Minimum size circuit for 45kva xfmr

[paul]

Customer has a new machine that needs (1) 240V 3-phase 30A circuit and (1) 240v single phase 30A circuit. They have a 45kva 480-240-120LT delta highleg transformer with 5.53% impedance thst they want us to use. Boss is wanting to feed it from a 480V 3 phase, 30A and use tap rule to feed fusef disconnects before the equipment. I'm worried that the inrush will trip the breaker. What's the minimum size circuit and overcurrent protection I should use?

 

[GoldDigger]

Boss is wanting to feed it from a 480V 3 phase, 30A and use tap rule to feed fusef disconnects before the equipment.

I hope that he is planning to use a transformer! :angel:
Seriously, inrush to the transformer might well be more significant that inrush or starting current to the motor.
Is the requirement for 240 single phase really a requirement for 120/240 single phase three wire? Are there any line to neutral loads on that circuit?

 

[jumper]

Why can't this transformer be protected at 70A as the primary protection.

This size is used in a few MH examples and it is 70A. Is the impedance a factor? If so, how do you calculate that in?

 

[paul]

I hope that he is planning to use a transformer! :angel:
Seriously, inrush to the transformer might well be more significant that inrush or starting current to the motor.
Is the requirement for 240 single phase really a requirement for 120/240 single phase three wire? Are there any line to neutral loads on that circuit?

That's what I am worried about, is the inrush starting up the transformer. Not the three phase loads.

After I brought this up to the powers that be, they did decide to install a panel. All is good now.

 

[bob]

70 amp