480V Fire Pump Fed From 208V Service

[Mevlevi]

We have a situation whereby we have to feed a 480V fire pump from a 208V service through a step-up transformer. Does the following setup comply with code?

Fire Pump HP: 40HP
Fire Pump FLA: 52A (480V)
Step-up Transformer KVA: 52 x 480 x 1.73 = 43.2kVA (45kVA is too close, use 75kVA)
Fire pump feeder on secondary side of the transformer: 52 x 1.25 = 65A (6 AWG)
Fire pump current on the primary side of the transformer: 43.2/(208x1.73) = 120.3 A
Fire pump feeder on the primary side of the transformer: 120.3 x 1.25 = 150.4A (2/0 AWG)
Fire pump OCPD on primary side of the transformer: 120.3 x 6 = 721.8A (next size up 800A)

The fire pump OCPD will be fed from the 208V utility service transformer. It is my understanding that the OCPD on secondary side of the transformer is not required in this situation.

 

[Jraef]

I have lost some of my knowledge on this, but you can double check in Article 695. I think the transformer must be sized at a minimumof 125% of the motor kVA. So 43.2kVA x 1.25 = 54kVA, so that validates your 75kVA decision, but your initial brief thought of 45kVA was never going to work.

 

[topgone]

Your power source should be able to carry the locked-rotor currents of the fire pump motors "indefinitely". The usual thing we do is size the transformer feeding the pumps at 125% of the biggest unit and the sum of the other loads of the pumphouse (52 x 1.732 x 480 x 1.25 =53 kVA~ 75 kVA. The catch is that you size the OCPD of the transformer to carry the pump load "indefinitely"; the locked-rotor amps of a 40 HP motor @ 480V is 290A, so 125A primary circuit breaker will suffice (secondary overcurrent protection is not allowed)!