Short circuit calc question

[fandi]

Hello All,
If the POCO available fault current is 65kAIC at POCO's 800A rated pullbox (208/120V, 3ph 4w) then what is the available fault current at the customer swbd which is 800A, 208/120V, 1ph 3w? POCO provide 3 phase service but the customer only uses 1 phase. The distance from the pullbox to the swbd is about 5ft.
A formula would be appreciated.
Thank you.

 

[don_resqcapt19]

Download the free Bussmann FC² program and plug in the numbers. Five feet of cable will not make much difference unless it is a small conductor.

 

[fandi]

Download the free Bussmann FC² program and plug in the numbers. Five feet of cable will not make much difference unless it is a small conductor.

Thanks. I have tried the app before posting the question but I can't find a way to calculate the single phase short circuit value base on a given three phase short circuit value.

 

[GarwoodV6]

Thanks. I have tried the app before posting the question but I can't find a way to calculate the single phase short circuit value base on a given three phase short circuit value.

The fault current is per phase, so 65KA is the same on any of the phase "legs". Wire size and length to the first point of disconnecting means factors into the final available fault current. Short service runs won't reduce this by very much.

 

[topgone]

Thanks. I have tried the app before posting the question but I can't find a way to calculate the single phase short circuit value base on a given three phase short circuit value.

Calculate using single-phase fault formulas. Since you haven't posted the details (conductor size, runs per phase, etc, should the service wires be 15 feet, 2 X 600MCM, you'll get about 59kA at the user's disconnect. That's just my quick calcs, please verify if correct.

 

[Julius Right]

First, 65kAIC it is not the maximum short-circuit available current, but the breaker capacity.
What you need it is the 3-phase initial symmetrical short-circuit current in r.m.s [Available Fault Current AFC].
This will give you the positive sequence short-circuit impedance [Z1]
However, in order to calculate the single phase short circuit current you need also Z2-negative sequence and Zo-zero sequence short-circuit impedance.

 

[kingpb]

First off, this is a perfect example of proper use of ANSI voltage designations, a 208V 3ph service is 208Y/120V. Using a 208V 1ph service from that is stated as 120/208YV. Anyway:

If you think of it in KVA or MVA the math becomes much easier. So, 65KA at 208V, 3ph = 23.42MVAsc
That is the starting number for the service. Now to determine short circuit at your 208V, 1 ph, then assume close coupled and ignore the cable impedance, it becomes 23.42MVAsc/ 208V = 112KAIC.

 

[Julius Right]

Let's say you want to go up to breaker capability.
In my opinion, the 3 w -single phase supply system it is close to line-to-line short circuit with earth connection.
See IEC 60909-0 for instance.
Let’s say the Utility transformer connection is Delta [high voltage] Yn [secondary] and no generator or induction motor is connected with.
Then Z1=Z2=Zo.
Let's take the Utility supply transformer as 1000 kVA.
Cooper-Brossmann indicates 3.5% impedance. But, according to IEEE see-for instance-Table 3 from E.T.N.[Eaton Catalogue Data CA202003EN] for 1000 kVA is 5.75%.
As Cooper-Brossmann also says, Utility voltages may vary ±10% for power services. Therefore, for highest short circuit conditions, one has to multiply values as calculated by 1.1. Also, in the same article is written the short-circuit impedance error it could be =+/- 7.5% for ANSI standard transformer. In this case uk%=5.75*(1-7.5%)=5.32%
If Irated=1000*1000/1.73/208=2779A then, short-circuit current is 1.1*2777*100/5.32=57.5 kA.
If we will take separate R and X instead of and use complex numbers we get 59.7kA.
This for three-phase short circuit current.
The same current we get if we consider line-to-line short circuit with earth connection. If it is line-to-line only, short-circuit the current will be Isc2=sqrt(3)*VL-L/(Z1+Z2)=0.866*Isc3=51.7 kA.

 

[topgone]

Let's say you want to go up to breaker capability.
In my opinion, the 3 w -single phase supply system it is close to line-to-line short circuit with earth connection.
See IEC 60909-0 for instance.
Let’s say the Utility transformer connection is Delta [high voltage] Yn [secondary] and no generator or induction motor is connected with.
Then Z1=Z2=Zo.
Let's take the Utility supply transformer as 1000 kVA.
Cooper-Brossmann indicates 3.5% impedance. But, according to IEEE see-for instance-Table 3 from E.T.N.[Eaton Catalogue Data CA202003EN] for 1000 kVA is 5.75%.
As Cooper-Brossmann also says, Utility voltages may vary ±10% for power services. Therefore, for highest short circuit conditions, one has to multiply values as calculated by 1.1. Also, in the same article is written the short-circuit impedance error it could be =+/- 7.5% for ANSI standard transformer. In this case uk%=5.75*(1-7.5%)=5.32%
If Irated=1000*1000/1.73/208=2779A then, short-circuit current is 1.1*2777*100/5.32=57.5 kA.
If we will take separate R and X instead of and use complex numbers we get 59.7kA.
This for three-phase short circuit current.
The same current we get if we consider line-to-line short circuit with earth connection. If it is line-to-line only, short-circuit the current will be Isc2=sqrt(3)*VL-L/(Z1+Z2)=0.866*Isc3=51.7 kA.

Do as suggested by kingpb-> use the MVA method for simplicity. Add all MVAsc for the elements in parallel/ add the inverse MVAsc of all elements in series. Once you have the total MVAsc, divide by the voltage at that point to get the SC available.