15kva 480V-120/240V Single Phase transformer protection primary/secondary calculations??

[TexasEE14]

I have a 15Kva 480V-120/240V single phase transformer. I am wanting to determine and calculate the required ocp protection and the conductor sizing. (Secondary is feeding power panel using both 120V circuits and 240V circuits.)

1.) What specifically determines if you are to use "primary & secondary" OR only "primary" ocp % values in table 450.3(B) ?

2.) How to size primary and secondary conductors? Is there a % above/below the rated primary/secondary current you go by?

Any help would be greatly appreciated. I have read through so many posts asking similar questions regarding sizing but cant seem to find a direct solution.

 

[TexasEE14]

I have a 15Kva 480V-120/240V single phase transformer. I am wanting to determine and calculate the required ocp protection and the conductor sizing. (Secondary is feeding power panel using both 120V circuits and 240V circuits.)

1.) What specifically determines if you are to use "primary & secondary" OR only "primary" ocp % values in table 450.3(B) ?

2.) How to size primary and secondary conductors? Is there a % above/below the rated primary/secondary current you go by?

Any help would be greatly appreciated. I have read through so many posts asking similar questions regarding sizing but cant seem to find a direct solution.

It is a 3 WIRE (L1,L2,N) AND GROUND secondary

 

[jim dungar]

I have a 15Kva 480V-120/240V single phase transformer. I am wanting to determine and calculate the required ocp protection and the conductor sizing. (Secondary is feeding power panel using both 120V circuits and 240V circuits.)

1.) What specifically determines if you are to use "primary & secondary" OR only "primary" ocp % values in table 450.3(B) ?

2.) How to size primary and secondary conductors? Is there a % above/below the rated primary/secondary current you go by?

Any help would be greatly appreciated. I have read through so many posts asking similar questions regarding sizing but cant seem to find a direct solution.

Transformer need primary side protection at 125%, per 450.3.
The secondary side conductors need protection per 240.21(C).
The secondary side panel need protection per 408.

It is possible for the panel protection to also protect the conductors, if sized and located appropriately.
It is also possible, but not required, for the secondary side conductor protection to allow the primary side device to be larger than 125% but less than 250%.

I find it easier to start with my loads and make equipment, conductor, and protection selections as I work backward to the transformer feeder.