How to size transformer when you have a 3 phase load as well as single phase loads?

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aric.martens

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lexington park MD
If I have a 480/208/120 volt transformer three phase and have the following loads: 100 KW of 208 volt 3 phase pumps, and 250 EA 0f 120 volt general purpose receptacles? How do I properly size this?
 
aric.martens said:
If I have a 480/208/120 volt transformer three phase and have the following loads: 100 KW of 208 volt 3 phase pumps, and 250 EA 0f 120 volt general purpose receptacles? How do I properly size this?
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A 480/208/120v transformer? Or, do you have a 480v Delta primary to as 208y/120 secondary trsnsformer which is a very common configuration?
 
You can distribute your 120v load between all the phases. Without knowing the exact load the NEC would consider it at 180va per outlet adding 45kw to your 100 kw 3 phase load. The closest size would be a 150kva but with motors involved and without knowing the exact receptacle load I would likely use a 225kva. More info on the motor load could help with the decision.
 
aric.martens said:
If I have a 480/208/120 volt transformer three phase and have the following loads: 100 KW of 208 volt 3 phase pumps, and 250 EA 0f 120 volt general purpose receptacles? How do I properly size this?
Click to expand...
electrofelon said:
How was the 100 KW figure for the motors arrived at? Was it by the proper NEC method?
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Along with electrofelon's query ....Why not run the motors 480V and just use the transformer for the 120V?
 
If you’re stuck with 208V motors there’s not much can be done. Even dual voltage DY motors will be 33% over voltage in Y.

The OP mentions 100KW of pump motors. 10x10KW, 4x25KW????

Far more information needed.
 
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