Hello, I have a 75KVA 480 Delta to 208/120 Wye transformer …
can I safely wire it to a 480 three phase breaker using only 10awg wire. … (and a 30A CB)
Will the inrush current, with no load, be significant enough to require larger gauge wire for the primary?.
No, this will not work reliably.
Inrush current for a 75kva, 480V can be as high as 11 x FLA = 990A.
The Instantaneous trip for a 30A cb is ~ 270A.
GE says the max inrush will happen 1 in six energizations. They don't say anything about minimum energization.
The connected load has no bearing on the inrush current - unloaded, loaded does not matter.
That being said, I have seen one case where a 225KVA was fed with temporary power, 100A CB. When energized, the xfm oozed on line, held the 50A load just fine. We never turned it off for the duration of the temporary power requirements.
For your installation, A long #10 feeder will help. The added impedance will reduce the inrush.
So, give it a try. Connect a peak reading clamp-on ammeter so you can see what you are doing to the CB.
Don't be surprised if the CB trips. Reset and try again.... and again ….. and again. You might get one to hold.
The installation is not particularly unsafe, although after 5 or 6 times of the CB tripping on the instantaneous, I'd be wondering what I was doing to the CB contacts.
For references, do a search on "transformer inrush". It is a statistical issue. It has to do with the residual flux in the core
I used a SquareD 30A FA for a typical time current curve for thermal magnetic 30A CB. That is pretty generic.