. 240.21(C) prohibits the use of 240.4(B)(The Next Size Up Rule) for transformer secondary's. Since 500 Kcmil copper conductors have an ampacity of 380 amps at 75 degrees C you would have to use 600s.

Chris


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## Southeast Power (Jan 18, 2009)

brian john said:


> You MAY blow the OCP upon startup if you get that transformer online then until the first outage you are OK.
> 
> If you go with a CB with adjustable instantaneous you may be able to overcome the inrush issue.
> 
> To the OP if the old transformer has been offline for any period of time you should megger the transformer. Transformers that sit can absorb moisture, if the readings are low you can dry out the transformer.


Brian,
Would you dry out a transformer electrically energizing it? or mechanically with a dehumidifier? 
I remember a few projects along the way where we hung a string of lights inside of a switchboard or dried out a busduct with a welder.

I really cant see doing that now unless a replacement was on the way.


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## Navyguy (Mar 15, 2010)

60 watt rough service light bulbs work good! (if you are not in a hurry...)

Cheers

John


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## brian john (Mar 11, 2007)

Southeast Power said:


> Brian,
> Would you dry out a transformer electrically energizing it? or mechanically with a dehumidifier?
> I remember a few projects along the way where we hung a string of lights inside of a switchboard or dried out a busduct with a welder.
> 
> I really cant see doing that now unless a replacement was on the way.


I have taken transformer that were submerged and dried them out. This was a rare emergency)

We used fan forced heat and dehumidification (multiple commercial dehumidifiers in the electric room), we were able to get the readings from "0" primary to secondary and primary and secondary to ground above 250 megohms. That took 3 days.


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## frenchelectrican (Mar 15, 2007)

brian john said:


> I have taken transformer that were submerged and dried them out. This was a rare emergency)
> 
> We used fan forced heat and dehumidification (multiple commercial dehumidifiers in the electric room), we were able to get the readings from "0" primary to secondary and primary and secondary to ground above 250 megohms. That took 3 days.


Brian John ., 

Do you recall what size that transformer it was ?


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## brian john (Mar 11, 2007)

frenchelectrican said:


> Brian John .,
> 
> Do you recall what size that transformer it was ?


112.5 kva and another time 75 kva


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## frenchelectrican (Mar 15, 2007)

brian john said:


> 112.5 kva and another time 75 kva


That about the smallest I will try to save it otherwise anything else .,, all I will say the door is open and drag it out. 

the smaller one it is easy to get new one and be done with it quick. unless ya hit a very oddball one like 50KW 2.4KV transfomer then it will be different game.,


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## brian john (Mar 11, 2007)

frenchelectrican said:


> That about the smallest I will try to save it otherwise anything else .,, all I will say the door is open and drag it out.
> 
> the smaller one it is easy to get a new one and be done with it quick. unless ya hit a very oddball one like 50KW 2.4KV transformer then it will be a different game.,


The issue with the 75 KVA was it was in a pit and the opening for the pit was too small for another transformer. The 112.5 was a difficult installation and we needed to make a room for a new unit.


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