Transformers

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coop3339

Senior Member
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NJ
yes must have same impeadance and winding ratios. Be careful to read about how to do it correctly. You have to connect the primary and then one secondary at a time, measuring for voltage before making the connection each time. Did it once on an office expansion about 20 years ago.
 

ron

Senior Member
I don't think the code prohibits it directly, but if you are not careful, you might consider that you are not going to be able to protect the conductors to the individual transformers at their respective ratings.

From an application standpoint, they both would need to match configurations (delta - wye) and you would want their impedances to be pretty close.

We do this on occasion when closed transitioning two sources on the transformer secondaries in a critical facility.
 

iceworm

Curmudgeon still using printed IEEE Color Books
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EE (Field - as little design as possible)
Can you parallel two 45KVA transformers to get 90KVA?

If not what code article?

I don't think the code prohibits it directly, but if you are not careful, you might consider that you are not going to be able to protect the conductors to the individual transformers at their respective ratings...
Adding to ron's post:

Assumption - three phase transformers, such as 480D/208Y

Yes. However you can't just tie the xfm primaries together and tie the transformer secondaries together and treat as a single transformer. You still need to protect each transformer per art 450.

Each transformer may require a separate primary CB.
Each transformer secondary may require a separate CB.
And, of course, the set of secondary conductors carring the combined xfm currents will require protection.
It can be done with three CBs. I don't think you can do it with two.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Whoops -- forgot another warning.

Depending on the loading you may wish to set up the transformer CBs for a common trip. If one xfm goes off-line, you likely will want the other to trip as well. This follows from the assumption that the design loading is more than 45kva - and one transformer would be overloaded.

ice
 

Carultch

Senior Member
Location
Massachusetts
yes must have same impeadance and winding ratios. Be careful to read about how to do it correctly. You have to connect the primary and then one secondary at a time, measuring for voltage before making the connection each time. Did it once on an office expansion about 20 years ago.

Is it easy to guarantee that a new transformer has the same impedance as an existing transformer? I thought that this is a value that the manufacturer builds within a certain specified range. It will be near a certain value, but you can't know it until it is built.

That is my understanding. Let me know where I am wrong.
 

kingpb

Senior Member
Location
SE USA as far as you can go
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Engineer, Registered
Is it easy to guarantee that a new transformer has the same impedance as an existing transformer? I thought that this is a value that the manufacturer builds within a certain specified range. It will be near a certain value, but you can't know it until it is built.

That is my understanding. Let me know where I am wrong.

The standard says within +/- 7.5% of the impedance specified; unless you ask for something else. You can specify another tolerance, but it will cost you $. But, for a 45kVA, I doubt you would buy custom made equipment.
 
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