10KAIC breakers in a 200A service

The AIC you give for the 25 kVA single phase transformer would have to be 0.99%Z or 1%Z. Do you have transformers this size with that low of an impedance value? Just curious. Kind of the same question on the 167 kVA with a 1.5%Z which also seems kind of low.
I think those values are unrealistically low. I dont recall ever seeing a 25KVA lower than 1.8%, typically 1.9-2.4. The one photo is the transformer serving my house, its 2.4%, I have a 15 KVA pad here that is 1.9%. Here is a 167 I came across with 1.8%, lowest I have ever seen.
 

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I've heard also that the input voltage plays a part too. I remember years ago in EC&M magazine they had an article about poco's upgrading line voltage (usually doubling the voltage) which affected the available fault current, and services that previously had sufficient aic ratings, no longer would be sufficient.
I dont see how primary voltage itself would matter. When they changed voltage they must have changed the transformers, and, very generally, newer transformers will have lower impedance. Certainly primary fault current could go up with a distribution upgrade, but we usually assume infintite primary fault current anyway.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
I dont see how primary voltage itself would matter. When they changed voltage they must have changed the transformers, and, very generally, newer transformers will have lower impedance. Certainly primary fault current could go up with a distribution upgrade, but we usually assume infintite primary fault current anyway.
That may be the case, the article did not mention it. When they upgraded the line at my house, they replaced the transformer with a dual voltage, so when they cut in the higher voltage line, they were able to quickly switch over everybody's transformer with minimum downtime.
 
That may be the case, the article did not mention it. When they upgraded the line at my house, they replaced the transformer with a dual voltage, so when they cut in the higher voltage line, they were able to quickly switch over everybody's transformer with minimum downtime.
That photo I posted of my transformer is a multi voltage, 4800 and 13.2/7.62. it does seem that that would effect Z a bit, but note it only shows one impedance value.
 

kwired

Electron manager
Location
NE Nebraska
Most the time calculations by us installers assumes infinite bus - which means the primary side the transformer can deliver unlimited amount of energy if called upon. Often times that is good enough for what we want as well, usually just trying to determine worst case amount of fault current that is available to determine if the equipment can safely handle such a fault.

Actual fault current can be further limited by impedance of the supply to that transformer. Closer you are the the substation the less impact there often is on the supply side. Substation transformer also has an impedance value and if they change it the entire system has a different value at all user points. We generally still assume infinite bus because any time the POCO makes changes to their distribution system that primary side impact can change.
 

Mevlevi

Member
Location
Massachusetts
Occupation
Engineer
The AIC you give for the 25 kVA single phase transformer would have to be 0.99%Z or 1%Z. Do you have transformers this size with that low of an impedance value? Just curious. Kind of the same question on the 167 kVA with a 1.5%Z which also seems kind of low.
These values are listed by the utility co. in our area. See attached. 73016175-D0FF-4CE9-AA76-06BB200AB6B3.png
 

kwired

Electron manager
Location
NE Nebraska
These values are listed by the utility co. in our area. See attached. View attachment 2553026
Did not look them over closely, but am guessing they are what that utility considers a "safe number" for all installations that meet the rest of the criteria. Chances are the actual values will be lower most cases but they give a worst case value as a CYA means yet don't need to individually calculate an exact figure for every request.

reality is over time they will have purchased different transformer model numbers and the actual impedances will vary, maybe not by all that much but they will vary.
 

Carultch

Senior Member
Location
Massachusetts
I've heard also that the input voltage plays a part too. I remember years ago in EC&M magazine they had an article about poco's upgrading line voltage (usually doubling the voltage) which affected the available fault current, and services that previously had sufficient aic ratings, no longer would be sufficient.
The fault current calculator that I linked, assumes "infinite bus" on the utility side of the transformer, which means the product of fault current and voltage on the utility side is infinite.

Examine the formula behind fault current across a transformer, and you'll notice that V_primary * Isc_primary come as a pair, multiplied together. Call this value X. If you graph Isc_secondary as a function of X, you will notice that there is a limit of this function, as the input approaches infinity. That is what the fault current calculator is providing. The upper limit of what the secondary fault current could be, regardless of what it starts at, before the service transformer. This feature of a function/graph is called an asymptote. This provides an upper bound on your fault current, as long as your service transformer remains unchanged, that covers you no matter what the utility does on their transmission and distribution system to adapt to the growth of other services.

The source formulas are here, in the section titled "Calculation of Short-Circuit Currents When Primary Available Short-Circuit Current is Known":

Here's an example, given the following specs:
100 kVA, 1-phase 240V, 2.5%Z, 13.8kV primary
1595784903852.png
 

NewtonLaw

Senior Member
These values are listed by the utility co. in our area. See attached. View attachment 2553026
Yes I see there listing. This is what the give to their Technician's similar to what we give ours to answer customer's questions quickly without performing an actual calculation. It is wrong for a 25 kVA transformer since there is no supplier of a 25 kVA transformer that I could find with an impedance below 1.2%. Interesting. Plus I see there AIC for 3 10 kVA units in a three phase bank of 30 kVA has 10,000 amps AIC. Not so.
 

NewtonLaw

Senior Member
If they increase the supply voltage, anything approaching 10% increase will cause overexcitation problems. So going from 12,470 to 13,200 works (and UGI did exactly that). This has the affect of changing the %Z, and the base kVA of the transformer since you get move kVA at the higher voltage and I base remains the same. Thus Isc for a 25 kVA unit at 240 volts and 1.2%Z is 8680.8 amps, at the new base voltage the fault duty increases to 9,193.2 since the new kVA base becomes 26.46 with a new voltage base of 254 volts and Z base of 2.44 amps si %Z becomes 1.13% instead of 1.2%. All this assumes a linear relationship for the increases when in fact the overexcitation causes some harmonics which affect the true %Z.
 

augie47

Moderator
Staff member
Location
Tennessee
You can tell from all the posts that there is no clear-cut answer.
I think many will agree that for single family dwellings, short circuit amps at the service load center is rarely over 10k
The jurisdictions in this area do not entertain fault current on 200 amp services unless there are extenuating circumstances such as large transformers feeding large loads other than the one or two dwellings.
 

kwired

Electron manager
Location
NE Nebraska
Yes I see there listing. This is what the give to their Technician's similar to what we give ours to answer customer's questions quickly without performing an actual calculation. It is wrong for a 25 kVA transformer since there is no supplier of a 25 kVA transformer that I could find with an impedance below 1.2%. Interesting. Plus I see there AIC for 3 10 kVA units in a three phase bank of 30 kVA has 10,000 amps AIC. Not so.
They are CYA values that they have pretty much assured will work for the worst possible case scenario. Better to be too high than too low with the available fault current.

The other problem here is inaccuracy in determining incident energy for arc flash calculations, as those not only need to consider the current but also the duration of the incident. If current is lower than what was used in the calculation it may mean longer time before overcurrent device opens - might mean more overall incident energy than if the current were higher.
 
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