Available fault current calculation

Status
Not open for further replies.
sunny1

sunny1

Member
Location
Washington State
Occupation
electrician
Greetings,
I used a fault current calculator I found on here to determine what AIC rating for breakers would be adequate for a panelboard I plan to install. I'm not sure of the power factor, so I used the guidance on the spreadsheet that says to use 100% if unknown. I played with that number a little as well but in all cases, I believe I should be plenty good with regular 10K AIC rated breakers. I have reached out to the POCO regarding the utility fault current amps, but all the other values are good.

Is it more accurate to use the utility fault current vs. the transformer impedance (Z value) or would the result be the same? I must admit I don't often do this calc and was hoping someone could tell me if I'm on the right track. Thank you ahead of time!


1689796266416.png1689796333926.png
 
I'm not an expert on this, and I haven't used that particular spreadsheet. But it's my understanding that when you use only the transformer impedance, you're assuming that the power lines feeding the primary side of the transformer can supply infinite current. Since the actual value will be less than that, you'll end up with less available fault current when you do the calculation with the power company's information.
 
I haven't used this calculator, but the 200 feet of conductor between the transformer and the service entrance is going to greatly reduce the available fault current at 120/240 V. As long as that distance is correct and the feeder size is correct, I would imagine your result is reasonable. But if the length is something like only 100 ft, then 10 kA probably is not sufficient. At the transformer terminals the fault current will probably be something like 25,000 A. At 240 V and 208 V, these calculations are highly sensitive to the length of the secondary feeder. You can play with it in your calculator to see this. I would use whatever you think the shortest possible distance could be.
 
Mr. Serious said:
I'm not an expert on this, and I haven't used that particular spreadsheet. But it's my understanding that when you use only the transformer impedance, you're assuming that the power lines feeding the primary side of the transformer can supply infinite current. Since the actual value will be less than that, you'll end up with less available fault current when you do the calculation with the power company's information.
Click to expand...
That's true, but it may not make any difference on the secondary. If you play with the calculator a bit you'll see that as you increase the available fault current on the primary, the available fault current on the secondary pretty quickly reaches what it would be with the infinite bus model on the primary.
 
The 1.5% is used as a multiplier of the full load amps of the trans.
100*1000/240= 416.66
1/.015= 66.66(multiplayer)
66.66 * 416.66= 27,774.556
Then factor in conductor,lenght ect.
 
letgomywago said:
If you're carefull alot of small load centers are series rated to 22kaic with their main and 10kaic breakers. Eaton has this for sure I think a few others do too
Click to expand...
Where would I find that information for a specific panel and breakers?
 
Last edited:
Tulsa Electrician said:
The 1.5% is used as a multiplier of the full load amps of the trans.
100*1000/240= 416.66
1/.015= 66.66(multiplayer)
66.66 * 416.66= 27,774.556
Then factor in conductor,lenght ect.
Click to expand...
FYI an easier way to do this is simply divide the KVA by the impedance:
100KVA/ 0.015 = 6.67MVAsc
Then you can divide by whatever voltage you will be utilizing, in this 240V:
6.67MVAsc / 240V = 27.78KAIC

This is at the 240V terminals of the Xfmr; assuming infinite bus.

It is easier to utilize MVAsc values, because it doesn't change, regardless of voltage.
 
Mr. Serious said:
…with the power company's information.
Click to expand...
That assumes that the PoCo doesn’t use infinite bus in their own calcs. I have found over the years a lot of laziness on their part in that regard. I’ve had to challenge their findings on one occasion where the difference between my 54kA and their 69kA was going to make a huge difference in the cost of gear.
 
kingpb said:
FYI an easier way to do this is simply divide the KVA by the impedance:
100KVA/ 0.015 = 6.67MVAsc
Then you can divide by whatever voltage you will be utilizing, in this 240V:
6.67MVAsc / 240V = 27.78KAIC

This is at the 240V terminals of the Xfmr; assuming infinite bus.

