Aic rating

[custom electrical]

Hi everyone, the inspector needs to know the aic rating of the power companies transformer in order to size the main breakers aic rating. The transformers rating is 26194, the distance from house is 50 feet, the size of wire is 2/0 aluminum. Is there a calculation out there that can help me?

 

[zog]

Hi everyone, the inspector needs to know the aic rating of the power companies transformer in order to size the main breakers aic rating. The transformers rating is 26194, the distance from house is 50 feet, the size of wire is 2/0 aluminum. Is there a calculation out there that can help me?

You need to know the available fault current from the utility (AIC is an OCPD interuption rating), the utility should provide you with that information, is that what your 26,194 is?

 

[nakulak]

are you working off approved plans ? normally the engineer puts this stuff on the plans (long before the gear is ordered)

 

[mcclary's electrical]

Maybe not the norm, but around here the poco guarantees all SFD to be below 10,000 AIC. Ask the POCO for a fault current letter AT THE METER, not the transformer

 

[Dennis Alwon]

Hi everyone, the inspector needs to know the aic rating of the power companies transformer in order to size the main breakers aic rating. The transformers rating is 26194, the distance from house is 50 feet, the size of wire is 2/0 aluminum. Is there a calculation out there that can help me?

I agree the fault current needs to be gotten from the power company. They can tell you what it is at the terminals of the transformer. From there go to mike holts online fault current calculator.

 

[Npstewart]

Best bet is to write a formal email to the power company. Call the service planner to see where you need to send it to. This will shift some liabilty to the power company, not to mention its standard practice to request it.

They wont tell you what it is without you sending something so they have proof of a date and time, with address.

I do this on almost every project, see the letter I attached for example.

 

[One-eyed Jack]

If you get it from the POCO here they will give it to you at the delivery point. Be sure you know where this rating is. Lateral and or drop size and length can significantly reduce this value.

 

[kwired]

If you get it from the POCO here they will give it to you at the delivery point. Be sure you know where this rating is. Lateral and or drop size and length can significantly reduce this value.[/quote]

Very true. I have found that you do not need to worry too much about available fault current on typical 200 amp or less service with at least 25 or more feet of aluminum conductor unless you have an unusually big transformer for some reason but that rarely happens except for maybe in a case where a large transformer is needed but you are setting up a small capacity temporary service for construction purposes supplied by the large transformer.

 

[hbendillo]

Looks like what you have is the maximum available fault current at the transformer secondary terminals assuming an infinite availability on the primary side. If you have this rating why do you need to do a calculation if all the inspector asked for was the transformer rating. Now if he asked for the available fault current at the meter a calculation would have to be done. There is an easy method called the point-to-point method that you can use to figure this out. Google it and you will probably find. Bussman used to put out a publication that had all the tables you needed and the calculation method.

I find most power companies will refuse to give a value at any point in their system. They just tell you to assume infinite so they will be safe and equipment will be overrated.

 

[augie47]

I agree. The 26194 sounds like the utilty transformer secondary available fault current. Using Bussmann's fomula, I obatin a AIC at the end of your run of 10,493.

Using Mike's formula (below) I get 10,499.
You might want to try your calcualtions.

http://www.mikeholt.com/documents/calculations/formulas/FaultV6.2.xls

 

[Dennis Alwon]

So if the available fault current is 10,400 amps does that mean you have to use 25K breakers or will the 10K standard breakers work okay?

 

[augie47]

So if the available fault current is 10,400 amps does that mean you have to use 25K breakers or will the 10K standard breakers work okay?

Locally 10k's would not be accepted.
5 ft added to the 2/0 would take it below 10k.
I think I'd look for a means to add 5 ft.

 

[jim dungar]

When you get fault currents that are within about 15% of the breaker's AIC rating (i.e above 8,500A for a 10kAIC) you really should get professional help in making your calculations.

All UL Listed breakers an adjustment multiplier which will effectively lower their published AIC ratings. The multiplier has to due with the X/R ratio of the breaker test circuit vs that of the installed system. The breaker's size and AIC also come into play a little bit.

This is rarely an issue, because so many 'short cut' calculators usually involve using an infinite amount of short circuit amps, so equipment chosen this way is not close to my suggested 15% margin.