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As to the SCA (Short Circuit Amperes) of a 25 KVA 1Ø 3 Wire Transformer with a Secondary Voltage rating of 120/240VAC, here's a "Worst-Case Scenario Figure":

25 KVA 120/240V 1Ø 3 Wire, Z = 1.58%:
SCA at the Transformer's Terminals would be 11,574 Amps.
Wow, thanks Scott, great explanation [Linked Image]

I just paced off the drop- 216' from my panel to the pole. There is #4/0 Al run into my box, but the aerial drop looks to be significantly smaller; going with the worst-case scenario, I'll assume #4/0, which is 0.1 Ohm/1000'. Resistance for this line would be 0.0432 Ohms. Coupled with 1.5% impedance gives a complex impedance of 0.045 Ohms at 19° (PF .95).

So, 120V bolted short on my panel bus would give a fault current of 25kVA/120V/.95PF/0.045 Ohms=4896A. 240V fault would give half that. In reality with a smaller conductor, resitance would be higher, and fault current would be lower with a PF closer to 1. Did I calculator that right?

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Gfret, I be that, if you look closely, you'll find "egg" insulators in the lines that separate the transformer secondaries from one another, meaning that any given house is only supplied by one transformer.

That's how they do it around here, anyway. I guess it's possible that they really do parallel the transformer secondaries, but that could lead to a larger outage or too much current to safely be inetrrupted.
In the Queens power outage thread, it was explained that the NYC grid is designed exactly like gfretwell described, with the secondaries tied together, with no means of disconnect in between. Strikes me as a dangerous situation, too! I'd hope there are egg insulators on the poles and the wires are there only for tension... Though, in theory, there are fuses on the primary side of all the pole pigs which should open if overloaded, right?

[This message has been edited by SteveFehr (edited 08-12-2006).]