20 kVA UPS installation and upstream transformer sizing

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We're installing a new 20 kVA UPS, with an input voltage of 120/208V.

Option 1: There is a 120/208V distribution panel with enough spares for the 3-pole breaker required for the new UPS. The concern is that the upstream transformer for this panel is 15 kVA. My reasoning is that the loads on the new UPS are less than 5 kVA and we won't experience any overloads. No new equipment will be planned to be added as loads on the UPS.

Option 2: The 480 V MCC has a spare bucket, where we can install a new circuit breaker, and we can feed a new 45 kVA transformer, and use that to feed the 20 kVA transformer. The concern here is scope creep and additional costs in terms of labor and equipment.

Is there any code violations for installing a 20 kVA UPS on a distribution panel that only has a 15 kVA upstream transformer, even though we only plan on putting about 5 kVA total load on the UPS? Other loads on the distribution panel are minimal.
 
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