It is easier to utilize MVAsc values, because it doesn't change, regardless of voltage.
Click to expand...
Thank you for an alternate method.
 
letgomywago said:
If you're carefull alot of small load centers are series rated to 22kaic with their main and 10kaic breakers. Eaton has this for sure I think a few others do too
Click to expand...
Thank you for your reply that is helpful for sure. It seems like I will be under 22kaic in the worst case scenario. We typically use square d QO like in the documents you provided but I didn't appreciate the difference between a series rating vs a full rating, so thanks!
 
Mr. Serious said:
I'm not an expert on this, and I haven't used that particular spreadsheet. But it's my understanding that when you use only the transformer impedance, you're assuming that the power lines feeding the primary side of the transformer can supply infinite current. Since the actual value will be less than that, you'll end up with less available fault current when you do the calculation with the power company's information.
Click to expand...
Jraef said:
That assumes that the PoCo doesn’t use infinite bus in their own calcs. I have found over the years a lot of laziness on their part in that regard. I’ve had to challenge their findings on one occasion where the difference between my 54kA and their 69kA was going to make a huge difference in the cost of gear.
Click to expand...
My experience is that even using infinite bus with transformer data, you will get a drastically lower value than the utility will provide. Every time I have had the opportunity to see the numbers from both methods, the POCO figure is always 1.5-2x what I get with actual transformer data and infinite primary. Of course it may be prudent to provide some headroom in case of transformer replacement with a larger unit and/or lower impedance. I evaluate what figure to provide for on a case-by-case basis.
 
letgomywago said:
If you're carefull alot of small load centers are series rated to 22kaic with their main and 10kaic breakers. Eaton has this for sure I think a few others do too
Click to expand...
ggunn said:
Where would I find that information for a specific panel and breakers?
Click to expand...
You can just about almost figure any load center is going to have a 22 kaic min main (I think Eaton's are 25k) that series rates to 10K branches. I remember digging through the catalogs a while back to verify this, and my hopefully correct recollection is that Siemens
is the one exception as they do show their cheaper ES series load centers as being available with a 10K main, but I have never encountered those in the wild or found anybody who stocks them.

Of course factory order panel boards you can get whatever you want, including 10K mains if you want it - that's actually what I have at my house, because it goes through (2) 15 KVA transformers (in addition to the Poco 25kva) and a 1900 ft 2400v wire run, then a 200' secondary run, so I'm not too worried about fault current 😇
 
electrofelon said:
You can just about almost figure any load center is going to have a 22 kaic min main (I think Eaton's are 25k) that series rates to 10K branches. I remember digging through the catalogs a while back to verify this, and my hopefully correct recollection is that Siemens
is the one exception as they do show their cheaper ES series load centers as being available with a 10K main, but I have never encountered those in the wild or found anybody who stocks them.

Of course factory order panel boards you can get whatever you want, including 10K mains if you want it - that's actually what I have at my house, because it goes through (2) 15 KVA transformers (in addition to the Poco 25kva) and a 1900 ft 2400v wire run, then a 200' secondary run, so I'm not too worried about fault current 😇
Click to expand...
I think GE standard 10kaic breakers say on the side not for series rating or something like that I'll have to look at one later to check.
 
electrofelon said:
My experience is that even using infinite bus with transformer data, you will get a drastically lower value than the utility will provide. Every time I have had the opportunity to see the numbers from both methods, the POCO figure is always 1.5-2x what I get with actual transformer data and infinite primary. Of course it may be prudent to provide some headroom in case of transformer replacement with a larger unit and/or lower impedance. I evaluate what figure to provide for on a case-by-case basis.
Click to expand...
The value provided by utility, although you may not agree with it, is based on calculations they have done for the system, that takes into consideration planning based on what they will see at some future time. Assuming they own the transformer, they may be considering they could change the transformer to a larger size and/or lower impedance in the future, which if you don't plan for you could be setting yourself up for problems later.
 
Status
Not open for further replies.
Top