Fault current calculation

[mightyjoe]

Trying to figure out available fault current for a transformer. Utility has not responded to any of my calls.
Transformer 500KVA,208/120,3ph,4wire,assuming Z=1.2%
500K/(208x1.73x.012)=115792
Existing switch gear is all fused and want to set an MDP within 50'.
Am I correct in the fact breakers must have a 100KAIC rating, without talking to utility.

 

[dkarst]

Trying to figure out available fault current for a transformer. Utility has not responded to any of my calls.
Transformer 500KVA,208/120,3ph,4wire,assuming Z=1.2%
500K/(208x1.73x.012)=115792
Existing switch gear is all fused and want to set an MDP within 50'.
Am I correct in the fact breakers must have a 100KAIC rating, without talking to utility.

What you have calculated is termed an "infinite source" fault calculation, i.e. assumed the source on the primary side of this transformer is capable of providing in infinite amount of current. In reality, of course the source has finite capability and this source impendance can be combined with the transformer impedance to calculate the fault current. What you have done should be worst case but I'm also wondering how you arrived at the transformer %Z since the utility apparently did not provide it. You need to be careful with assumptions as if for example you had assumed 2.5%Z and the utility later replaced the transformer with 1.5%, you'd be in deep trouble.
As it stands, given your result, the 100kAIC equipment is insufficient since your fault current is > 100kA. In reality, you MAY be under 100kA fault current but you need more information on the source impedance, confirm the transformer %Z (and potentially tolerance), and add the 50' of conductor from transformer to complete your work. You really need this answer from the utility.

 

[bob]

You have calculated the max fault current assuming an infinite source. Of course, the source is not infinite and the % impedance seems low to me. I would think it would be 3 t o 5 % but check to be sure. 3% = 46 ka 5% = 28 ka.

 

[jim dungar]

If the utility is not responding how do you know the transformer is 500kVA?

There is nothing drastically wrong with your infinite bus SC value, but it is at the transformer terminals, have you really need to calculate what it will be at the end of your 50' of conductors.