Primary xfmr Conductors

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ron

ron

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A client intends to fully utilize the capacity of a 300kVA 480-208/120V D-Y low inrush transformer.
FLA is 361A.
If we use the calculated 361A for the 480V current, and we have an upstream 400A 100% rated breaker, do the 480V conductors need to be sized for 400A or some multiple of the 361A?
 
ron said:
A client intends to fully utilize the capacity of a 300kVA 480-208/120V D-Y low inrush transformer.
FLA is 361A.
If we use the calculated 361A for the 480V current, and we have an upstream 400A 100% rated breaker, do the 480V conductors need to be sized for 400A or some multiple of the 361A?
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The primary conductors need to be sized for the primary overcurrent protection, 400A in your example.
 
Since you are allowed the 125% (at least) factor on the transformer and there is nothing to prevent the load from reaching 400 amps, I would say 400 amp conductors.
 
ron said:
A client intends to fully utilize the capacity of a 300kVA 480-208/120V D-Y low inrush transformer.
FLA is 361A.
If we use the calculated 361A for the 480V current, and we have an upstream 400A 100% rated breaker, do the 480V conductors need to be sized for 400A or some multiple of the 361A?
Click to expand...
If you want to use the transformer to full capacity, your conductors prior to the application of adjustment and correction factors should be not less than 361A ? 125%, or 451A, allowing for 100% continuous loading.

If using a Table 310.16 75?C wiring method, the smallest you could use is 700kcmil@460A. For 90?C, the smallest you could use is 600kcmil@475A. The 75?C termination temperature limitations would not be a factor: 361A only requires 500kcmil or greater.

Parallel conductors is also a consideration. If you want to factor for combination continuous, non-continuous loading, that too is a design decision.
 
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Smart,
If you want to use the transformer to full capacity, your conductors prior to the application of adjustment and correction factors should be not less than 361A ? 125%, or 451A, allowing for 100% continuous loading.
Click to expand...

Exception#1 to 215.2(A)(1) seems to exclude the need for the conductors at 125% for continuous loading.
 
ron said:
Smart,


Exception#1 to 215.2(A)(1) seems to exclude the need for the conductors at 125% for continuous loading.
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Your earlier statement is confusing me. You either have a 100%-rated breaker assy, or you do not. The breaker by itself does not constitue a 100% application. It must be matched with an appropriate enclosure.

If you do have a 100%-rated application, then your conductors can be sized to not less than 361A... but, as I understand it, they must be 90?C conductors, and the ampacity must be based on the 75?C column ampacity... requiring minimum 500kcmil conductors.

Here's an application paper from Eaton...

http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&dID=225062

Here's a data bulleting from SqD, but in glancing through it I did not see the 90?C wire requirement....

http://ecatalog.squared.com/pubs/Circuit Protection/0600DB0101.pdf
 
Smart is correct about the conductor sizing, but I think the more important issue here is making sure you use a rated assembly as stated. I'm also curious as to the OCPD: how low is the in-rush exactly? or are you using a starting method that prevents high inrush or perhaps using an expensive breaker with adjustable short circuit trips? 400A rated breaker may not be sized correctly.
 
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