Does AHJ require fault current calcs?

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sceepe

sceepe

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All of the recent posts about fault current made me think of a related question....

How often are you guys asked to provide fault current calculations to the AHJ for commercial work. What about breaker coordination calculations...

I know NEC requires these but I have never been asked to produce them. I do them to insure the system is designed correctly and stick them in a file incase there is ever an issue.

Has anybody found a good way to get fault current calc done before its too late. For example, I can't get POCO xfmr size / impedance or available Issc until I can give them the full load of the building. I don't know full load until design is done. Sometimes I have to go back and change fault current ratings or OCPD types by addendum. anybody got a better way.
 
Re: Does AHJ require fault current calcs?

I do a lot of design for one local town that requires fault calculations for every new service. (That still misses some subpanels that are added during remodels and expansions).

I do plan reviews for a local village, and I have followed our neighbors lead in requiring fault calcs for new services.

Has anybody found a good way to get fault current calc done before its too late
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That's always a problem, especially since the city wants the calcs before they even issue a permit, and the POCO usually does their design long after the contract has been awarded. I usually just guess large for the load, push the POCO into stating what their standard design would be for that load, and round anything close up.

Oh, and that still requires a recalculation if anything changes on the final POCO design.

Steve

[ August 10, 2005, 09:01 AM: Message edited by: steve66 ]
 
Re: Does AHJ require fault current calcs?

The Washington Department of Labor and Industries was the AHJ for most of my design projects here. They require the plans to show the available fault current at the main service panel, alongside the fault current rating of that same panel. I have not been asked by an AHJ to provide coordination calculations.
 
Re: Does AHJ require fault current calcs?

I have been asked ,by the AHJ, for the fault current calculation. I was doing a service upgrade to a church / school. The AHJ came out and did a extremely intensive inspection , told me I did a great job but wanted to see the calculation for fault current, and until it is produced theres no passing the inspection. I had that calculation for him the next day.
 
Re: Does AHJ require fault current calcs?

I require a fault current study at the time of plan review.

I have never had to require coordination studies, as we are under the 2002 and haven't had a hospital, nor have we had 2 elevators on one feeder. I am sure I will have to require the coordination studies when we adopt the 2005, however, given the changes to 700 and 701.
 
Re: Does AHJ require fault current calcs?

I have a question, something I have wondered about before.
In the back of Ferm's, there is the Busman tables giving maximum fault current values for various distribution transformers, as a funtion of secondary size and length.
Can you simply determine the available fault current using these tables?
Thanks
Buck
 
Re: Does AHJ require fault current calcs?

You can use whatever you wish to determine the available fault current, even a WAG. However, if the serving electric utility has a published chart or has given you a value for the expected available fault current and it is higher than the number you are using, you need to use the number from the electric utility.

The chart we use was developed over several years and is based on the lowest impedance transformer nameplate we have in the transformer yard over several years. The engineer may give you a higher figure because he expects the transformer to be replaced in the future to serve additional load. If it is on our downtown network, we may expect an additional vault to be installed that is very close to this one.

Additionally, you need to understand how the chart was developed, "The standard calculated available fault currents are given in amperes, RMS symmetrical, at the secondary bushings of the Company's transformer, assuming an infinite bus and a bolted fault." This is the statement that we put out with our numbers. The bus will not be infinite and you will not have a bolted fault except in the most unusual circumstances.

In other words, the simple chart that Bussmann developed is not adequate for all circumstances. :D
 
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