Transformer OA/FA/FOA secondary feeder size amps

[JEFF MILLAR]

The transformer is sized for the design running load. plus 25 %. or 20% for future expansion. You size the secondary. conductors for this design load. i.e. AMPS x 1.25 %. This is my experience. Your comments as always welcome.

 

[JEFF MILLAR]

The question is simple as written, How do you size the secondary conductors for OA/ FA / FOA. 55/ 65C TRANSFORMER.
Two possible answers , the OA Rated KVA at 55 C , OR the max possible Rated KVA at FOA, 65 C. And a reference addressing this would be much appreciated. I have searched all over for a simple technical to this problem and can find none, I know how to size a transformer, calculate the short circuit, size the overload protection. Answers how to start the largest motor are all easy to find. But no where is there a simple written answer how to size the secondary conductors. The answer could be to size the conductors for the nameplate largest Rated KVA. Motor starting where is it written that you can use the FA or FOA Rated KVA in the calculation. Please for give my lack of field experience. This must be a simple answer based on daily industrial applications. Thank you

Live and learn, we learn from the shared experience of others who have been there and done that. I have learned that Transformers are sized for the design running load plus a future expansion factor. ( And not. the total connected load ) Why because some loads are CONTINUOUS some INTERMITTENT and some STAND - BY.

 

[don_resqcapt19]

The question is simple as written, How do you size the secondary conductors for OA/ FA / FOA. 55/ 65C TRANSFORMER.
Two possible answers , the OA Rated KVA at 55 C , OR the max possible Rated KVA at FOA, 65 C. And a reference addressing this would be much appreciated. I have searched all over for a simple technical to this problem and can find none, I know how to size a transformer, calculate the short circuit, size the overload protection. Answers how to start the largest motor are all easy to find. But no where is there a simple written answer how to size the secondary conductors. The answer could be to size the conductors for the nameplate largest Rated KVA. Motor starting where is it written that you can use the FA or FOA Rated KVA in the calculation. Please for give my lack of field experience. This must be a simple answer based on daily industrial applications. Thank you

Assuming the transformer is sized to supply the calculated load, you simply ignore the transformer and size and protect the secondary conductors based on the calculated load.