KAIC of Panel

[tryinghard]

If I have a 15kva 480-120/240 cu transformer feeding a panel 10? away with #10 cu in RMC what would a likely KAIC be?

I?m looking at a Fault Current Calculator from Mike Holts ?Free Stuff? and I don?t think I understand how to use the impedance %?

 

[benaround]

Tryinghard,

Wouldn't it depend on how close to the main service distribution that it was located?

 

[don_resqcapt19]

The secondary rated current divided by the impedance in decimal form will give you the maximum available fault current. This calculation assumes an infinite primary current and will be the worst case. There will also be a reduction in the current available at the panel as a result of the impedance of the consuctors between the transformer and the panel.
The #10s from the transformer to the panel seem too small for a 15KVA single phase transformer.

 

[tryinghard]

Yah 8's not 10's; so 15kva secondary 240v rated current would be 36a and if the impedance is 2% the available fault current would be up to 1800 AIC, is this correct?

If I use the attached calculator #8's at 15' I get 1713 AIC. A lower impedance raises the AIC, is this correct?

 

[tryinghard]

Tryinghard,

Wouldn't it depend on how close to the main service distribution that it was located?

Maybe but this 120/240 panel is fed from the 15kva only 10' away, the 15kva is fed by a 480v 20a circuit.

 

[benaround]

Tryinghard,

For 480to240/120 15kva single phase I get 31a pri and 62a sec. 62/.02=3100 ?

 

[ron]

The likely rating of the downstream panelboard is 10kAIC. The fault current will never be higher than 3125 (guessing that the transformer impedance is 2%), even if the utility generators where located adjacent to the transformer primary, thus the infinite primary method.

 

[jim dungar]

Just for the record.

The term AIC means Amps Interrupting Capacity it is a term that only applies to protective devices that 'break' current (i.e. fuses or breakers). Equipment that does not interrupt fault current (i.e. conductors, panels, and VFDs) has short circuit current ratings (SCCR).

You have been discussing how to calculate the amount of available short circuit amps (SCA).

 

[tryinghard]

So the L-L is all that’s used in the calc not sq rt of 3 even though it’s 3ph (15kva/240v/2%), got it.

I may be misusing the acronym AIC but only because it is what I hear the vendors question/refer to for panel ratings.

So does the Available Fault Current Calculator work for this type of application?

 

[jim dungar]

I may be misusing the acronym AIC but only because it is what I hear the vendors question/refer to for panel ratings.

That does not make it a good practice.

Asking what SCA you have or what SCCR equipment needs is like asking how big a load you want a conductor to carry. Asking what AIC is needed is like asking only what gauge conductor you want.

Would you ask a vendor for a 16A conductor?

 

[tryinghard]

That does not make it a good practice.

Asking what SCA you have or what SCCR equipment needs is like asking how big a load you want a conductor to carry. Asking what AIC is needed is like asking only what gauge conductor you want.

Would you ask a vendor for a 16A conductor?

No, not for this project anyway I think the vendors ask about the AIC rating of a panel actually meaning the breakers.

 

[don_resqcapt19]

So the L-L is all that?s used in the calc not sq rt of 3 even though it?s 3ph (15kva/240v/2%), got it. ...

You said 120/240 in the original post. That is a single phase voltage, not a 3 phase voltage. If you have 208Y/120 which is a 3 phase voltage you would use the square root of 3 in the current calculation.

 

[tryinghard]

You said 120/240 in the original post. That is a single phase voltage, not a 3 phase voltage. If you have 208Y/120 which is a 3 phase voltage you would use the square root of 3 in the current calculation.

Sorry about that it's 3 phase delta secondary, so would it be up to 1804?

 

[Pierre C Belarge]

Breakers without identification are rated at 5k. The minimum rating I have seen that is identified is 10k.
The rating should not be just the breakers, the enclosure and bus should also have the same rating.

 

[tryinghard]

Terminology; I suppose the best way to refer to the rating of a panel is "Short-Circuit Amps", this seems to encapsulate everything like breakers, bracing & whatever?

I think I?ve seen 2 KAIC breakers; anyway I just want to understand the terminology better. Mike Holt?s site also has a Fault Current doc to help me here, I don't think I'm using the spreadsheet calculator correctly anyway.

 

[tryinghard]

I do notice lower impedance raises the short circuit amps, if one stresses $ then high impedance is cheaper for distribution. Is it still most prudent to get low impedance on a transformer?

 

[ron]

tryinghard,
Higher XFMR impedance impacts the one time purchase price, but effects the long term losses that you pay everyday.

 

[tryinghard]

Yah, probably so :roll: