208Y/120V 3PH 4W 150 KVA transformer located close to the building. The available fault current at the secondary of the transformer will be over 40K.
Abbreviations:
"SCA" = Short-Circuit Amperes,
"Z" = Impedance,
"LRA" = Locked-Rotor Amperes,
"WCS" = Worst-Case Scenario
Not trying to be an A$$ here, but when I saw the SCA -vs- the Transformer Size, I had to run a quick Calculation; as the SCA seemed too high.
Per the Commercial use of a 150KVA PMT (Pad-Mount Transformer), installation for So. California based Utilities (SCE, SDG&E, LADWP, APU, Etc... the EUSERC crowd) would normally be as follows:
- Primary: 12,470V, Secondary: 208Y/120V 3PH 4W,
- Z = 4.0% (no less than 3.0%),
- Secondary Feeders = (2x) 4 #300MCM AL; 3" PVC 40 (depends on Load Calcs) - at least 40 feet of Conductor length,
- Typically Feeds into 600A Switchboard Section, with 600/3 Main as Service Disconnect,
- Service Disconnect rated min. 35KAIC@240VAC, Bus Kit braced for 42K.
I ran a quick calculation using a 150KVA 208/120V Transformer with 2.0% Z (WCS rated to 1.955% Z), and came up with a SCA of 20,819A at the Secondary Terminals.
With 50% KVA Capacity for LRA (Motor Contribution), the SCA at the Terminals increased to 21,651A (added 75KVA fault contribution).
With 100% KVA Capacity for LRA, the SCA at the Terminals increased to 23,733A (added 150KVA fault contribution).
Then figuring Feeder distance = 40 Feet, using Copper Conductors in Magnetic Ducts (2 sets of 300MCM CU), The SCA at the Service would be:
w/o LRA Contribution: 16.9KA,
w/ 50% LRA Contribution: 17.6KA,
w/ 100% LRA Contribution: 19.7KA.
Utility would likely quote the SCA at the Line Side of the Service Disconnect to be 22KA.
By the time the Fault Level has traveled to the First Panelboard, it may easily be down to 10KA - or even less... depending on the length of run and Panelboard Feeder Size.
That's my 2¢ worth for today...
Leaving the Soapbox...
--Scott (EE)
