Transformer is 75kva 3 phase 120-208. The panel does not have a main breaker just (4) 100 amp breakers. I'm trying to figure out if setting a 200 amp single phase subpanel is going to be to much for the transformer to handle. I am calling the local power company to see what the usage in Kwh's. What is the best way to determine if the transfomer can handle the additional load? How do I turn Kwh's into useful information? Thanks!
iwire, You are right about not using a single phase panel. A 3 phase panel will be used. Here is what I see when I go there. A transfomer that has the meter and a lock that has the power company name on it. A main panel that has 3 (not 4, I looked back at my notes) 100 amp breakers on it. The owners showed me 3 sub-panels inside the building. What useful info. can I get from the POCO? According to UGLY's a 3 phase 75kva transformer is capable of producing 180 amps per leg. Even though each leg has to be protected by overload protection at that amount, does that mean the transformer is capable of producing 540 amps overall?
A 75kVA delta/wye transformer is capable of ( 90.2 x 4 = 360.8 line to neutral amperes at 120 volts per each phase.
The 90.2 amps is the line to line draw of a delta connected 480 volt source. The turns ratio is 4:1 going from delta to wye. ( 480/120 )
Typical practice is to only partially load these transformers, ie max the draw at 225A line to neutral -- each phase.
Even if the delta voltage were higher... the load side calculations would remain the same for a 75kVA transformer.
Your latest info indicates that you could double tap this transformer. Do some load calculations. If the transformer belongs to the utility present them and the AHJ these load calculations in your submission for increased service.
I sure hope that the three 100 amp breakers have a common handle tie; in which case it's a single three pole breaker ... not 3 breakers.
This must be the first time through for you. Hook up with someone more experienced as soon as possible. Farm it out.
You guys are using line to line; not line to neutral.
The vast bulk of the loads in the panel will be at 120V. These are line to neutral loads.
When you inspect a dry type transformer you will note that the high side is delta connected -- line to line -- 480 volts. The load side windings are connected wye with a turns ratio of 4:1.
The line to line of 208A at 208V would be used if you had a delta/wye transformer running 208 delta to 208/120 wye. They are made typically as isolation transformers when the Service is 208/120 wye but isolation is desired. Such a transformer would still produce 360.8 line to neutral amps if it was 75kVA.