NEC short circuit calculations

[mbrooke]

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/352073226/Lecture-12-Voltage-Drop-and-Short-Circuit-Calculation

 

[mbrooke]

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



https://www.mikeholt.com/files/PDF/Short_Circuit_Calculatons_Book.pdf


It looks to simple vs a symmetrical calculation.

 

[Julius Right]

I think we discussed about this in this thread:
Thread: Adding Z of transformers and wiring and X/R ratios of 19 March 2019

 

[romex jockey]

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

~RJ~

 

[mbrooke]

I think we discussed about this in this thread:
Thread: Adding Z of transformers and wiring and X/R ratios of 19 March 2019

Missed it, I need a link.

 

[kwired]

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.

 

[Julius Right]

Missed it, I need a link.

https://forums.mikeholt.com/showthread.php?t=197322

 

[mbrooke]

https://forums.mikeholt.com/showthread.php?t=197322

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.

 

[mbrooke]

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

 

[Julius Right]

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.

 

[mbrooke]

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.

6% is good enough for me.




However- what to make of this:

Note 5:The calculated short-circuit currents above represent the bolted faultvalues that approximate worst case conditions. Approximations of Bolted faultvalues as percentage of 3-Phase (L-L-L) bolted fault values are shown below.

Phase-Phase (L-L):87%

Phase-Ground (L-G) 25-125% (Use 100% near transformer, 50% otherwise)

Phase-Neutral (L-N) 25-125% (Use 100% near transformer, 50% otherwise

:?


25-125% is a big gap, at what length does 100% become 50%? And where do I use 125%? Only at the terminals?