Xfmr Primary cable sizing
Xfmr Primary cable sizing
Keri,
There's some leeway in sizing the primary feeder cables to your 750 kVA transformer.
First determine the xfmr's primary fla, @ 750000/12470/1.732 = 35 amps.
The actual minimum size would be 125% of the 35 amps, or 44 amps.
Next, look at NEC 450.3 which gives you the required xfmr overcurrent protection, which in this case is anywhere from 300-600% of the primary fla.
Then the issue is further confused by 240.100, Overcurrent Protection over 600 Volts, which says you can go from 300-600% of the cable rating.
But there are other factors to consider, like Voltage drop associated with your anticipated loads, and perhaps the most significant other factor - practical cable application. That's to say that a 44 amp, 12470V cable is not likely available.
If it were a Utility, they would run whatever was on their truck, which would likely be either 1/0 or 4/0. But in this case, I would suggest investigating the available cable assemblies for your application. As I look at NEC table 310.71, that might translate to a 3/C #6, which has an ampacity of 93 amps. That gives you ample capacity to run the xfmr above nameplate and tolerate some voltage drop.
But if you were attempting to start a 500 HP motor on this unit, I would probably upsize again. In that case I would do a Transient Motor Starting Analysis, or a simple Voltage drop calc to assess the drop.
.. not simple answer, but an easily rationalized oversize!
JM
