How is the AIC rating figured

[Dennis Alwon]

I know there is a formula to determine the AIC from trany to service etc however, what determines the aic rating of the actual transformer? If 2 transformer are rated the same kw would the aic rating have to be the same? If not, what would change that value?

For instance, if I have a 480-208 trany that is rated 5000 kw would that have the same aic rating of all trany's marked as such?

 

[david luchini]

I know there is a formula to determine the AIC from trany to service etc however, what determines the aic rating of the actual transformer? If 2 transformer are rated the same kw would the aic rating have to be the same? If not, what would change that value?

For instance, if I have a 480-208 trany that is rated 5000 kw would that have the same aic rating of all trany's marked as such?

Not sure what you are asking... transformers don't have an AIC rating. If you are thinking about availabile short circuit current through a transformer, then the transformer % impedance would come into play. Two transformers with the same voltage and kva ratings, but with different impedances would have different available short circuit currents.

 

[Russs57]

In the past you shorted secondary and measured voltage required on primary to reach rated current.

 

[jim dungar]

Not sure what you are asking... transformers don't have an AIC rating. If you are thinking about availabile short circuit current through a transformer, then the transformer % impedance would come into play. Two transformers with the same voltage and kva ratings, but with different impedances would have different available short circuit currents.

As David said, you can determine the amount of available Short Circuit Amps (SCA) that a source can provide.
You then select protective devices with an Amps Interrupting Rating (AIC) rating higher than the available SCA. Likewise, non-interrupting equipment must have an Short Circuit Current Rating (SCCR) that is also higher than the SCA.

AIC ratings are determined by actual testing per standards. SCCR is usually determined by testing or by using look-up tables, although there are sometimes ways to calculate it.

 

[Dennis Alwon]

Not sure what you are asking... transformers don't have an AIC rating. If you are thinking about availabile short circuit current through a transformer, then the transformer % impedance would come into play. Two transformers with the same voltage and kva ratings, but with different impedances would have different available short circuit currents.

So if 2 tranies are rated the same but have different impedance then the sca is different, then what causes the different impedance? Is it the material used or many different factors

 

[GoldDigger]

So if 2 tranies are rated the same but have different impedance then the sca is different, then what causes the different impedance? Is it the material used or many different factors

Yes, both, sort of.....

The more copper you use in the transformer for a given power level, the lower the % impedance of the transformer. You could also design a stupidly high % impedance in a transformer by putting too much of your copper on the primary side (increasing unloaded efficiency) and too little on the secondary side (reducing full load efficiency.)
The efficiency of the transformer will also be affected by the size and material of the magnetic core and the geometry of the windings around the core, but that would not necessarily have any large effect on the % impedance and therefore on the available SCA from the transformer.

In a real life calculation the SCA at the secondary will also depend on the source impedance of the POCO lines connected to the primary.
You may be able to get a legitimate estimate of that from POCO or they may just tell you to use an infinite bus model (i.e. assuming no series resistance for the POCO network on the primary side, as if the primary were connected via zero resistance to an infinitely large generator or array of generators.)

As a practical matter, more modern high-efficiency transformer designs (i.e. green) will also have a lower % impedance.

 

[smoothops10]

Breakers have an AIC rating
Busses have an SCCR rating
ATS have a WCR rating
Disconnects have a SCCR rating (take note of this, often missed and only 10kA per UL 98)
Switchboards, panelboards, MCC have a SCCR rating unless a breaker or starter with lower AIC or SCCR rating is installed - then the whole assembly is the lower rating
Conductors have a withstand rating, not really addressed by NEC IMO but mostly cover by breaker listing, should check in some cases like transformer secondary, reference ICEA withstand chart

....

trannies - nothing, no rating I’m aware of.