AIC Ampere interrupting capacity

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JoshuaTN

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Tennessee
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Facility Manager
System specs: 120/208y 3 phase- (2) 200amp 3phase panels fed from a tap box (1) straight into a panel with a 200amp main disconnect breaker. The other is a Main lugs panel fed from the generator transfer switch with built in 200amp disconnect the withstand rating is 18KA. The main breaker is 25,000 AIC. Is this acceptable I thought the short circuit value should be the same or the first breaker be the highest.

someone shed some light on this for me.

Always willing to learn.
knowledge that is given is knowledge that is passed on.
 
The AIC is just a rating of the OCPD (breaker, fuse, etc). It is what it can interrupt (safely). As long as they are greater than the available short circuit current, you're good.
 
Thanks bwat.
So where is the available short circuit current? Is that on the transformer? This system have ( 2) 400 systems ran off the same pad mounted transformer. 50kva.
 
The transformer is where you begin with your calculation as that determines the short circuit rating but depending on many factors the rating will change down the line
 
JoshuaTN said:
Thanks bwat.
So where is the available short circuit current? Is that on the transformer? This system have ( 2) 400 systems ran off the same pad mounted transformer. 50kva.
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You need to contact your local power company and ask them for the available short circuit current or the impedance of the transformer.
If this is a company owned transformer, then you need to look at its nameplate to determine its impedance.

Once you have the impedance you can calculate the worst case, called infinite bus, short circuit current. but use this value only for selecting equipment AIC ratings, DO NOT use it for arc flash calculations.

For example, assuming a 50kVA 208V secondary 3-phase transformer with a 2.5%Z impedance:
The output amps is 50000VA/(208V*1.732)=139A
The infinite bus short circuit amps would be 139A/.025 = 5.6kA

My guess is that the 25kAIC for the main breaker is a boilerplate specification, although it could be a utility requirement, in which case you are stuck with it.
 
If I'm understanding this correctly; you have two opportunities here: you have the short circuit current contribution from the utility and the short circuit current contribution from the generator.

If your generator will never be in parallel with the utility then the ratings will need to be greater than either the generator short circuit contribution or the utility contribution; whichever is greater. If they can be in parallel, then the rating must be greater than the two contributions combined.

The maximum available fault current (Isc) for the utility will be found by taking the transformer KVA divided by its impedance. (See response #6) This will give you the short circuit MVA. Then simply divided by your voltage and sqrt 3. The generator contribution is a little trickier because you will need to know the sub transient reactance of the machine; Xd".
 
kingpb said:
If I'm understanding this correctly; you have two opportunities here: you have the short circuit current contribution from the utility and the short circuit current contribution from the generator.

If your generator will never be in parallel with the utility then the ratings will need to be greater than either the generator short circuit contribution or the utility contribution; whichever is greater. If they can be in parallel, then the rating must be greater than the two contributions combined.

The maximum available fault current (Isc) for the utility will be found by taking the transformer KVA divided by its impedance. (See response #6) This will give you the short circuit MVA. Then simply divided by your voltage and sqrt 3. The generator contribution is a little trickier because you will need to know the sub transient reactance of the machine; Xd".
Click to expand...

The generator and transfer switch came as a matched set. If I understand correctly. If I have a higher short circuit capacity like the 25. I have on the breaker now. instead of lets say a 10 that is better. example: the transformer is a 50. The disconnect has a 25. T he main panel has a 25 then the subpanel after that has a 10. (of course I would use main lugs on any subs in real life but this is just a line up.) high to low good, Low to high bad.
am I getting this right.
 
Not sure I'd say it bad, but maybe just wasted (you bought equipment with a higher AIC rating than needed). That being said, the available fault current reduces as you go through wires. More wire length and smaller wire size both start nipping away at the available fault current and reducing it. So yes, the higher AIC rated stuff should be closest to the power source since the available fault current is highest there. It can't get any higher downstream unless you have another transformer in which case you need you recalculate things from that point.
 
Yes, the AFC drops with distance, A LITTLE. But as a general rule, ALL equipment must be rated for the AFC in the system. So if the AFC is only 5.6kA, then anything 10kAIC or greater is fine, it makes no difference if some is 10kAIC, some is 18kAIC or some is 25kAIC, because they are all greater than 5.6kA.

Now, if your AFC was let's say, 18kA, then your 10kAIC rated devices would NOT be acceptable. UNLESS they have a "series rating" from the manufacturers to be used in conju8nction with that SPECIFIC main breaker at a level that is more than 18kAIC. That's actually not uncommon, but you MUST be able to document it with data from the mfr.
 
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