Short Circuit Calculation (What Is Correct Method?)

32Lateralus

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West Coast
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Electrical
I have seen two different ways of running a short circuit (fault) calculation to determine ratings for gear. Normally designing commercial buildings, selecting AIC ratings.

The first method Ive seen used by many places around town is Isc=V/Z. In this case you calculate the total impedance starting from NEC 'table 9' and then divide the voltage by total calculated impedance. However I cant seem to find where this full calculation is written out or described as the acceptable method.

The second method is the point-to-point method used by Eaton/Bussman calc:

Is one of these a more accurate/correct method? Have you seen AHJ's every require one over the other?
 

Hv&Lv

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Our AHJ doesn’t even care if the numbers are correct, as long as the label is on there with some numbers on it
 

32Lateralus

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West Coast
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Electrical
Our AHJ doesn’t even care if the numbers are correct, as long as the label is on there with some numbers on it
Kind of a matter of safety, is it not? For example, if you're providing 10kAIC rated gear with actual available fault current exceeding that amount?
 

petersonra

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Northern illinois
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engineer
Kind of a matter of safety, is it not? For example, if you're providing 10kAIC rated gear with actual available fault current exceeding that amount?
For the first 100 years of electricity no one cared anything about SCC. I wonder if there was so much as a single fatal incident related to equipment not rated for the available SCC.
 

Hv&Lv

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Kind of a matter of safety, is it not? For example, if you're providing 10kAIC rated gear with actual available fault current exceeding that amount?
Does the inspector make sure your liability numbers are correct on anything? You think he goes behind you to check your load calculations?
As a professional it’s up to you to make sure the numbers you post on a piece of equipment are correct.
 

32Lateralus

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West Coast
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Electrical
Does the inspector make sure your liability numbers are correct on anything? You think he goes behind you to check your load calculations?
As a professional it’s up to you to make sure the numbers you post on a piece of equipment are correct.
Right, Which is why my original question was, "which method is actually the more correct method"?

The AHJ portion was just a curiosity if both methods are somewhat accepted as valid solutions, I could see a jurisdiction requiring the more conservative estimate. I am just not sure to begin with if one is correct over the other or if they are both acceptable ways of solving for the fault current.
 
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mbrooke

Batteries Not Included
Location
United States
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Electricity
Right, Which is why my original question was, "which method is actually the more correct method"?

The AHJ portion was just a curiosity if both methods are somewhat accepted as valid solutions, I could see a jurisdiction requiring the more conservative estimate. I am just not sure to begin with if one is correct over the other or if they are both acceptable ways of solving for the fault current.

If it makes you feel better I have been asking for 10 years on every engineering forum. I've asked at least 100 times, and gotten 100 different answers.

At this point I'm just using chapter 9 table 9 with some correction factors. I'm just clueless to be honest.
 

32Lateralus

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Location
West Coast
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Electrical
If it makes you feel better I have been asking for 10 years on every engineering forum. I've asked at least 100 times, and gotten 100 different answers.

At this point I'm just using chapter 9 table 9 with some correction factors. I'm just clueless to be honest.
Okay, so Ive been doing a lot of research. I actually just found in the IEEE Standards 141 for industrial and 241 for commercial it lists the Isc=V/Z as a solution for solving short circuit. The NEC handbook actually references the IEEE for these calculations. So I think youre right with the NEC Ch9 T9 method, as thats how you solve for total impedance and then use the equation above. Seems good as it all uses info published in or referenced by the NEC.

And then I think Eaton/Bussman has their point-to-point method ive seen used a few places, it seems to always be on the conservative side, higher AIC ratings in the end. So im guessing fine to use, but makes me think the IEEE method is the most correct as it appears most referenced by NEC.
 

