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Discussion Starter · #1 ·
Ok, I think I understand that:
When you have primary and secondary ocpd, Primary ocpd can be up to 250% of the transformers ampacity. Also, the secondary ocpd can be no more than 125% of the load being served. Also, I understand that what ever ocpd you install on the primary, the primary conductors must match. This being said and hopefully correct then....What determines if you upsize the primary fuse and feeders? I understand the inrush of the transformer when energized can blow the fuses. So at the sake of getting the job with the smallest conductors/pipe/fuses, how do I know if I am going to have an issue on start-up? Experience?

Thanks all!
 

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Ok, I think I understand that:
When you have primary and secondary ocpd, Primary ocpd can be up to 250% of the transformers ampacity. Also, the secondary ocpd can be no more than 125% of the load being served. Also, I understand that what ever ocpd you install on the primary, the primary conductors must match. This being said and hopefully correct then....What determines if you upsize the primary fuse and feeders? I understand the inrush of the transformer when energized can blow the fuses. So at the sake of getting the job with the smallest conductors/pipe/fuses, how do I know if I am going to have an issue on start-up? Experience?

Thanks all!
I believe it could be calculated but many factors apply, from the design of the transformer to the available fault current etc.

The calculations are beyond me so I would try to find specs from EEs for similar sized transformers.
 

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Discussion Starter · #3 ·
Thanks BBQ, just another layer of confusion on my part.
CAT# DA51H-N1/K4/BE
KVA 51
TYPE:ANN
3 PHASE ISO
480DELTA-400Y/231

MAX LOAD 68A
 

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What determines if you upsize the primary fuse and feeders? I understand the inrush of the transformer when energized can blow the fuses. So at the sake of getting the job with the smallest conductors/pipe/fuses, how do I know if I am going to have an issue on start-up? Experience?

Thanks all!
I have always struggled with this too.
Most engineered drawings I see, the primary wire/ocpd are sized at 125%.
That's what I use when there are no engineered drawings.
 

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Discussion Starter · #5 ·
I have always struggled with this too.
Most engineered drawings I see, the primary wire/ocpd are sized at 125%.
That's what I use when there are no engineered drawings.

Thanks!! Really! Glad to feel Im not alone on this. This machine is going to be continuous duty, so I already sized to 125%. Im wondering of this is all the adjustment needed, or would you go an additional 125%.

Im so confused! lol:icon_confused:
 
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