I have a customer who has purchased a machine for his business which requires a 50 amp 3-phase 480 volt feeder, he also went and purchased a 75 KVA dry transformer to step his voltage up from the 208 3-phase he has availiable from an existing 3-phase 200 amp load center. My problem is I can only get a 125 ampere breaker in this panel to feed this 75 KVA transformer which should give me 54 amps at the high voltage side, my question is would this be a code violation and is the transformer output proportional to the input, will the transformer operate properly?
Under powered transformer
- Thread starter dphilippa
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Starting from the fixed CB rating of 125A, table 450.3(B) says that that value is 125% of transformer current -> 100A is maximum primary load. This rates the transformer at 36KVA (100A x 208V x 1.732). Taking this to the secondary gives 43.3A (36000/480/1.732) So not exactly 50A, but if the machine is meant to be fed by a 50A breaker, it's actual operating current should be around 40A (50A x .8) . Transformer should operate properly at about half its actual listing, just not quite as efficient. Think all this meets code as long as your conductors are sized to the OCP.
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- Wisconsin
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- PE (Retired) - Power Systems
dphilippa said:
You did not say if theis is a step-up or a reverse-fed transformer.
Reverse fed transformers usually have very high inrush currents. Your 125A breaker will probably not start your 75kVA if it is reverse fed.
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