transformers

[stanleycochran]

On a 75 KVA transformer, 480 volt, 3 phase primary, 240/120 volt 3 phase, 4 wire secondary what is the value of the neutral conductor ?

Or where is this specified in the CODE ? I understand it is 5% of the value of the transformer - is this correct and if so where can I find it.

 

[jim dungar]

Re: transformers

This is not a code issue.
In a single core transformer the laws of physics set the practical limit at 5%.

Three (or two) individual transformers do not have this problem. That is why the utilities can supply much larger 120V loads than you can get from a commercially available single unit.

 

[brian john]

Re: transformers

Jim:

I appear to be missing something in this conversation, Where is the 5% value for the neutral derived from?

Thanks in Advance

 

[bob]

Re: transformers

Brian
you are not alone in your confusion.
Jim
Is the 5% maybe the transformer impedance?
I do not understand the question.

 

[jim dungar]

Re: transformers

Every major transformer manufacturer of single core 480V delta to 240V delta with a 120V center tap units, has a single phase loading limit of 5% of the rated KVA.

This loading limit has to due with circulating currents in the core of the transformer. The only practical (read non-special) way around this 5% limit is to use individual single phase units connected into a three phase bank.