# Thread: Short Circuit Calculations Help

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Originally Posted by david luchini
They divide the transformer rated secondary current by the percent impedance...But since the percent impedance can have a 10% tolerance, the use the low side of 0.9%Z, which would give the highest possible short circuit value.

208.19A/0.9%=23132A
How do you figure out the percent impedance is 10%Z and use the low side of 9%Z

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Originally Posted by BatmanisWatching1987
How do you figure out the percent impedance is 10%Z and use the low side of 9%Z
When the utility gave you the available short-circuit value of 27kA at the transformer terminals, it means the %Z being considered was FLA/Isc = 624A/27,000A = 2.31%!

3. Originally Posted by BatmanisWatching1987
How do you figure out the percent impedance is 10%Z and use the low side of 9%Z
I'm not sure what you are asking. The table that you referenced lists 1%Z for the 75kva transformer.

1%Z +10% gives a range of 0.9%Z to 1.1%Z.

4. The short circuit value wouldn't utilize a tolerance from the utility. They know what the transformer impedance is, or is going to be. The tolerance is only used by manufacturing to IEEE standards.

A utility transformer with an impedance of 2.31% is very realistic.

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Is the transformer primary voltage relevant?

Thanks

6. IMO, once the utility gives you the available current at their transformer terminals, the transformer, impedance, etc. become irrelevant. All you need is their number.

7. Originally Posted by augie47
IMO, once the utility gives you the available current at their transformer terminals, the transformer, impedance, etc. become irrelevant. All you need is their number.

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Originally Posted by don_resqcapt19
You have all the information you need to use the Bussmann app for this calculation. You just start with "add a conductor" to my system.
Using your information in the app, and assuming steel conduit, I get 17,191 amps at the end of your 100' run.
I'm not sure what I am doing incorrectly?

I selected "No, Assume Infinite Current on Primary"

What was the Z% value you inputted into the system

Or did I fill out the conductor information incorrectly.

Also if I wanted to add my MDP at the end of the line, why does it ask for distance again when inputting a Bus Run to the system?

9. Originally Posted by BatmanisWatching1987
I'm not sure what I am doing incorrectly?

I selected "No, Assume Infinite Current on Primary"

What was the Z% value you inputted into the system

Or did I fill out the conductor information incorrectly.

Also if I wanted to add my MDP at the end of the line, why does it ask for distance again when inputting a Bus Run to the system?

Last edited by david luchini; 01-11-19 at 02:48 PM.

10. Originally Posted by don_resqcapt19
You have all the information you need to use the Bussmann app for this calculation. You just start with "add a conductor" to my system.
Using your information in the app, and assuming steel conduit, I get 17,191 amps at the end of your 100' run.
same

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