Short Circuit Calculations

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new_ee

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What do you use for POCO transformer sizes when you dont yet know the size they will install? Do you just use the first one that will cover your calculated load?

This has always been a problem with me because we never know what size transformer or its location the POCO will use when we are designing our system. How do you do short circuit calcs in this situation?
 
You have to drop a dime on them. They never use the size that is large enough to cover the load you calculate. Every POCO has a different way of diversifing your load (I always provide connected, undiversified load so they can figure it out themselves).

You have to call them. Short circ calcs are the last thing I do. I send them the loads and find out what they want to do and then go back and do the short circuit calcs and make adjustments to equipment ratings as needed.
 
You need to base your design on what is required, not on what you think they might provide. The reason they usually supply a transformer that is smaller then what you think is becasue they have a lot of historical data and have done economic analysis to determine the best fit. That usually means a smaller transformer, that may or may not not last as many years.

The fault current you use will most likely be greater then avaiable, but you won't be low either. You should go back and check after the do their install just to make sure your OK.
 
I agree. Start with the transformer that will handle the calculated load. If the short circuit calculation gives you acceptable results (e.g., every bus in the facility has an available fault current that is lower than the rating you had chosen for that bus), then the final results (with the final transformer size) will also pass. If you have a problem with a bus, however, that does not mean it is time to specify a higher rated bus. It is possible that the calculation will pass with a lower sized transformer. Sometimes you just have to investigate all available options, even the unlikely ones, before making a decision that will cost the owner more money.
 
It also depends on where you work. Most places where I work the POCO has a standard table that specifies a fault current value at the service, and fault calcs start at the service, not the utility transformer.

Martin
 
Martin,
Most places where I work the POCO has a standard table that specifies a fault current value at the service, and fault calcs start at the service, not the utility transformer.
Click to expand...
How is that? The service conductors between the utility transformer and the service equipment has an effect on the fault current that is available at the service equipment.
Don
 
We provide a table of maximum available fault current of various sizes of transformers based on infinite primary bus, and the minimum impedance of transformers that we buy.

Charlie, one thing that needs to be considered under the unlikely options you mentioned is that for different sized transformers, the company I work for specified different minimum impedances. For example, at 480 V, 3 phase, secondary, 500 kVA and smaller, the minimum impedance we will buy is 2.5%. For larger sizes, the minimum impedance we buy is 5.5%.

So if the calculated load calls for a 750 kVA transformer with a maximum fault current available at the transformer secondary of 20,011 amps, assuming 5.5% minimum impedance and infinite primary bus, but it turns out we install a 500 kVA transformer with a minimum impedance of 2.5%, and a maximum available fault current of 26,462 Amps, there could be enough of a difference to warrant changing OCPD device ratings.

I know, it's a stretch...:)
 
don_resqcapt19 said:
Martin,

How is that? The service conductors between the utility transformer and the service equipment has an effect on the fault current that is available at the service equipment.
Don
Click to expand...

Excellent question. I can't speak officially. The common practice is that the contractor installs the raceway(s) for the lateral, but the utility sizes, provides, installs, and owns the conductors. A typical table is at http://esp.aps.com/resource/metering/esrm/800.pdf

While you are correct that the conductors between the utility transformer and the service equipment have an effect on available fault current at the service, we are essentially required to use the table values (with rare exceptions) regardless of the length of the lateral. It means we frequently install equipment with heavier bracing than would be required if we could calculate from the transformer secondary, but I'm not so sure that that is a bad thing. The values seem to be conservative -- I can't recall any projects in areas where I have to size the lateral where the table values have been too low.

Martin
 
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