Transformer AIC Let-Thru

[Scott Pemberton]

When determining the AIC of a transformer on the secondary terminal, and using the Bussman approach, should i use the base KVA or the Fan Cooled (largest) KVA?

 

[wbdvt]

First off, let's get the terminology correct. AIC stands for Ampere Interrupting Capacity which is a rating for devices such as molded case circuit breakers relating to their ability to withstand fault current. This allows for breakers to be properly rated and installed on a system. For example: A mccb rated for 10KA cannot be installed in a location where the fault current is 12kA.

Now to your question: The amount of fault current on the secondary side of a transformer depends on the available fault current on the primary side and impedance of the transformer. The fan cooled rating is for loading of the transformer without affecting transformer life. The impedance is on the base kVA.

 

[kwired]

I'd say probably the base rating, if it has a fan cooled rating that is likely just how much you can overload it so to speak as long as you have the added cooling.

 

[kingpb]

If you use the fan cooled rating then the impedance must increase proportionally, so really no advantage of using those numbers as they will result in the same short circuit value. Base KVA and rated Z is all you need.