# Thread: NEC short circuit calculations

1. ## NEC short circuit calculations

Using page 7 and onward as a reference, are the short circuit calculations shown accurate enough for breaker AIC sizing purposes? If so, how much do they deviate from symmetrical components and X and R calculations? Will they ever produce values below a "real" short circuit calculation?

What about 3 phase grounded wye?

Also why don't molded case breakers list the X/R ratio they base their fault current on? Is there ever a time when the X/R ratio of a 600 volt and under scenario will exceed that of the breaker?

For the sake of the discussion lets assume the POCO transformer primary is an infinite source.

https://www.scribd.com/document/3520...it-Calculation

2. Here is the Mike Holt paper those equations are derived from:

https://www.mikeholt.com/files/PDF/S...atons_Book.pdf

It looks to simple vs a symmetrical calculation.

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Thread: Adding Z of transformers and wiring and X/R ratios of 19 March 2019

4. iirc, someone posted a free arc calulator here in the past, which i lost...can anyone update that here?

~RJ~

5. Originally Posted by Julius Right
Thread: Adding Z of transformers and wiring and X/R ratios of 19 March 2019
Missed it, I need a link.

6. Originally Posted by romex jockey
iirc, someone posted a free arc calulator here in the past, which i lost...can anyone update that here?

~RJ~
Look in free stuff on Mike Holt's main website.

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Originally Posted by mbrooke
Missed it, I need a link.

8. Originally Posted by Julius Right
But it doesn't answer all that well how far those equations stray from the symmetrical component analysis and if they can ever produce values below "classical" equations.

9. Also, how were these values derived? Are the conductors twisted? Pulled flat? Worse case? Average?

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If we are speaking about Point-To-Point Method Of Short-Circuit Calculation Cooper-Bussmann pg.7 then up to point 3 [including Notes 1 to 6] I agree with.
I don't like to use the transformer data table. Usually the impedance is less than known.
The factor f system it can be discussed. The C factor is calculated using IEEE 241/1990 pg.320 Table 65 Approximate Impedance Data Insulated Conductors 60 Hz.
Let's take 5 cables from the table:
1/0,2/0,4/0,500 mcm and 1000 mcm and let's compare with NEC 2017 Table 9, and Okonite catalogue.
First, the conductor resistance is less than "correct" calculated since factors as temperature, skin effect and proximity effect are neglected. The reactance is also less than other sources [NEC, Manufacturer]. However, recalculated the examples from page 7 using "correct" parameters from NEC 2017 Table 9 I got these results:
System A point F1 “correct” Isc1=49611A ,page 7=49803
System A point F2 “correct” Isc2= 33531A ,page 7= 35445
System B point F1 “correct” Isc1= 33088A ,page 7= 33215
System B point F2 “correct” Isc2= 30715A ,page 7= 30059
System B point F3 “correct” Isc3= 30299A ,page 7= 29731
The difference between “correct” and the way using C constant it is only 6% maximum.

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