Transformer conductor sizing?

[PsychicViking]

I have a 50 KVA single phase transformer (480V primary 240V secondary) that I will use to feed a 200A Main Breaker 240V/120V panel.

I want to run (2) #3 THHN copper feeders 275' on a 100A breaker.

Can I use 80A to calculate Primary conductor size, OCPD, and voltage drop instead of the 104A rating of the transformer.

The 80A is 80% of the panel rating and allows for future expansion.

I did a 30-day demand recording. Highest recording was only 45A.

I want to keep the cost down and if I size everything @ 104A it doubles the cost e.g.... larger wire, larger disconncet and breaker jumps up to next frame size.


Thanks

 

[don_resqcapt19]

As long as the load is less than the ampacity of the conductors and the conductors are properly protected there is no issue with that part. There is one possible issue of the smaller breaker not being able to handle the transformer inrush without tripping.