AIC rating

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electrofelon said:
In my experience no. Not that that never happens of course, but I find it rare.
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Commercial ... a good friend owned a 12 unit strip mall, 1 transformer, 12 meters for the spaces, 1 meter for house loads. I would expect that to be common?
 
GeorgeB said:
Commercial ... a good friend owned a 12 unit strip mall, 1 transformer, 12 meters for the spaces, 1 meter for house loads. I would expect that to be common?
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if you have a service conductor run from source to each space, impedance of conductor will likely knock the available fault current down to below 22k or even below 10k.

If you have common service location and a meter center with 12 feeders leaving it, might have significant available fault current at that service equipment, maybe even at a occupant that has short feeder, but most those occupants still likely have significantly reduced fault current levels because of impedance of the feeder.

Place where multiple commercial customers are commonly supplied by one transformer is in old school "downtown districts", often they are only 200 amp or less services and often is 120/240 high leg delta - with many services being single phase but an occasional three phase service maybe because of somewhat limited three phase loads. The 120/240 unit in the transformer bank is often pretty large compared to the high leg transformer.
 
petersonra said:
How what's you suggest someone go about doing that in a cost-effective way?
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Two ways come to mind. Line reactors or coils of service or feeder cable if conductors for the distance alone (as kwired pointed out) are not sufficient.

Depends on how expensive it would be to replace the main and downstream breakers. Certainly not the solution for a residence, but for multi-tenant commercial or an industrial switchboard, maybe.
 
GoldDigger said:
Two ways come to mind. Line reactors or coils of service or feeder cable.
Depends on how expensive it would be to replace the main and downstream breakers. Certainly not the solution for a residence, but for an industrial switchboard, maybe.
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do residences even have to worry about it?
 
petersonra said:
do residences even have to worry about it?
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If you have higher capacity service and the source is very close to the panel, possibly.

It doesn't take all that much conductor to make a significant reduction in fault current level at the equipment, plus most typical mains in a dwelling are 22 or 25 kA rated and series rated to be used with 10kA branch breakers.

Most typical dwellings with only a 200 amp main will often have at least 75-100 feet of service conductor between the main and the source, and possibly only a 25 or 37.5 kVA source supplying them. If you only have a 100 amp service you likley have even smaller conductor which means more resistance and less available fault current even if there is a big transformer feeding many customers.
 
hhsting said:
Would the initial fault currents subject to change at the utility xfmr be acceptable or not? Would not one need wise case short circuit fault current based in poco xfmr impedance, size and infinite bus at primary?
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I see a lot of these letters from utilities and this is the standard boilerplate language they all seem to use. We just have to take it for what it is and use it to design the system. There is a recommendation in NFPA 70E to revisit an arc fault analysis at least every 5 years to catch if the utility makes changes that increase the available fault current, but I'll give you 3 guesses how often the typical commercial customer does this. One of my worst fears is doing the analysis and finding out that the utility has increased the available fault current of a service and the existing service equipment no longer complies. It can change what was a simple electrical system project and make it into an expensive upgrade of the existing service equipment.
 
pv_n00b said:
I see a lot of these letters from utilities and this is the standard boilerplate language they all seem to use. We just have to take it for what it is and use it to design the system. There is a recommendation in NFPA 70E to revisit an arc fault analysis at least every 5 years to catch if the utility makes changes that increase the available fault current, but I'll give you 3 guesses how often the typical commercial customer does this. One of my worst fears is doing the analysis and finding out that the utility has increased the available fault current of a service and the existing service equipment no longer complies. It can change what was a simple electrical system project and make it into an expensive upgrade of the existing service equipment.
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How often is there an increase in transformer capacity without an increase in load though?

If you have say 400 amp service then add more load on a separate allowed service disconnect from same source - there you maybe could have this kind of trouble. But say same application you had a 1000 amp service though the POCO sized existing transformer to the load - pretty good chance that 1000 amp gear already has high enough fault current rating if they up the transformer.

Then as I often comment, if there is some length to the service conductors, as little as 25 feet in some cases, that available fault current is reduced by the resistance of the conductors, so my initial example with existing 400 amp service may still be fine with a transformer upgrade unless the supply conductors are really short in length.
 
kwired said:
How often is there an increase in transformer capacity without an increase in load though?
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So far I have never found that the available fault current at an existing service entrance has increased above the rating of the existing equipment. In theory, it's possible if the service shared a utility transformer with other customers and the utility put in a larger transformer to add additional services or increase someone else's service rating. So I can see why the utility puts in the boilerplate text. I never want to have to be the one to tell a customer that they need to be an expensive service equipment upgrade because the available fault current now exceeds the AIC rating of their service SWBD.
 
pv_n00b said:
So far I have never found that the available fault current at an existing service entrance has increased above the rating of the existing equipment. In theory, it's possible if the service shared a utility transformer with other customers and the utility put in a larger transformer to add additional services or increase someone else's service rating. So I can see why the utility puts in the boilerplate text. I never want to have to be the one to tell a customer that they need to be an expensive service equipment upgrade because the available fault current now exceeds the AIC rating of their service SWBD.
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And unless your gear is extremely close to the transfomer, you likely have enough resistance in the supply conductors to lower fault current enough it isn't a problem for you.

Most the time when you have multiple customers on a single transformer you are talking 200 amp and less services, maybe an occasional 400 amp that isn't really loaded that much though. The 200 or less amp services have good chance of having at least a 22kA main, the 400 amp service has good chance of having even higher rated main. If you have at least 25-50 feet of conductor the conductor will lower the available fault current at the load end with the somewhat small conductors likely to be used. Start going over 1000 amps service and then you may run into too high fault current levels a little easier, or connecting small services within 10-15 feet (of conductor) of the transformer and you need to watch it more closely.
 
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