AIC ratings >200kA

[Pierre C Belarge]

Ron
I find this thread fascinating... as well as there are no derogatory comments being made.

I do not understand all of the processes, but I do know what you are trying to accomplish... and in NYC no less.

Have you tried to call UL in Melville? I bet they may be able to help you locate a 300,000 AIC enclosure and equipment if it does exist. If they say it does not exist, a new design may be necessary.

UL, Melville
1 877-854-3577
1 631-439-6464 fax

Good Luck, keep us posted!!!

 

[kingpb]

From your other two posts I was able to fine tune it. Notice the current limiting reactor ahead of the CB on the left side. It doesn't take much to lower the current.
View attachment 197

 

[bob]

Using Z = (%Z * E) / I, for (Ohms / 6), I still get 0.96 %Z = (I * Z / E) for the Xfmr bank, on both primary & secondary buses.

Plugging 1k ft of 13.8kvac upstream-feeder impedence, into a 0.96 %Z, 15000KVA, Pri., 50kA finite, SCC manages nothing less than 750kA SCC after the 10 ft, 480vac, bus feeders.

Ramsy
You can not use %Z in this way. If you want to caculate the X and R
from %Z you can do it this way:

We can make the assumption that Z is equal to X since R is small compared to
X. If so then X = 10 * %Z *(Secondary Voltage KV)?/ Transformer KVA
Assume the transformer has a Z = 5.75% then
X = 10 * 5.75 *[ 0.48]?/2500 = 0.0053
Fault Current = 480/(0.0053 * 1.73) = 52350 amps x 6 = 314 ka.

 

[ron]

King,
Thanks
That's it, but no ties on the secondaries.

 

[ramsy]

If you want to caculate the X and R from %Z you can do it this way:
(Xfmr) R is small compared to X. So, X = 10 * %Z *(Secondary Voltage KV)?/ Transformer KVA
X = 10 * 5.75 *[ 0.48]?/2500 = 0.0053 Fault Current = 480/(0.0053 * 1.73) = 52350 amps x 6 = 314 ka.

Yes I see, Xfmr reactances near capacity may choke more than a linear resistor, so crude parallel-resistance calcs may ask for more amps, but they're not always going to get them.

This was a generous solution to provide, which solves the Ohmic method for SCA with Xfmrs.

Skipping point-to-point proofs, ad nausia, E/Zsqrt(?) lets contractors go anywhere in the building, log Z from a constant-load test, and include unforseen ambients, cable reactance, & termination resistances, invisible to design/inspections.

By documenting real measurments for AIC, I believe Ron can get his equip. AIR's without expensive impedance analysis, assumptions from utilities, or finite ASCC's for svc points. Without a TC-of-O for his service, Ron could visit a similar, occupied-one-spot nearby, and start loggin Z's.

Oddly, Ron's 13.8kv one spot, 6-bank 2500kva is identical to the "typical 60-story office buildings" described by Shultz, George. "Transformers and Motors" 1989, bh.com/newnes, Woburn. p.101, except with this 4000A 480/277 secondary bus, "Fault currents at each service would be close to 100,000 amperes." Maybe newer Xfmr impedances are lower, than in 1989, especially in NY. But, then again, maybe not.

 

[steve66]

Ron:

Is it absolutely required to tie all 6 transformer secondaries together? I would think you could have two services with 3 transformers feeding each. That should cut the fault current down below 200KAIC.

Its a shame they won't provide a medium voltage service up to say the 11'th floor, and save some of the copper you are going to have to run vertically.

Steve

 

[ron]

Steve,
I agree.
Unfortunately, the POCO wants (6) xfmrs tied together. They need (4) to carry load. Con Ed has a N+2 contingency in force in many areas of NYC.