AIC Rating
- Thread starter GSXR600
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- Logan, Utah
Keep in mind that what you have at the transformer is not AIC but available fault current or short circuit current. AIC (Amps Interrupting Capacity) is the maximum amount of current that an overcurrent device is capable of interrupting.
So in your example if you have a utility transformer that has an available fault current of 55,000 amperes and your breaker has an AIC rating of 65,000 amperes you are OK.
Now as you move from the transformer to the service you have impedance of the conductors which will reduce the available fault current at the service equipment. As you move farther down the electrical system from the service to the downstream panel your available fault current will again be reduced. So the available fault current at the panel may be low enough that you can use a lower rated OCPD.
You can download a free available fault current calculator on Mikes main page under "Free Stuff".
Now there is a way to use the upstream OCPD to protect the downstream OCPD and this is called series rating. Series ratings must meet the requirements of 240.86.
Chris
So in your example if you have a utility transformer that has an available fault current of 55,000 amperes and your breaker has an AIC rating of 65,000 amperes you are OK.
Now as you move from the transformer to the service you have impedance of the conductors which will reduce the available fault current at the service equipment. As you move farther down the electrical system from the service to the downstream panel your available fault current will again be reduced. So the available fault current at the panel may be low enough that you can use a lower rated OCPD.
You can download a free available fault current calculator on Mikes main page under "Free Stuff".
Now there is a way to use the upstream OCPD to protect the downstream OCPD and this is called series rating. Series ratings must meet the requirements of 240.86.
Chris
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The proper terms are:
Amps Interrupting Capacity (AIC)- a term that applies only to protective devices that interrupt current. The value comes from the manufacturer via testing.
Short Circuit Amps (SCA)- the amount of current that will flow as the result of a short circuit fault (some times called Short Circuit, Available). the value comes from field calculations or measurements.
Short Circuit Current Rating (SCCR) - a term that applies to equipment that does not interrupter a fault, like panels, conductors, drives, and transfer switches
So,
You have a transformer/source that can produce a SCA of 55K.
You need to determine how much the impedance of your 150' of conductor will reduce this current. You also need to see if there are other sources of fault current, like motors.
Then you compare resultant SCA to the AIC or SCCR of your equipment, to see if they are at least as large as the computed SCA where it is installed. Some people like to see a 10% safety margin.
For the vast majority of situations (maybe 99.99% of the time), you should not consider that an upstream device does anything to limit the amount of fault current to downstream protective devices.
Unless you have had special training or are working under UL508A standards, the series-combination of protective devices should only be done by actual manufacturer testing.
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