Sizing of Three Phase

Hi, I recently graduated as an Electrical Engineer so I'm new to the industry. The original load computation was a single phase but due to the additional motors, we switched it to 230V Single Phase, Line to Line. I've had a conversation with the Engineer of the Distribution Company and they said that I need to use a 3 - 37.5 KVA transformer. I was thinking that it is better to use a 2 - 50 KVA because it was more economical but my boss said that I should better follow and listen to the engineer to avoid the blame game. The decision to use 3 - 37.5 KVA is final but I just want to clear if it is an overkill or just the right size of transformer for the load computation because my curiosity kills me . The highest phase current of my 230 V 3 Phase line to line system is 170.37 A.​
 

topgone

Senior Member
Clue: What if you lose one of you transformers?

If you lose 1 of the 3 X 37.5, you can still operate your three-phase loads! Here's why: Total capacity = 3 X 37.5 kVA = 112.5 kVA;
Lose 1, the total capacity will be 57% of the total (if you banked the three single-phase transformers (37.5 kVA) = 112.5 X 0.57 = 64.5 kVA! Your requirement was = sqrt(3) X 170.37 X 230 = 68 kVA!
But, if you have 2 X 50 and you lost 1 transformer, you will be left with 50 kVA 3-phase transformer, which might not be enough of your needs!
Follow the "engineer" and live with no worries, IMHO.
 
Clue: What if you lose one of you transformers?

If you lose 1 of the 3 X 37.5, you can still operate your three-phase loads! Here's why: Total capacity = 3 X 37.5 kVA = 112.5 kVA;
Lose 1, the total capacity will be 57% of the total (if you banked the three single-phase transformers (37.5 kVA) = 112.5 X 0.57 = 64.5 kVA! Your requirement was = sqrt(3) X 170.37 X 230 = 68 kVA!
But, if you have 2 X 50 and you lost 1 transformer, you will be left with 50 kVA 3-phase transformer, which might not be enough of your needs!
Follow the "engineer" and live with no worries, IMHO.
Do we size the transformer based on my highest phase current or should I just based It on my total VA?
 

topgone

Senior Member
Do we size the transformer based on my highest phase current or should I just based It on my total VA?
Size them according to your requirements plus some fudge factor to account for surprises! But clients with deep pockets only want to get what they wanted.
 
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