Dsg319

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West Virginia
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Journeyman Electrician
Okay, so Ive been doing a lot of research. I actually just found in the IEEE Standards 141 for industrial and 241 for commercial it lists the Isc=V/Z as a solution for solving short circuit. The NEC handbook actually references the IEEE for these calculations. So I think youre right with the NEC Ch9 T9 method, as thats how you solve for total impedance and then use the equation above. Seems good as it all uses info published in or referenced by the NEC.

And then I think Eaton/Bussman has theirs, it seems to always be on the conservative side, higher AIC ratings. So im guessing fine to use, but makes me think the IEEE method is the most correct.
You should do an example of one using the same numbers on both paper and the bussman app, I’m curious to see how close.
 

32Lateralus

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Location
West Coast
Occupation
Electrical
You should do an example of one using the same numbers on both paper and the bussman app, I’m curious to see how close.
So it doesnt seem too bad. Just ran a few quick ones, and worst I saw was almost 4,000A difference and down to the under 500A difference.

Ran a few with different voltages, run lengths, and multiple points along the system.

But still 4k could be the difference between gear sizes, so if the Isc=V/Z method is valid and keeps the final fault numbers down, could mean less expensive gear for end users over the course of multiple projects. But overall, nothing gamechanging.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Okay, so Ive been doing a lot of research. I actually just found in the IEEE Standards 141 for industrial and 241 for commercial it lists the Isc=V/Z as a solution for solving short circuit. The NEC handbook actually references the IEEE for these calculations. So I think youre right with the NEC Ch9 T9 method, as thats how you solve for total impedance and then use the equation above. Seems good as it all uses info published in or referenced by the NEC.

And then I think Eaton/Bussman has their point-to-point method ive seen used a few places, it seems to always be on the conservative side, higher AIC ratings in the end. So im guessing fine to use, but makes me think the IEEE method is the most correct as it appears most referenced by NEC.

Right, but I'm still trying to convert POCO transformers into an ohm value with great grudge from others in the community.

However, assuming the POCO trafo is an infinite source with 75*C conductor Z will typically (IMO) give worse case values even when using nominal voltage vs +6% voltage and the actual source impedance with a 30*C conductor Z.

The IEEE method is basically what BS7671 and most other codes follow, so I'd say you are good in using that method :)
 

32Lateralus

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Location
West Coast
Occupation
Electrical
I second that.

And one where the POCO trafo is turned into an ohm value, preferably X and R.
Idk what utility companies you deal with, both in my area provide us with starting fault current values that we are to use at the Service Entrance Section. The utility assumes infinite primary and shortest distance to service. So I normally do not deal with poco xfmr in my calcs

EDIT: Well i guess im actually kinda doing that cause im taking the fault current at my service and turning into a staring value to put into my total impedance. Doesnt matter if the starting value is at the xfmr or SES really, calc is still the same.
 
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mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Idk what utility companies you deal with, both in my area provide us with starting fault current values that we are to use at the Service Entrance Section. The utility assumes infinite primary and shortest distance to service. So I normally do not deal with poco xfmr in my calcs

EDIT: Well i guess im actually kinda doing that cause im taking the fault current at my service and turning into a staring value to put into my total impedance. Doesnt matter if the starting value is at the xfmr or SES really, calc is still the same.

I know most sparkies do not deal with trafo calcs, but IMO if the NEC provided a simple table condensing them into a ohm value (as done with chapter 9 table 9 conductors) that could be added to your calcs to give a more realistic fault current value.

In reality the trafo will "choke" and "hold back" current vs an infinite source.
 

32Lateralus

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Location
West Coast
Occupation
Electrical
I know most sparkies do not deal with trafo calcs, but IMO if the NEC provided a simple table condensing them into a ohm value (as done with chapter 9 table 9 conductors) that could be added to your calcs to give a more realistic fault current value.

In reality the trafo will "choke" and "hold back" current vs an infinite source.
I think what you are looking for exists in the IEEE 241
 

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Hv&Lv

Senior Member
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Since POCOs don't care, and NFPA refuses to add a table, I think its time to just by a multi function tester.
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.

 
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