Interrupting rating?

[FORK]

I am getting a price quote on 600amp 480volt I-line main. My supplier needs to know what interrupting rating I need, 10k...22k & so on. I'm drawing a complete blank on calculating what I need. Any thoughts?

 

[jim dungar]

I am getting a price quote on 600amp 480volt I-line main. My supplier needs to know what interrupting rating I need, 10k...22k & so on. I'm drawing a complete blank on calculating what I need. Any thoughts?

You need to ask your POCO.
There are ways to estimate worst case values, but even those require knowledge of the equipment that the POCO is actually supplying.

 

[necnotevenclose]

try looking up Bussman short circuit calculations, keep in mind that the Bussman formula does not include motor contribution.

 

[Pierre C Belarge]

This is really something he should go to the POCO for, even though getting this info from the poco is sometimes harder than pulling teeth.

Otherwise he will need some info off of the transformer supplying his service, the size of the conductors and the length of the conductors.

 

[davidr43229]

Fork,
If you tell us the size of the main transformer, incoming & outgoing votage, Impedence rating, then the size of the wire, length of the run, pairs per phase, in conduit or not, we may be able to help you.
Just my $.02

 

[kingpb]

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This would be the minimum impedance that you can have @ 500KVA. Keeping that in mind, the utility could provide a smaller transformer, and often times they do. Which means you might be able to use lower impedance and still be under the 22KAIC. However, this is risky, because they could also come and change the transformer later, and then you might have a problem.

The short circuit determination above does not take into consideration cable, and therefore the actual short circuit value will normally be less then the calculated. However, this will get you in the ballpark, and thinking in the right direction. Regardless, I do not see how you could go with anything less then the 22KAIC.

 

[FORK]

Thanks to all that have replied to my post. I have resolved my question.

 

[jim dungar]

Keeping that in mind, the utility could provide a smaller transformer, and often times they do. Which means you might be able to use lower impedance and still be under the 22KAIC. However, this is risky, because they could also come and change the transformer later, and then you might have a problem.

Many utilities try to use transformers that have impedances lower than 5.75%. These low impedances result in high available fault currents. So, given the possiblity a utility may change their transformer the utility may say they have available fault currents in excess of 42kA.