# Thread: Short Circuit Calculations Help

1. Originally Posted by kingpb
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.

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

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you are close kingpb

Here is what I get in ETAP

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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)"

5. Originally Posted by Designer69
you are close kingpb

Here is what I get in ETAP

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

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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

7. Originally Posted by Designer69
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.

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