# 208/120 Transformer



## McClary’s Electrical

caliguy said:


> My background is in mainly in residential, but I have a client who has a warehouse they wanted me to take a look at upgrading one of it's panels from a 200 amp to 400 amps. Right now they have a 208/120 3 phase 200 amp main breaker panel that is feed with 250 copper from a Sylivana transformer. The transformer's label has been removed. The transformer is feed by a 125 amp 3 phase breaker with 1/0 copper. Has anyone ran into this type of transformer. If so can you point me in the right, so I can point my client in the right direction to getting this working? Will I need to change out the wire from the 3 phase breaker to the transformer? Where can I find out if this transformer is capable of handling 400 amps
> Thanks


 

What voltage is the primary? This can get very expensive very fast. The last upgrade I priced similar to this came in at $26,000, and that was with a used transformer. You possibly can get a new service drop for less depending on the situation with poco. Yes, most definately would need to change the wire, xfmr, and breaker feeding the xfmr.


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## BBQ

Based on the 125 amp primary breaker and the 200 amp panel it is likely a 75KVA transformeer with a rated output of 208 amps.

For a full capacity 400 amp panel you would need150 KVA transformer which would have a rated output of 416 amps.

Primary current 

150x1000/480/1.73=180amps rated input.

Primary breaker

180x1.25=225 amps

180x2.5=450 amps

So the primary breaker and conductors should be rated at least 225 and as high as 450 amps.


On the secondary side

150x1000/208/1.73=416 amps rated output.

416x1.25=520 amps

The breakers and conductors can be as small as you want but cannot exceed around 520 amps.


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## frenchelectrican

Bob.,

I am not disagree with you but with your qoute this kinda got me a second.,,



> The breakers and conductors can be as small as you want but cannot exceed around 520 amps.


I know the conductors can go larger than the transfomer however the Interrupteurs { breakers } can not go over the transfomer rating which I did recall when I was reading in the NEC. { French part is very simauir but slightly diffrent methold }

Merci,
Marc


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## BBQ

frenchelectrican said:


> Bob.,
> 
> I am not disagree with you but with your qoute this kinda got me a second.,,
> 
> 
> 
> I know the conductors can go larger than the transfomer however the Interrupteurs { breakers } can not go over the transfomer rating which I did recall when I was reading in the NEC. { French part is very simauir but slightly diffrent methold }
> 
> Merci,
> Marc


The secondary side overcurrent protection can be 125% of the rated current or even more.




> Notes:
> 1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.


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## frenchelectrican

That figures and I know in French part we can go much as 140% depending on the conductor but typically 120% is pretty common with standard building conductors.

Merci,
Marc


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## Techy

you may not even have that much primary current available, need to do a load calc.


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## vizzolts

Sounds like your regular old 480 to 208WYE 75KVA as others have said. It has been my experience that most (not all) of the time when a customer says they need '400AMPS' their actual usage is much much less. Do a little survey of how loaded the 480V service is (odds are pretty low you don't have the extra capacity on the primary side) and more importantly do a load calc for what is going to be fed from your 120/208V transformer. Chances are good you could use the existing 125A primary to feed a 112.5KVA transformer (150A primary would be ideal), full load on a 112.5 would be about 311 AMPS. Since you can size your secondary overcurrent protection at 125% that puts you at 389AMPS aka 400A breaker. In this case all you would have to do is swap out your transformer, run a bigger secondary, rerun your GEC, and put in a 400A panelboard. If their load really is up towards 400A or more you'll be redoing the primary and putting in a bigger transformer than 112.5, oh well.


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## RIVETER

caliguy said:


> My background is in mainly in residential, but I have a client who has a warehouse they wanted me to take a look at upgrading one of it's panels from a 200 amp to 400 amps. Right now they have a 208/120 3 phase 200 amp main breaker panel that is feed with 250 copper from a Sylivana transformer. The transformer's label has been removed. The transformer is feed by a 125 amp 3 phase breaker with 1/0 copper. Has anyone ran into this type of transformer. If so can you point me in the right, so I can point my client in the right direction to getting this working? Will I need to change out the wire from the 3 phase breaker to the transformer? Where can I find out if this transformer is capable of handling 400 amps
> Thanks


If you have been at this for a while, just contact your normal supplier and they will give you the specifics.


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