# Fault current quick and dirty estimates



## matt1124 (Aug 23, 2011)

Noticed outside I’ve got 75kva pots in a wye formation, and the existing main breakers are 10kAIC. That didn’t seem right, so I did a little googling. I don’t know the impedance of the transformers or input voltage, but I found this chart randomly.








I’ve found similar from other providers. My provider is OG&E, and yes I can call them and try to get an answer, but mainly just curious if that 10k is enough, I don’t believe it is.

I’ve seen some charts, and they seem to indicate on this arrangement, I should be considering it as 225kva but this one seems to be 3 75kva pots, the way it’s formatted and that footnote at the bottom. Do I go by total KVA of the arrangement or biggest pot size.

I know, I know. Estimates only. Approx 60’ of 500MCM Al., 208Y/120


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## matt1124 (Aug 23, 2011)

This one doesn’t show individual or total. I just can’t imagine having to use 65kAIC mains in 320 class service…


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## matt1124 (Aug 23, 2011)

Mainly… Should I be giving those 10k a second thought is the point


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## micromind (Aug 11, 2007)

Since the chart is for a 3Ø bank of 3 - 75KVA units, you'd use the 225 rating. 

The short-circuit of a transformer is VA ÷ voltage = full-load current. FLC ÷ impedance expressed as a decimal = short-circuit current at the transformer assuming full voltage at the primary. 

In your case, it'd be 75,000 ÷ 120 = 625. If the impedance is 2%, it'd be 625 ÷ .02 = 31,250. 

In my experience, 2% impedance is pretty low, usually a 75 KVA pole-mount pot will be closer to 4% but they vary a lot. You'd need to get the actual impedance rather than a stock number that the PUCO uses to cover its hind-end. 

Also, this is only valid at the transformer, wire size and length will cut back on current a lot. 

If it's a long ways from the substation and then primary wire is not huge, the primary voltage will sag a bit, this will cut back on current as well. 

If an actual study was done, I wouldn't be surprised if it's under 10K. If the pots are 2.5% impedance, that would likely come out slightly less that 10K.

Also, is the 60' of 500 all the way from the transformers to the panel bus or just to the weatherhead?


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## matt1124 (Aug 23, 2011)

The weatherhead is about 10’ from the pots and it wraps the building in 4” PVC, my line side service conductors are the 60’ of 500MCM to the top of the meter. There’s not 5’ from the meter to the panels.


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## 460 Delta (May 9, 2018)

micromind said:


> If it's a long ways from the substation and then primary wire is not huge, the primary voltage will sag a bit, this will cut back on current as well.


AEP around here (Ohio Power, Columbus &Southern) tell you to assume unlimited power available at the primary of the xformer doing fault calcs. I suppose it makes it easier to calculate everything as worst case.


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## SWDweller (Dec 9, 2020)

Your working in a facility and do not know the available fault current at your service.
Silly rabbit.

The cost of a study can be expensive but it all starts with the numbers given by the POCO.
Ask for the stated value to be on the POCO letter head. Then keep a copy and give it to the person in charge. Do not expect much, I have seen so many companies/facilities ignore basic electrical safety because they feel it costs to much. Their insurance companies do feel differently. 
Basic safety will tell you once you have the value if you need to be wearing a blast suit when switching breakers on an off. 
Until you find out, never switch under a load. The results can be life altering. It is your life.
Are you wearing FR clothing now?


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## matt1124 (Aug 23, 2011)

22,108 is what the utility’s “distributions circuit analyst” has sent me. Good luck to the other 10kAIC main services that are served by this station 🤣🤣


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## SWDweller (Dec 9, 2020)

22K is not bad at all. I worked in a facility that had the fault current in Mva. 
Our switching proccedures took a long time to shed loads and not blow ourselfs up.


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## paulengr (Oct 8, 2017)

As far as accurately doing short circuit Bussman created the “point by point” method. It is a very simple set of formulas and the entire instructions I think are 2 or 3 pages. It is so good that the original “IEEE 1584 spreadsheet” for arc flash used it. It is much more than the infinite bus version.


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