[RE-wrenches] Utility Transformer Sizing

[Drake]

Hello Wrenches,

Does anyone have any insights on utility transformer sizing. We are 
building a micro grid and connecting two, 200 A services, which are 
each currently fed by15 kVA transformers. Ohm's Law says that these 
transformers would be able to supply 62.5 A at 240 V.  A 200 A 
service requires a 4/0 AL feed. The  transformer can supply 31% of 
the service rating.

The upgrade is to a 400 A service. The transformer is rated at 37.5 
kVA, which would produce 156.25 A at 240 V, and will require 350 
kcmil AL cable. This transformer will supply 39% of the rated 400 A.

This could only make any sense with fully imbalanced loads at nearly 
max output, and we are required to balance the phases. The larger 
loads are supplied by 240 V. Even in a fully imbalanced condition, 
the main breakers would trip to clear any overloads if the 37.5 kVA 
transformer were run to a 200 A service.

Is there really a good reason for these specifications?

Thanks,

Drake

Drake Chamberlin
Athens Electric LLC
OH License 44810
NABCEP Certified Solar PV
740-448-7328
<http://athens-electric.com/>http://athens-electric.com/

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