breaker size

[vinod]

What will be the breaker size for 75KVA (480/277-120V 3phase) transformer primary side. calculated primary current will be 90Amps. Can I use 100Amps breaker? Or is there any calculation to find out the breaker size?

 

[ron]

It is dependent on the continuous load you plan to draw and the inrush current that you need to overcome. A 125A breaker minimum is my suggestion.

 

[mayanees]

NEC Table 450.3(B)
I would use 2.5*90 = 225A primary protection..... perhaps 200A since it might be a more readily available breaker in your panel, but maximum 225.
JM

 

[vinod]

thanks

thanks

thanks for yours help

 

[mayanees]

..to clarify Ron's and my comment, Ron's referring to primary only protection, and I'm referring to primary and secondary protection, both illustrated in 430.52(B).
Primary only: 125% of 90 = 112.5, rounded up to 125A
Primary and secondary: 225A primary (max) as described above; secondary - 75kVA/208/3-phase = 208A * 1.25 = 260A.. use 300A per note 1

JM

 

[wasasparky]

Industry standard would be 125A on the primary and 225A on the secondary.
If money is no object, oversizing is possible as described above...

 

[infinity]

Industry standard would be 125A on the primary and 225A on the secondary.
If money is no object, oversizing is possible as described above...

I agree. This seems to be the standard by which we install most 75 KVA transformers.

 

[vinod]

..to clarify Ron's and my comment, Ron's referring to primary only protection, and I'm referring to primary and secondary protection, both illustrated in 430.52(B).
Primary only: 125% of 90 = 112.5, rounded up to 125A
Primary and secondary: 225A primary (max) as described above; secondary - 75kVA/208/3-phase = 208A * 1.25 = 260A.. use 300A per note 1

JM

I think the article is 450.3 (B). Am I correct?

 

[mayanees]

Yessir you are correct, 450.3(B)
Don't know where that reference came from! Thanks for the correction.

Since I'm posting again on this topic, I'll address a previous comment that said if money was no object, 450.3(B) is okay to follow. I say that in some cases it's necessary, particularly oversizing of the primary breaker to accommodate inrush current.
A good application for the oversized primary breaker is if the transformer were located outdoors, and exposed to freezing temperature. Initial energization would have maximum inrush. In that case, the required primary cable would be kept as short as possible in order to minimize expense.
JM