Short Circuit Protection and Existing Load HELP

[tolind]

My head engineer is out of town and I got some problems.

I recently submitted a Short-Circuit Calculation to a City for permits. They asked specifically for it so I worked it out according to the Cooper Selecting Protective Devises book. I have an existing 300KVA 208v 3phase Transformer that leads to a meter and then leads into an existing 1000A 208v 3phase fusible distribution panel. When Calculated I have a I(sc sym rms)=74,426A.
The City planner says that this is an extremely high number and that I need to indicate the Let Thru Current (which I thought was the 74,426A) and need to indicate how I am protecting against this. With every other project I've worked on I have never had any questions in regards to this.
Secondly, He also states that I need to indicate the existing load on existing panels that are gaining circuits. I understand why he would want this(to show that I am not overloading the panels) but how can I figure this out when half the circuits are being removed in the field and most are just receptacle ciruits anyways.
Thank you for any help, I always receive great information from this site.

 

[DanDel]

How did you arrive at 74426A as the fault current? A quick calculation for the maximum possible fault current from a 300kVA transformer (even using a transformer %Z of 2, which is very low) is 41636A. Typical impedance for this size transformer is 4.8%, which would give you an absolute maximum fault current of 17348A.
The let-through current is what fault current is left after being (theoretically) limited by any current-limiting devices. This is not a quick analysis, and I don't recommend this as something that you should try to perform.
There must have been some loading calculations performed originally in the design to help determine what size equipment needs to be installed. An estimate of what is being used minus what is being lost(maybe based on the NEC for lighting loads) is all he needs.

 

[tolind]

According to the Cooper Bussman Electrcial Protection Handbook - table 1 p48 - a 300KVA 3phase Transformer has a %Z of 1.11. This sets the Let thru at 83,357A. This is a worse case value but I can't find any more data on this. What litrature did you find the 4.8 value in??

Do you have a reference where I can find the analysis just so I can look at it later to help my own knowledge?

All the equipment(for the most part) is existing. It was done by another firm years ago and we have no way of seeing what they designed. What do you mean by being lost?? Sorry but I'm not to familiar with this.

Thanks again

 

[wareagle]

IEEE Buff Book on protection lists a 300 kva with 4.5% min.
The "load lost" I think is the circuits you said were being removed.

 

[DanDel]

The best way is to get the %Z from the transformer nameplate. Are you sure that 1.11%Z wasn't for a 1-phase 300kVA transformer?
wareagle's comments about the IEEE Buff Book (Std 242) for a reference and lost loads are accurate.