Available Fault

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Toros

Senior Member
Location
Tujunga, CA
Hi
How Can I determine the availble Fault value at my commercial service (277/480v) when it is supplied by a utility, pad monted 500kva-750 kva xfrmer ????

Thank you
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Toros said:
Hi
How Can I determine the availble Fault value at my commercial service (277/480v) when it is supplied by a utility, pad monted 500kva-750 kva xfrmer ????

Thank you
Click to expand...

You normally get it from the power company.

Or if you know the transformer impedance I think you can calculate it but you have to assume an infinite supply current because you do not really know what current is available on the primary.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Use a spreadsheet designed to calculate this. There is one on Mike Holt's website that is a free download.

You will need to input kVA of the transformer, voltage, impedance. This much allows calculating the available fault current at transformer.

Next steps on the spread sheet you will need to enter conductor size, type, length, number per phase (for parallel sets), whether in magnetic raceway or non magnetic raceway. This will allow it to calculate how much fault current is available at the load end of those conductors.

Other spread sheets out there would be pretty similar in what information they will ask for.

This method assumes infinite supply ability of the primary side of the transformer, real world, there is reductions there as well but they will vary, but this does leave you for the most part with a worst case fault current level.
 
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templdl

Senior Member
Location
Wisconsin
Toros said:
Hi
How Can I determine the availble Fault value at my commercial service (277/480v) when it is supplied by a utility, pad monted 500kva-750 kva xfrmer ????

Thank you
Click to expand...

Without any other input assume that there is an unlimited available fault current at the primary from thE POCO. Then assume that you have 100% motor contribution. For a 500kva transformer with 5%z @ 480v figure on 14.4ka max. A 750kva transformer with 5.75%z @480v would be 19.3ka.
Start there. If you have devices started at 14kaic you have as issue. With he 500kva by finding out what the actusl available fault current from the POCO is and the actual motor contribution you may be able to reduce the actual fault current to less than 14ka.
The 750kva unit may be a bit of a stretch. If your existing devices were rated 18kaic you would have a better chance to get the 19.3ka reduced to 18 ka or less.
But what I always advise is to figure on the worst posible case first before you go through the effort and expense to calculate what the actual available fault current is.
 
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