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Discuss Unbalanced Load in the Electrical Forum area at ElectriciansForums.net

F

freakydeakie

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Would someone please advise me on the correct way to calculate the main protection device required when I have two 25KVA 2 phase 380vac transformers with a secondary voltage of 120vac.
The First transformer would be connected across phase 1 & 2, the second across phase 2 & 3, therefore giving me an unbalanced load.
I understand how to protect it if it was evenly balanced, but what are the calculations when the loads are unbalanced??
Thanks in advance
 
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25KVA / 380V equals the primary demand. Are you sure it is not a 415V supply?
don't worry too much about unbalanced load as long as your supply can take the load of twice the primary demand current for the one phase.

Each tranny protected by dual pole mcb, preferably a c type or d type ( high inrush current )
 
F

freakydeakie

  • Thread Starter Thread Starter
  • #3
Hi and thanks for the reply. OK Yep its 415vac. From what you say above am I correct in the following??
25000/415 = 60.24A, therefore phase 2 would be 60.24A x 2 = 120.48A.
Supply requirement would be 125A, or 86KVA approx.
Appreciate your help. thanks
 
K

Knobhead

  • Thread Starter Thread Starter
  • #5
The two transformers and their individual protection should be regarded as separate with a DP MCCB each.
If you can get adjustable MCCB’s they can be matched to the transformers, if not then 60A type D may be pushing your luck.
 
Yeah, you are right, might be pushing it a bit, 'pents really where it is fed from and what make of breakers you can use.
 

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