42 AIC Rating

[Socalshocker]

The San Diego City Inspector has asked me to give him the AIC rating of 2 series rated, and identified 200amp, 208volt (3/O wire) sub-panels, using 42kAIC as the base rate to calculate with. The transformer size and impedence
is unknown, only 42kAIC is given as a starting point. The Power company ( SDGE ) has given the 42kAIC rating of the main disconnect breaker requirements. The sub-panels are 95 and 65 ft away from the Main disconnect. My question is, is there a way to calculate this without the transformer KVA or Z rating if so, how do I do it? Thank you for your help.

 

[drcampbell]

Pretend that the transformer impedance is zero and calculate the maximum available fault current. This will yield a wrong answer, but we know that the right answer will be less. If the panel can tolerate that, we know that it can withstand that actual available fault current.

If not, you'll need to investigate further and be more precise, but this technique will let you move forward some of the time.

 

[kwired]

The San Diego City Inspector has asked me to give him the AIC rating of 2 series rated, and identified 200amp, 208volt (3/O wire) sub-panels, using 42kAIC as the base rate to calculate with. The transformer size and impedence
is unknown, only 42kAIC is given as a starting point. The Power company ( SDGE ) has given the 42kAIC rating of the main disconnect breaker requirements. The sub-panels are 95 and 65 ft away from the Main disconnect. My question is, is there a way to calculate this without the transformer KVA or Z rating if so, how do I do it? Thank you for your help.

Go to this page on Mike Holt's main site.
You will find a free excel fault current calculator that you can download.

since you are supposed to use (for whatever reason) 42,000 amps as the available fault current, that is what you would enter on the first field of the spreadsheet, and leave transformer data blank. By entering the 42000 amps you are disregarding transformer capacity and impedance and telling the sheet that the available fault current at your starting point (presumably the service equipment) is 42000.

Then enter conductor details of the next segment of the circuit. Result will tell you available fault current at the end of the next segment.

Assuming some values I came up with about 15,000 amps available at the end of 65 foot run.



But you and your inspector need to get some things straight on terminology and specifically what is being asked for.

First is there really 42000 available at the service? Who stated 42,000 and why? If you have a 42k rated main that doesn't mean 42 k is what is available it just means that is how much that device was designed to safely be able to interrupt.

If there actually is only 25000 available at the service then the end of the 65 foot run will be even less.

If there actually is 42000 available at the POCO transformer, then you need to plug in service conductors data and figure out what is actually available at the service disconnect. If it were 65 feet of 3/0 conductor - then my result of about 15000 would be what is available at the service equipment.

Some of those details can make a huge difference in what the rating of equipment you use needs to be - as well as can make a huge difference in cost.

Long service lateral can significantly reduce what is available at service equipment compared to what is available at transformer.


Add: some of what I said was a result of the "42000" that was given. That is a fairly common rating tier for such equipment and is probably what rating is being reccommended and not the actual available fault current. With that recommendation the actual value could possibly be anywhere between 25k next lower common rating in many cases and 42k.

 

[kwired]

And to add some more, when mentioning series rating - the device at the end of the run could have a general rating of only 10k, but possibly can still be used if it is actually series rated with the device at the beginning of the run. That series rating is manufacturer dependent.

 

[GoldDigger]

And to add some more, when mentioning series rating - the device at the end of the run could have a general rating of only 10k, but possibly can still be used if it is actually series rated with the device at the beginning of the run. That series rating is manufacturer dependent.

If the inspector actually used the words "series rated", then you probably need to find the manufacturer's data on series rating of the exact breakers you are using. If for some reason they are from different makers you will be SOL because there is no incentive to cross rate breakers with another brand. Testing is expensive.

Sent from my XT1585 using Tapatalk

 

[kwired]

If the inspector actually used the words "series rated", then you probably need to find the manufacturer's data on series rating of the exact breakers you are using. If for some reason they are from different makers you will be SOL because there is no incentive to cross rate breakers with another brand. Testing is expensive.

Sent from my XT1585 using Tapatalk

Yes kind of what I was trying to say. Would be nice to get more clarification on OP's actual situation, as I believe there is possibly some misunderstanding possibly by OP or even his inspector on how available fault current, AIC ratings, and series ratings all work together, and seems a little fishy on how they determined they needed to start with 42kA presumably at the service equipment. Unless this is a pretty large capacity service (like at least 1200 amps) and/or the supply transformer is only say 10 feet of conductor away from the service equipment, chances are there is less then 42kA available at the service equipment.

 

[jcassity]

i agree totally on the series rating contention you have.

my local power co rattled off this ,, what they call a quick down and dirty way of getting inside the goal posts of the AIC size.
the actual formula for properly calculating this is about as complicated as time travel! but.................. hope below lines up with what you see or perhaps can stare and compare past projects to see how the below shakes out.


========================
AIC info
Xfmr size in amps divided by .03 = ball park aic (down and dirty way)
Or reverse the KVA of xfrmr down to amps then divide by .03

Use .03 for pole mount xfrmr, .05 for pad mt xfrmr

500kva / 208/1.73 / .03 = @ 46kaic
Or 1200A / .03 = 40kaic
Use .05 for pad mounted xfrmr