Transformer Calculations Check

[laketime]

Have a customer that wants power set up for MRI unit. MRI unit calls for 480v 3 phase 125kva fused at 150 amps. The building has 120/208v 3 phase power available. By my calculations I will need a 150kva transformer. The primary side of the transformer needs to be fed with 350 amps? :jawdrop:

 

[Ingenieur]

125000/(480 x sqrt3) = 150 A
125000/(208 x sqrt3) = 347 A

iirc 150 kva is a std size

talk to the mri mfg, they may have special requirements, shielding, etc

 

[JoeStillman]

My calcs show 417 amps on the primary of a 150 kVA, 208V-480V step-up transformer. I wouldn't feed it with less than a 500A breaker so it can hold the inrush. Check your TCC curves; you might want to use a higher AIC than necessary so that you get a higher instantaneous that's to the right of your inrush point.

 

[Tony S]

Don’t you have to base calculations based on the transformers FLC. I get 416A for 150kVA @208V. Sorry for making your predicament worse.

 

[laketime]

Don’t you have to base calculations based on the transformers FLC. I get 416A for 150kVA @208V. Sorry for making your predicament worse.

No worries...they want me to feed the system off a 200 amp service lol...

 

[Ingenieur]

since you need a new service get a dedicated 480 for the mri?

 

[laketime]

since you need a new service get a dedicated 480 for the mri?

That will be my suggestion. Thank you all for your help again.