Transformer 480 3phase to 120/208 3 phase

[JFLOAT]

have a 480v 3 phase to 120/208 3 phase 30KVA transformer. tranformer to feed a 120/208 100 amp panel with 15 amp load. Engineer wants to feed the transformer with a 3/90 amp breaker which is 250% using NEC 450.3(B). my concern is the note #3 which states the transformer to have a thermal overload protection and arranged to interrupt the primary current. the transformer not equiped with overload protection or a way to interrupt the incoming primary current.so if the transformer does not have these do we use the 250% or do we use 125% for the breaker size

 

[MSILKS]

Note 3 is used if you want to protect the transformer at greater than 250%, namely 600 or 400% depending on transformer impedence. Using a primary rating of 250% permits the transformer inrush current that occurs anytime power is turned off then back on to pass the breaker without tripping it (power outage). If the primary is protected at 250% the secondary should also be protected at 125% by either a single device or a combination of upto 6 devices that total no more than 125%.

 

[charlie b]

Take a closer look at the note. We can use up to 250% without further considerations. But if there is thermal overload protection, we can go as high as 400% or even 600%. That said, I always go with 125% on both primary and secondary.

 

[charlie b]

I see that I was typing too slowly again, and was beaten to the draw. :happyyes: