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 Post subject: Short circuit calculations for specific equipment
PostPosted: Mon Nov 13, 2017 4:03 am 
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Joined: Sat May 17, 2008 5:00 pm
Posts: 77
The NEC seems to be requiring short circuit calculations for more locations with each code cycle. Didn't 110.9 that requires equipment to have an adequate interrupting or withstand rating achieve this objective? Now the code seems to be singling out specific equipment for detailed calculations.

Here is the latest addition in 430.
430.99 Available Fault Current. The available short circuit
current at the motor control center and the date the short
circuit current calculation was performed shall be documented
and made available to those authorized to inspect the installation.


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 Post subject: Re: Short circuit calculations for specific equipment
PostPosted: Tue Nov 14, 2017 4:22 am 
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Location: North Carolina
There is a difference. 110 requires labels on the equipment for selection purposes. 430 requires that you actually do the calculation and post it, not just buy something and hope that it's right.

Unlike for arc flash purposes where the traditional and very accurate ohm's law method is mostly used, there is a quick and dirty method where you basically assume infinite bus on the line side of the transformer, ignore all the resistances, ignore smaller motors, and use impedance only plus some multipliers that are in the ANSI standard for shielded cables and the larger motors. This is the essence of the ANSI method that is commonly used.

The results will be conservative which is a polite way of saying always higher than reality, but at least the equipment will survive a bolted fault. This is also the reason that it should not be used for arc flash calculation purposes and once you've already gone that far in terms of power system analysis, you might as well drop ANSI and just use the more accurate ohm's law calculations.


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 Post subject: Re: Short circuit calculations for specific equipment
PostPosted: Tue Nov 14, 2017 12:03 pm 
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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.


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 Post subject: Re: Short circuit calculations for specific equipment
PostPosted: Tue Nov 14, 2017 4:36 pm 
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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.


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