# Short Circuit Calculation (What Is Correct Method?)

#### jim dungar

##### Moderator
Staff member
One thing that is rarely considered by people performing DIY AIC selection, is the requirement to make adjustments for when the short circuit X/R ratio differs from the one used in the UL listing for molded case breakers. Software packages like SKM show several short circuit values depending on the AIC 'range' of the molded case breaker, A high interrupting breaker, like a 22kA, may have been tested with a different X/R than a 10kA unit. The difference is not great so many people were taught to leave a fudge factor such as selecting a 65kA breaker even though the quick and dirty SCA result is 21kA.

Having to update or confirm arc flash calculations on a regular basis will often catch any issues resulting from utility changes (assuming someone revisits the available fault current).

I would guess that maybe 55-60% of the existing facilities, I have ever evaluated, have had at least some equipment that was inadequate for the realistic fault values I calculated.

#### mbrooke

##### Batteries Not Included
What "Both Windings"? The chart doesn't say anything about "both windings."

See this diagram, there are two ways to connect a pole pig:

#### mbrooke

##### Batteries Not Included
Maybe this will be more clear:

50kVA, 2.0%Z, 120V secondary: max short circuit current = 20,828A

50kVA, 2.0%Z, 240V secondary: max short circuit current = 10,414A.

50kVA, 2.0%Z, 120/240V secondary: max short circuit current L-L = 10,414A.
..........................................................................max short circuit current L-N = 15,621A.

I get what you are saying- and while I could be wrong- I was taught that a L-N short circuit would be close to 20,000 amps in your example.

#### tortuga

##### Code Historian
One thing that is rarely considered by people performing DIY AIC selection, is the requirement to make adjustments for when the short circuit X/R ratio differs from the one used in the UL listing for molded case breakers. Software packages like SKM show several short circuit values depending on the AIC 'range' of the molded case breaker
Good point, I think as electricians & engineers its important we continue to be able to do these calculations by hand to check the software, hence these are useful discussions.
There are always edge cases, glitches, custom stuff, odd systems (DC, 2 phase..) that this has not been checked well in the software, and of course software bugs.

#### mbrooke

##### Batteries Not Included
Good point, I think as electricians & engineers its important we continue to be able to do these calculations by hand to check the software, hence these are useful discussions.
There are always edge cases, glitches, custom stuff, odd systems (DC, 2 phase..) that this has not been checked well in the software, and of course software bugs.

Problem is there is no accepted way of doing them by hand.

#### winnie

##### Senior Member
Problem is there is no accepted way of doing them by hand.
The software has to be following an algorithm, and one should be able to follow that algorithm by hand. Of course software can repeat calculations over and over with ease, so in reality 'follow that algorithm by hand' really means have a simplified version of the algorithm that gives you a ballpark figure that lets you check/understand what the software is doing.

IMHO software for such safety calculations should document the algorithm being used and the basis for selecting that algorithm.

-Jon

#### mbrooke

##### Batteries Not Included
The software has to be following an algorithm, and one should be able to follow that algorithm by hand. Of course software can repeat calculations over and over with ease, so in reality 'follow that algorithm by hand' really means have a simplified version of the algorithm that gives you a ballpark figure that lets you check/understand what the software is doing.

IMHO software for such safety calculations should document the algorithm being used and the basis for selecting that algorithm.

-Jon

Right but you've got the bussman mathethod, the chapter 9 table 9 method, the sequence component method the DC equivalent method, ect.

#### winnie

##### Senior Member
I wonder how the variations in the results of these different algorithms compares to the real world difference in fault current magnitudes.

You have a transformer with its design or tested impedance. But that impedance probably changes depending on things such as temperature, it is probably subject to aging, etc. At the same time you can have a solid bolted fault or some sort of fault which has some impedance. I am sure there are other variations. So the real world 'short circuit' will have different values.

Also the calculation can be problematic if it gives a number that is too high or too low, so you can't 'be conservative'. If the real world fault current is higher than calculated, you can have OCPD that fails. If it is lower than calculated then OCPD response could be slower, increasing arc fault energy.

-Jon

#### topgone

##### Senior Member
See this post. I just don’t understand why some POCOs use charts that assume a particular Z that is generally a lot lower than actual impedance. I made a comment here about the ease of some modeling software that is super easy to give fault numbers that are real.

Except for really big power transformers where customers have option to specify their desired transformer impedances (ordering transformers for paralleling with existing transformer/s), pole pig impedances fall under a certain range!

Anyways, the fault current available at the distribution transformer secondaries are limited up to the value obtained by assuming infinite impedance of the POCO's system! Please play with numbers on your software and see that available fault currents at the transformer secondaries stay within a rather close range even if you input utility SCMVAs by 100X the original POCO published values!