Transformer Sizing

[A/A Fuel GTX]

I know a 75kva transformer with a 480v primary and a 208/120v secondary is typically used to feed a 200A loadcenter. I often see a 100A OCPD protecting the primary feeders going into the transformer. What are the calculations I need to justify the 100A OCPD? What calculation would prove to me that a 75kva transformer with a 480v primary is adequate to feed a 200A loadcenter @ 208/120v?:-?

 

[Keri_WW]

75KVA / 480V / sqrt(3) = 90 Max Full Load Amps

75KVA / 208 / sqrt(3) = 208 Max Full Load Amps

I typically protect the primary with a 125A breaker and the secondary with no more than a 300A breaker.

 

[Keri_WW]

I did leave out one vital tidbit, see Table 450.3 for the maximum settings for OCPD on transformers.

 

[A/A Fuel GTX]

Thank You.....:smile: