stevenal wrote:
Using source impedance on the primary rather than infinite bus means there will be no allowance for utility reconfiguration or conductor upgrading. And using actual service transformer impedance rather than an assumed minimum means no allowance is left for transformer replacement. I also suggest that load growth be considered when looking at interruption ratings. I see the two calculations as entirely different. The fault current calculation for arc flash is good for today and must be periodically rechecked. The fault current calculation for equipment sizing should be good for the life of the installation. Suggest the two not be combined.
While future planning sounds good it doesn't void the fact that you still have to check it every time you go to do an installation. And taken too far it pushes you into the 65-100kA ratings where you start having to have backing fuses in your circuit breakers...not a good place to be. And this being the arc flash forum...you're already going to be doing a short circuit analysis as part of the arc flash study so the engineering data is already available in the report.
Taking some of the issues:
1. Utilities don't support customers anyways so it's kind of a moot point.
2. Service transformer sizing is nearly always close to ANSI and to a certain degree not controlled. The impedance is always a guess prior to building the transformer and that's why the actual value is stamped after the fact during testing.
3. Load growth...pray tell what X/R or X/Z or X/E do you want to use? 20? 50? That pushes you into derating circuit breakers and a whole host of other issues. Fine for a brand new main substation but gets pretty questionable after that, and I've done almost entirely retrofits. We spent a lot of capital in the 1960's. All of that infrastructure is now hitting pretty much some absolute no-turning-back age limits and having to be replaced. At least over my career electrical work is semi-permanently in demand due to this "aging out" issue. It's just as good as laying fiber was 10 years ago except it lasts longer.