Why do our electrical utilities use transformers way over their rated KVA?

by Nick Williams   Last Updated October 18, 2018 16:25 PM

My parents' house and my house have different electric utilities, but both are rural electric cooperatives. We both have 120/240V (60 Hz) split-single phase service (this is in the United States state of Tennessee). My service is sized 600A. My dad's service is currently sized 200A, but it's getting upgraded to 400A because his property has outgrown 200A.

My understanding is that transformer sizing should be fairly simple. Volts times amps equals volt-amps. Size your transformer for the volt-amps. I also get that utilities tend to under-rate things and run them "hot," but this just seems extreme.

(Related question: I was never clear whether the "volts" in this equation should be 120 or 240. Is the transformer rating based on the combination phase voltage, or the split phase voltage?)

My transformer has "37.5 kVA" stamped on the side of it. 600A x 120V = 72 kVA. My transformer is 52% the size it should be. Not terrible, but still seems very under-sized.

My dad's transformer (before this morning) had 1.5 kVA stamped on the side of it. They "upgraded" it to prepare for his 400A service upgrade. Now it says 2.5 kVA. 200A x 120V = 24 kVA. 400A x 120V = 48 kVA. His old transformer was 6% the size it should be. Now his new transformer is 5% the size it should be. That's even worse, by a huge amount. That's just ... crazy small.

This white paper backs up my calculations. What am I missing here? This can't possibly be correct.



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