# Fault Current Calculations



## telsa (May 22, 2015)

AIC is normally determined by physical test in a laboratory.

There is no math // physics equation that will suffice.

It's not as if AIC is a 'closely' determined value.

It's hugely quantized.

( ie there are staggering jumps in the AIC value// rating between designs. )


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## fly.explore (Jul 4, 2017)

Thank you! Do you know how ISC can be calculated in my drawing?


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## Big John (May 23, 2010)

I can't load the link on my phone, but remember that the ISC calculation you posted is worst-case at the transformer lugs.

It assumes infinite bus so doesn't include any voltage drop on the utility primary, nor does it include any cable impedance to your service point. 

The actual available fault current will always be less than what that equation gives you, so that may explain why the values you were given are lower.


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## fly.explore (Jul 4, 2017)

Big John said:


> I can't load the link on my phone, but remember that the ISC calculation you posted is worst-case at the transformer lugs.
> 
> It assumes infinite bus so doesn't include any voltage drop on the utility primary, nor does it include any cable impedance to your service point.
> 
> The actual available fault current will always be less than what that equation gives you, so that may explain why the values you were given are lower.


That was very helpful, Big John! So would you know how the actual ISC in the drawing is obtained? Is it obtained through testing, or is there a specific way to calculate it taking into account the factors you mentioned?

Thanks again!


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## Big John (May 23, 2010)

Look up "Bussman Short Circuit Calculations."

It's a PDF that gives a much more in depth explanation of how to get an accurate ISC. 

Try plugging your info into those formulas and see if it gets you closer to the given number.


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## RePhase277 (Feb 5, 2008)

Here's some good info:

http://www.electrical-installation.org/enwiki/3-phase_short-circuit_current_(Isc)_at_any_point_within_a_LV_installation


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