AA/FA Transformer Rating for Sizing

[minesh21]

Hi,

I have a 4160/480V transformer that is 1500/2000KVA rated. The 2000KVA is with fan assist. Do I size my conductors and overcurrent protection based on the standard 1500 KVA rating or the FA 2000KVA rating?

Also, I read the impedance (5.75% in my case) is based on the 1500 KVA rating. Do I need to consider the 2000 KVA impedance rating for short circuit calcs? If so, how is this determined?

 

[ron]

The primary OCPD and conductors for MV transformers are typically sized for SC protection and to allow the FA amount of current to pass continuously. Which makes sense because you would want the fans to be able to come on and the transformer to operate at its full load rating.

Regarding the impedance, it is per unit based on the base rating, which is the AA value (1500 in your case). The fans coming on do not change the transformers impedance impedance or internal working other than getting rid of heat, so it can push a bit more power throw its parts.

 

[publicgood]

2000kva sizing conductors and OCPD. Impedance stays the same, no additional short-circuit for fan rating.

 

[minesh21]

The primary OCPD and conductors for MV transformers are typically sized for SC protection and to allow the FA amount of current to pass continuously. Which makes sense because you would want the fans to be able to come on and the transformer to operate at its full load rating.

Regarding the impedance, it is per unit based on the base rating, which is the AA value (1500 in your case). The fans coming on do not change the transformers impedance impedance or internal working other than getting rid of heat, so it can push a bit more power throw its parts.

Ok well in my situation the NEC allows 300% for the fuse, which puts the ampacity at 624A if we are referencing the 1500 KVA. Square D says to use 133% on the primary fuses, which is only 276A. I was specifying a 300A fuse, but now that you mention sizing based on the 2000KVA, then I will need to account for a higher amperage. So I would need 277 x 1.33 = 368A, so a 400A fuse?

 

[minesh21]

The reason I mentioned impedance values for FA, is because I came across this article:

http://www.ecmweb.com/design/how-impedance-values-can-fool-you

It says that you should account for a higher impedance value and adjust your short circuit values (otherwise you are over sizing the short circuit rating of equipment).