Interrupting capacity

[Juanvaldez]

I have a new 45 kva transformer being feed off an old existing GE 480V panel. Old Gear does not list AIC on it, but plans show it as having 14000 AIC. Am I OK putting a new 10K AIC GE panel off this transformer, or does it all need to be 14K AIC?

 

[augie47]

You would have to calculate the AIC taking the transformer impedance into account, but I'ved never seen a 45 kva transformer with an AIC anywhere clsoe to 10K.

 

[jim dungar]

Actually you calculate the amount of short circuit current available (SCA)from the transformer, then you choose an AIC rating that exceeds that value.

Do you know the impedance of your transformer?

But, I agree with Augie, it is all but improbable that you would have a 45kVA transformer with a 208Y/120 secondary that would require equipment to have an AIC rating >10kA.

 

[augie47]

Actually you calculate the amount of short circuit current available (SCA)from the transformer, then you choose an AIC rating that exceeds that value.

Do you know the impedance of your transformer?

But, I agree with Augie, it is all but improbable that you would have a 45kVA transformer with a 208Y/120 secondary that would require equipment to have an AIC rating >10kA.

I should have waited
Try as you may, you just can't teach this ole dog the proper terminology.
Thanks for ignoring it.

 

[jim dungar]

Thanks for ignoring it.

Merry Christmas.

 

[LarryFine]

Juan, welcome to the forum!

The conductors from the transformer to the gear also add a bit of impedance, which also lowers the available current at those terminals.

 

[Jraef]

We are Ass-u-me-ing that by "off this transformer" you meant on the load (low voltage) side.

Quick, dirty and rough* estimate using the "infinite bus" method, which means we ignore upstream issues and assume unlimited SCA on the line side.

45kVA
Ass-u-me-ing 120/208V secondary
Ass-u-me-ing 5.75% Z
Ass-u-me-ing std efficiency

208V FLA = 125A {45kVA/(.208 x 1.732)}
SCA = (125 x 100) / 5.75
SCA, worst case scenario, = 2,173A

Motor contribution may take it up a notch, but not much.
Wire impedance may take it down, probably more than any motor contribution anyway.

Bottom line, not even close to 14,000A. Where did that number come from? Was that the SCA on the LINE side of the transformer? If so, the SCA on the secondary would be even lower.

*How I like my women too...

 

[jim dungar]

Ass-u-me-ing 5.75% Z

This a good assumption for transformers over 150kVA, but probably not for one this small. 75kVA and smaller are more likely around 3.5%.

If you are assuming an infinite bus on the primary there is little need to add an assumed motor load unless the motors are over 50HP. If you are trying to select an appropriate AIC rating, then ignoring the cable impedance saves a little bit of effort, with no compromise of safety.

If your results come close, say within 15%, to a device break point (i.e. assumed 15kSCA versus 14kAIC devices or 9kSCA versus 10kAIC), I suggest refining your assumptions.

 

[bluesmoke...oops]

Bottom line, not even close to 14,000A. Where did that number come from?

the 14000 is the panel being used to supply the transformer.

 

[topgone]

This a good assumption for transformers over 150kVA, but probably not for one this small. 75kVA and smaller are more likely around 3.5%.

If you are assuming an infinite bus on the primary there is little need to add an assumed motor load unless the motors are over 50HP. If you are trying to select an appropriate AIC rating, then ignoring the cable impedance saves a little bit of effort, with no compromise of safety.

If your results come close, say within 15%, to a device break point (i.e. assumed 15kSCA versus 14kAIC devices or 9kSCA versus 10kAIC), I suggest refining your assumptions.

If I may add, even if the transformer percent impedance is just 1% (granting there is such a transformer with a very low impedance), you will get a kA of 125/0.01=12.5 kA (same assumption with an infinite bus and neglecting cable impedances) --<than 14 kA, IMHO.