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

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freakydeakie

EF Member
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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
 

stef

Regular EF Member
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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 )
 
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freakydeakie

EF Member
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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
 

stef

Regular EF Member
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576
Location
Gosport
Yes, sounds about right. What are the trannys for if I may ask? Supply for a machine?
 
K

Knobhead

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.
 

stef

Regular EF Member
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576
Location
Gosport
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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