what is the maximum AIC on the secondary side of the transformer

[jmartinez1959]

this data is from a 12KVA tansformewr with 480/277VAC secondary feed an 800A switchgear from a distance of 10ft and I am trying to answer the question. what is the maximum AIC on the secondary side . how do I calculate the secondary full load current of the transformer and which value I use for the transformer impedance, is it the X1 ohms?

 

[jim dungar]

this data is from a 12KVA tansformewr with 480/277VAC secondary feed an 800A switchgear from a distance of 10ft and I am trying to answer the question. what is the maximum AIC on the secondary side . how do I calculate the secondary full load current of the transformer and which value I use for the transformer impedance, is it the X1 ohms?

You cannot calculate an AIC. The AIC is determined by the protective device manufacturer.

I think you are asking what is the maximum available Short Circuit Amps (SCA).
Transformer impedance is typically abbreviated %Z. If you have OHMs you need to convert them into a single Per Unit value, combining both the X and the R components.

After you calculate the SCA you will select equipment with a higher AIC or SCCR.

 

[don_resqcapt19]

The impedance is 3.5%. You find that in the installed transformer values on the left side of the data sheet. Even with an "infinite bus" calculation the available fault current at the secondary of the transformer is only 11,457 amps using the worst case of the impedance being 10% less than what is on that sheet.
However they have also given you the available fault current on the primary side from two different sources and that will reduce the secondary fault current from what you get on the infinite bus calculation.
Bussmann has a free app that lets you do the calculations. It is called FC² and available for both apple and android.

Secondary full load current = 300000/ (480 times the square root of 3)

 

[jmartinez1959]

I just got some secodary data from the utility and the max Amps says is 8195 and the X1 ohms is .0320 . secondary full load current of the transformer?

 

[d0nut]

It states 8195A for a line to ground fault and 9004A for a three phase fault. That document also states that the values given are for arc flash calculations only and not to be used for AIC service rating and to contact SMUD line design for the AIC rating data. To me that means that the numbers given are accurate for the as-installed condition but the utility company wants you to use a more conservative number in case they swap out the transformer, upsize some equipment, or feed you from a different substation. Give them a call and they will tell you what value to use.

 

[don_resqcapt19]

It states 8195A for a line to ground fault and 9004A for a three phase fault. That document also states that the values given are for arc flash calculations only and not to be used for AIC service rating and to contact SMUD line design for the AIC rating data. To me that means that the numbers given are accurate for the as-installed condition but the utility company wants you to use a more conservative number in case they swap out the transformer, upsize some equipment, or feed you from a different substation. Give them a call and they will tell you what value to use.

This is one where there is not a lot of difference...the infinite bus fault current for that transformer is 10,300 amps.

 

[d0nut]

This is one where there is not a lot of difference...the infinite bus fault current for that transformer is 10,300 amps.

They might be looking at something more drastic than just infinite bus. It might be considering a completely different transformer that is larger or has a significantly lower impedance.

 

[jmartinez1959]

so if the infinite bus fault current for the transformer is 10300 amps can I use a 3P150A breaker with an Interrupting Rating : 35kA@480V?

 

[d0nut]

so if the infinite bus fault current for the transformer is 10300 amps can I use a 3P150A breaker with an Interrupting Rating : 35kA@480V?

Absolutely, but again:

 

[don_resqcapt19]

It states 8195A for a line to ground fault and 9004A for a three phase fault. That document also states that the values given are for arc flash calculations only and not to be used for AIC service rating and to contact SMUD line design for the AIC rating data. To me that means that the numbers given are accurate for the as-installed condition but the utility company wants you to use a more conservative number in case they swap out the transformer, upsize some equipment, or feed you from a different substation. Give them a call and they will tell you what value to use.

Wonder how common that is? ...everything around here is the exact opposite and it is almost impossible to get data from our utilities that says it is suitable for arc flash calculations.