short circuit analysis

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bccodes

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A few weeks back I posted a question about Kaic ratings on a project in our downtown section due to a transformer network with 100k available fault current. The response I got back was that the main fuses in the spec are approved by the utility company and that therefore the short circuit study is not needed. I don't like this response but can't seem to put my finger on the code section that requires this submittal. Can I ask them to provide documentation to prove 110.10. Is that not what the short circuit analysis is all about? The rest of the plans are so full of holes that I want to make sure someone is on top of this .Thank for all your help
 
"The main fuses are approved by the utility company" means nothing.

Electrical gear must be designed and maintained to handle the calculated available fault current. NEC 110.10 covers this. Your engineer MUST calculate the available fault current (AFC) at every panel in the distribution system to ensure the available interrupting current (AIC) rating specified is greater the calculated AFC. If your engineer is refusing to do this calculation, you could have gear installed that is incapable of handling a fault. This is extremely dangerous. I suspect your engineer either doesn't know how to do this or doesn't want to spend the time.

If the engineer refuses, remind him of his duty to protect the public safety first. If he still refuses, I would remind him of his duties as a professional engineer and that you can pose the question to the board of professional engineers with whom he/she is licensed. I assure you any P.E. on the Board will agree this is a requirement of the design professional.
 
bccodes said:
A few weeks back I posted a question about Kaic ratings on a project in our downtown section due to a transformer network with 100k available fault current. The response I got back was that the main fuses in the spec are approved by the utility company and that therefore the short circuit study is not needed. I don't like this response but can't seem to put my finger on the code section that requires this submittal. Can I ask them to provide documentation to prove 110.10. Is that not what the short circuit analysis is all about? The rest of the plans are so full of holes that I want to make sure someone is on top of this .Thank for all your help
Click to expand...

NEC 110.9 and 110.10 effectively say that ALL equipment, not just service entrance equipment , must be applied correctly based on the available fault current. A short circuit study is the primary method (but not the only method) for determining the short circuit current requirements at different points in the facility.
 
Harold-eng said:
This is extremely dangerous.
Click to expand...
But done ALL THE TIME - especially in todays "design-build" world. I'm 100% for the design-build method done right, but when it comes to Div 16, in many cases it ain't done right.
 
Often times, because the designer does not know specifcally what the final installtion may be, especially on a design build project, the requirement to do the fault analysis is specified to be performed by the Contractor. This is unfortunate, becasue they do not often have the capability to do such. The Contractor has two choices, have the equipment manufacturer do it as part of the equipment package, e.g. SqD, Cutler Hammer can both do it, or hire an engineer capable of doing it.

If you don't have many motor loads on the secondary side of the transformer, then the available fault current is very very cloase to simply the rated current on the secondary side divided by the impedance (not percent) of the transformer. for example:

LV = 480Y/277V
Transformer KVA = 2000 KVA
Transformer Impedance Z = 5.75%

I rated = 2000KVA/(1.732 x 480)
I rated = 2406 A

I sc = 2406/(.0575)
I sc = 41,837 A

You can see how the impedance affects the sc current by changing the Z to 2.3%:

I sc = 2406/(0.023)
I sc = 104,610 A

Hope this helps
 
There are various factors that affect the short circuit for a system.
Transformers, panel ratings, fuse/breaker ratings, cable types, cable lengths, etc.

If you are unfamiliar with doing short circuit calcs, then I would suggest hiring a professional to put together a short circuit study and coordination report. It may be pricy depending on the complexity of the one-line, but it is worth knowing that you are covered.

Would you rather spend a few grand to be assured or held accountable for having the panels blow off the wall and/or possibly injure someone?
 
IMO the location of the service point is a critical factor. If the overcurrent device is on the utility company side of the service point, I can't see how NEC rules would apply

Brian Dolan
 
also consider....

also consider....

In addition to your need to assure that the electrical system is rated to handle the available fault current, there is significant value in having the electrical components coordinate.
By insisting that a Power System Analysis be performed, you'd not only get electrical components rated to operate in your high available fault current environment, but you'd have recommended breaker settings to isolate the fault to the lowest level in the system, thereby minimizing th e impact to the entire facility.
It always amazes me when I walk through facilities and see breakers that are adjusted to the maximum on all of their settings - from the main breakers to the lowest level.
And the cost of the Power Study is a mere pittance in comparison to the cost of the electrical system.
JM
 
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