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GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
In my experience no. Not that that never happens of course, but I find it rare.
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?
 

kwired

Electron manager
Location
NE Nebraska
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?
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.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
How what's you suggest someone go about doing that in a cost-effective way?
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.
 

kwired

Electron manager
Location
NE Nebraska
do residences even have to worry about it?
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.
 

pv_n00b

Senior Member
Location
CA, USA
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?
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.
 

kwired

Electron manager
Location
NE Nebraska
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.
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.
 
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