Transformer wire sizes

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PCEC

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B.C. Canada eh.
I'm looking at some drawings. there is a 125A breaker feeding a 150kVA 600V-120/208V tx with 3#1/0, on the other side of the Tx is 2 X 4#3/0 to a 400A PDC. It's a 3 wire PDC so already we can get rid of the 4th conductor.

Based on the 125A breaker can we reduce the wire sizes?
 
Yes the primary conductor size can be reduced in size. What's a PDC? Is there an OCPD on the secondary?
 
PCEC said:
I'm looking at some drawings. there is a 125A breaker feeding a 150kVA 600V-120/208V tx with 3#1/0, on the other side of the Tx is 2 X 4#3/0 to a 400A PDC. It's a 3 wire PDC so already we can get rid of the 4th conductor.

Based on the 125A breaker can we reduce the wire sizes?
Click to expand...
You must provide a supply side bonding jumper between the transformer and the first OCPD. This could be a non-flexible metal raceway, or a conductor.
 
So, the transformer is effectively limited to being a 130kVA transformer, via the 125A primary side OCPD. You should be able to treat the secondary side as thought it were being fed from a 130kVA transformer. That limits your rated secondary current to 360A. Because of that, (3)#3/0 conductors would give you an ampacity of 495A, which is plenty.
 
PCEC said:
I'm looking at some drawings. there is a 125A breaker feeding a 150kVA 600V-120/208V tx with 3#1/0, on the other side of the Tx is 2 X 4#3/0 to a 400A PDC. It's a 3 wire PDC so already we can get rid of the 4th conductor.

Based on the 125A breaker can we reduce the wire sizes?
Click to expand...
The 125 amp primary OCPD is less than the full load primary current...I would expect it will trip on inrush when you energize the transformer.
 
Yikes, I misread the question. I agree about the potential for tripping the primary side breaker when power is applied, unless there is a premag circuit involved. Also agree there needs to be some sort of conductor protection, even if the transformer protection doesn't require secondary protection.
 
Tip DS said:
Yikes, I misread the question. I agree about the potential for tripping the primary side breaker when power is applied, unless there is a premag circuit involved. Also agree there needs to be some sort of conductor protection, even if the transformer protection doesn't require secondary protection.
Click to expand...
Only two transformer types allow for no secondary protection.
 
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