Short Circuit Calculations Help

[BatmanisWatching1987]

Awesome. Thanks for the help

Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

Do I select Bus Run and select add as feeder with the distance of 100 ft

FYI...I just need the hand calculation and I got 18074.

 

[david luchini]

Awesome. Thanks for the help

Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

Do I select Bus Run and select add as feeder with the distance of 100 ft

FYI...I just need the hand calculation and I got 18074.

Just add another Conductor Run for the next panel.

 

[BatmanisWatching1987]

Awesome. Thanks for the help

Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

Do I select Bus Run and select add as feeder with the distance of 100 ft

FYI...I just need the hand calculation and I got 18074.

I re did my hand calculation and I got the same answer as the Busmann Program now of 17191 Amps

 

[david luchini]

I re did my hand calculation and I got the same answer as the Busmann Program now of 17191 Amps

18068 is correct for PVC conduit. (The 17191 is for metallic conduit.)

 

[augie47]

Its an entirely different conversation and you may already be familiar with the method but, for downstream panels, there is a process called "series rating". If you are not familiar, it will be worth your time to explore.

 

[electrofelon]

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.

Although, it is my opinion that nothing requires us to use that number, I am free to calculate from the transformer info if I wish. I know some disagree with me on that.

 

[augie47]

Although, it is my opinion that nothing requires us to use that number, I am free to calculate from the transformer info if I wish. I know some disagree with me on that.

I don't disagree but, locally, our POCO advises they arrive at their number by taking into account the transformers they keep on hand in case of a failure so they will be in the clear if they have to replace the one actually in place so we use their number in inspecting.

 

[electrofelon]

I don't disagree but, locally, our POCO advises they arrive at their number by taking into account the transformers they keep on hand in case of a failure so they will be in the clear if they have to replace the one actually in place so we use their number in inspecting.

I usually go with their number too, because it is rare I get to see the transformer or get accurate specs. I have used my own calc a few times when there number was unreasonably high.

 

[Julius Right]

Calculating Zsys for 208 V and 27 kA Zsys=208/sqrt(3)/27000=0.004448 ohm
According to NEC Table 9 3*350 MCM copper in p.v.c. conduit:
r=0.038 and x=0.040 ohm/1000ft then for 100 ft two parallel cables:
Rcb=0.038*100/1000/2=0.0019 ohm; Xcb=0.040*100/1000/2=0.002 ohm
If Xsys=Zsys [Rsys=0] the short circuit current will be maximum:
I=208/SQRT(3)/SQRT(0.0019^2+0.006448^2)=17865 A
If Rsys>0 then for Rsys=Xsys I=16666 A for instance.

 

[kingpb]

I usually go with their number too, because it is rare I get to see the transformer or get accurate specs. I have used my own calc a few times when there number was unreasonably high.

That is a risky proposition. Utilities use system planning numbers that have future load growth and many other parameters that would not be known to someone not having access to their information. So, what may seem "unreasonably high" to you today, it could very well be down the road a few years that changes to the distribution system means those numbers are now accurate.

For my own knowledge, what would be the advantage of not using the utility numbers; simply to save a few bucks on equipment ratings? That makes no sense to me. The risk is too high that someone could be injured.

 

[electrofelon]

That is a risky proposition. Utilities use system planning numbers that have future load growth and many other parameters that would not be known to someone not having access to their information. So, what may seem "unreasonably high" to you today, it could very well be down the road a few years that changes to the distribution system means those numbers are now accurate.

For my own knowledge, what would be the advantage of not using the utility numbers; simply to save a few bucks on equipment ratings? That makes no sense to me. The risk is too high that someone could be injured.

In situations where I have or would do this, the transformer is dedicated to the one customer, and my calcs are based on infinite primary, so distribution changes would have no effect. I'll usually provide enough equipment rating for the next size up transformer. Yes absolutely it's about cost. All engineering weighs cost is benefit. I recently had a job where the utility used twice the transformer size they had said they would install, and didn't tell me. So there are no guarantees with anything.

 

[kingpb]

This is the MVA Method, assuming no motor contribution at either the 600A MDP or 225A Panel.

 

[Designer69]

you are close kingpb

Here is what I get in ETAP

 

[Julius Right]

Hi,Designer: See at first in the open post:
"that is running is 2 Sets of ( 4 # 350 kcmil + 1 # 2/0 kcmil G in 4" Conduit PVC)"

 

[kingpb]

you are close kingpb

Here is what I get in ETAP

View attachment 22075

Since you don't have anything numbered or named I'm not sure what your results mean.

 

[Designer69]

Thanks kingp yours is a lot better.

Did you put 5000MVAsc on the grid to basically indicate infinite? What did you put for the grid X/R?


Thanks

 

[kingpb]

Did you put 5000MVAsc on the grid to basically indicate infinite? What did you put for the grid X/R?

Good questions.

I usually use X/R = 15 for distribution when I don't have specific numbers.

Yes, the 5000MVAsc is to force an infinite bus. I assigned the 13.8kV on the HV side of XFMR since it had no consequence to the evaluation.