I need help figuring out available fault current!

[Rsteenson]

I am having a difficult time finding out how to calculate my afc on a transformer. Is there a good cheat sheet i can find or can somebody just give me the rundown if it isnt too timely? I will be installing a 800 amp 480v service in a couple of months with 3 runs of 300 mcm about ten feet from the ct can. The transformer impedence is 4.7%. Any insight would be greatly appreciated.

 

[jumper]

I am having a difficult time finding out how to calculate my afc on a transformer. Is there a good cheat sheet i can find or can somebody just give me the rundown if it isnt too timely? I will be installing a 800 amp 480v service in a couple of months with 3 runs of 300 mcm about ten feet from the ct can. The transformer impedence is 4.7%. Any insight would be greatly appreciated.

Here is one.The 7th one down.

http://www.mikeholt.com/freestuff.php?id=freegeneral

I here many just ask POCO, I would.

 

[templdl]

I am having a difficult time finding out how to calculate my afc on a transformer. Is there a good cheat sheet i can find or can somebody just give me the rundown if it isnt too timely? I will be installing a 800 amp 480v service in a couple of months with 3 runs of 300 mcm about ten feet from the ct can. The transformer impedence is 4.7%. Any insight would be greatly appreciated.

This should give you a good start based upoin unlimited fault current on the primary of the transformer.
If the POCO can provide you with the actual available fault current that figure can go down. As such all that there is left is the cable's impedence which you should be able to determine.

 

[ramsy]

Wondering how close my Point-to-Point spreadsheet comes to an engineer's numbers:

Engineering supervision or approved plans may use Point-to-Point methods for fault current.

If xfmr size is known => 125% max load, then I figured 850kVA min. or next size up, 1000kVA xfmr.

Assuming 1mW xfmr (25592 Amps) my Point-to-Point calc drops (346 Amps) after 10ft of #300 x 3

If you could find an 850kVA xfmr, you might get under 20,000 Amps.

 

[ptonsparky]

MHs works well. I had made my own back when this first showed up in the nec.
One of the local POCOs now has a request form on their website. We just give them the meter # and in a couple days we get the #s for at the transformer. Put those into the spreadsheet, add your additional info and nothing to it.

 

[templdl]

What are the common general groups of kaic ratings of breakers? 10, 14, 18, 22, 25, 35, 42, 65,100, 200 kaic? It would be interesting to find out where the actual available fault current ends up to be after doing an actual fault current study comparing it the devices. That is how out of wack your systen really is.
If on considers 100% available fault current at the service entrance and then assumes that all the system must be 100% rated because there has been no coordination study done then at least you know what trouble you may be in. This would justify starting with the PO first to find out what the actual available fault current is from the POCO if you are still in deep dodo have a professional do a coordination study for you in order to pinpoint those devices that are definitely not rated for the actual available fault current.

 

[ramsy]

MHs works well. I had made my own back when this first showed up in the nec.

Thank you. Rather than assume PF=1, Mike Holt's fault calc asks for power factor, which can change fault current significantly.

Assuming PF=1 is a 15% error for typical 85% power factors, which errors by 2 categories, as templdl points out.
The OP's equipment kaic requirement of 35k @ PF=85%, is way off from 22k @ PF=1.

One of the local POCOs now has a request form on their website. We just give them the meter # and in a couple days we get the #s for at the transformer. Put those into the spreadsheet, add your additional info and nothing to it.

Nice way to save engineering-approval fees, however, do you know if the POCO measures Power Factor or assumes PF=1?

 

[ptonsparky]

Thank you. Rather than assume PF=1, Mike Holt's fault calc asks for power factor, which can change fault current significantly.

Assuming PF=1 is a 15% error for typical 85% power factors, which errors by 2 categories, as templdl points out.
The OP's equipment kaic requirement of 35k @ PF=85%, is way off from 22k @ PF=1.



Nice way to save engineering-approval fees, however, do you know if the POCO measures Power Factor or assumes PF=1?

That's a pretty good question. This POCO requires all 3ph motors above 10hp to have PFC installed so that may figure into their calcs. I doubt that PF is static on multi motor loads or any system so using a specific PF is going to be exact only for a brief moment in time anyway. My WAG. Educate me if this wrong.

I'll ask. It will make me sound like I know way more than I do. (Part of the salesperson persona I have had to develop over the years,)

 

[jim dungar]

Short circuit current calculations are concerned with the power factor (the X and R impedances) of the conductors and transformers, not the Power Factor of the load.

The only time loads figure into short circuit calculations is when they act as sources (ie. motors acting as generators).

 

[ptonsparky]

Short circuit current calculations are concerned with the power factor (the X and R impedances) of the conductors and transformers, not the Power Factor of the load.

The only time loads figure into short circuit calculations is when they act as sources (ie. motors acting as generators).

OK, so will the PF stay the same on the POCOs side, and if so wouldn't they already know this? How would outside engineering help us solve for a factor that none of us know, other than at best the POCO?

 

[jim dungar]

The utility knows nothing about your side of their transformer.
A good short circuit calculation requires knowledge of the impedance of conducotrs and their length (including the insulation properties). An excellent calculation includes the properties of raceways and conduits.

When it comes to free software, remember: 'you don't get what you don't pay for'.

For specifying equipment, the absolute worst case will be: assume an infinite source of current and no impedance from the conductors. This method will negate the need for all of these other considerations. Of course, this may produce excessively conservative and expensive results.

 

[T.M.Haja Sahib]

Is it possible to calculate roughly the fault level across two points by relating it to voltage drop caused by a known load?

 

[petersonra]

Is it possible to calculate roughly the fault level across two points by relating it to voltage drop caused by a known load?

no.

if you put a 1 ohm resistor across a 115V line you will still have 115V there, at least until the OCPD trips.

 

[rcwilson]

Is it possible to calculate roughly the fault level across two points by relating it to voltage drop caused by a known load?

This is theoretically possible. Practically it would not give accurate results. You could monitor the load amps, volts and power factor with accurate metering, then trip the load and measure the voltage rise. A theoretical calculation could be done to determine the Thevanin impedance of the system at the measuring point. Accuracy would be poor unless you could get a 10% or higher voltage drop. Measurements have to be made quickly before system condition change, such as a another load switching or a voltage change from a utility voltage regulator or generator.

I doubt many inspectors would accept this approach for short circuit calculation.

 

[templdl]

Personally I would start with the 100% at the service entranc and 100% throught the facility and see where that gets you.
If you see that you may have an issue I would do some rough calculations tith the means that you have to see how much in trouble you might be in. At least you would have educated yourself with your system.
Then it's time to call in the professional, one who is able and qualified such as a PE to do a coordination study. Having done some calculations yourself you would be better equiped to understand and discuss the coordination study with PE.
Most likely it isn't you who is paying the bill for the study and upgrade but you will probably have to sell the idea to the one that is.