Three Phase Short Circuit Calculation

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BatmanisWatching1987

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I am trying to work backwards and see if I can get similar answer based on the information provided from the Electrician.
The system is 208V/3Ph system.

We have a value of 41,748a at the secondary taps of the utility transformers provided by the utility company.

Length of run from transformers to tap box is 185’. The conduit is (3) sets of 750 aluminum PVC conduit. The fault at the Tap Box is 22,874A

Length from Tap box to meters is 20’ of copper bussing. The conduit is (3) sets of 750 aluminum PVC conduit. The fault at the meter is 21,986A

Length to unit A is 100’ (1) set of 3/0 copper steel pipe. The fault at Meter A is 10,962A.

I am trying to use the Eaton app and do the hand calculation by hand. But I don't think I am getting the right KVA or Z% for the system.

I am assuming the electrician used a infinite bus for his calculation.
 

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That worksheet needs a transformer to calculate fault currents. I recommend to you to use the MVA method, it's easier.
 
Without knowing all of the exact inputs in the electrician's calculation, it will be difficult to nail down why there is a difference between the two values. It could be in the primary current, the transformer impedance, the conductor lengths, rounding, or any of the other inputs.

Also, in my opinion, a 1000A difference in a fault calculation is close enough to say the numbers are the same.
 
There are lots of different factors involved in short circuit calculations, so much so that many of the 'by hand' methodologies take short cuts making it difficult to compare the results. One major impact item, which is rarely mentioned, is the X/R ratio of the source. Insulation and conductor temperature also come into play, but I find they have little effect until the circuit lengths are extremely long (ha - I just took short cut).

For example, when I model your values in ETAP I get different values based on the assumed source X/R and insulation types. When I used a value of 8 (typical for utilities) the SCA at the Meters was 20.894kA, and a X/R of 20 (typical for generators) resulted in an SCA = 21.154kA
 
jim dungar said:
There are lots of different factors involved in short circuit calculations, so much so that many of the 'by hand' methodologies take short cuts making it difficult to compare the results. One major impact item, which is rarely mentioned, is the X/R ratio of the source. Insulation and conductor temperature also come into play, but I find they have little effect until the circuit lengths are extremely long (ha - I just took short cut).

For example, when I model your values in ETAP I get different values based on the assumed source X/R and insulation types. When I used a value of 8 (typical for utilities) the SCA at the Meters was 20.894kA, and a X/R of 20 (typical for generators) resulted in an SCA = 21.154kA
Click to expand...
I played around for about 3-4 minutes with the numbers, but like you say the variables were too many to pin down how they got those numbers.
I could get close, and even change a variable or two to get closer, but it was a waste of time so I quit…
 
I considered a 1 MVA trafsormer and 6.65% short circuit voltage and I got 41704 A three-phase solid short-circuit at transformer terminals. I took the cable resistance and reactance from Table 9 for pvc conduit 0.019 and 0.034 ohm/kft and -after some iteration-I got 22700 A short-circuit at end of 185 fts of 3 parallel cables.
 
Julius Right said:
I considered a 1 MVA trafsormer and 6.65% short circuit voltage and I got 41704 A three-phase solid short-circuit at transformer terminals. I took the cable resistance and reactance from Table 9 for pvc conduit 0.019 and 0.034 ohm/kft and -after some iteration-I got 22700 A short-circuit at end of 185 fts of 3 parallel cables.
Click to expand...
I didn't see the reactance of 750 MCM Aluminum as 0.019 to 0.034 ohm/1000 ft! My old NEC here lists as 0.148 to 0.045 ohms/1000 ft on PVC conduit.
Let's do the math: (Using the MVA Method)
  • Source transformer MVA sc = 1.732 x 208 x 41,748 = 15.04 MVA
  • Conductor MVAsc = (0.208)^2/(185 x 0.045/1000/3) = 15.59 MVA
  • Available SC MVA at end of 185 ft. run = 1/(1/15.04+1/15.59) = 7.66 MVA
  • And the available fault current = 7.66 X 10^6/(1.732 x 208) = 21,248 A!
 
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