Available Fault Current

[JdoubleU]

We have a building with 12,470 volt switch gear in it. A contractor is putting in a new chiller in with the gear. They are replacing an old one. The electricians want to know the Available fault current of the gear. Would someone be able to help me with the steps in getting this information to the contractors

 

[richxtlc]

The available short circuit current to the gear would only change if the contribution from the chiller motor is different then that of the old one. Short circuit current is a function of the available short circuit current from the utility, the impedance of the transformer supplying the facility, the size, length, impedance of the cable supplying the gear, the type of conduit the cable is in, and the settings of the OCPDs, and the speed of the circuit breakers.

 

[augie47]

The rating of the "gear" itself should be available from the gear manufacturer.
Determining the available fault current supplying the gear is an engineering function. If it is supplied by the utility, the utilty should be able to give you the current at their point of supply.
Given that number you can apply the factors that reduce the AIC (length on runs, wire sizes, etc; and current limitimng devices) and factors that can increase the AIC (such as motor contribution).
If the gear in question is fed by a transformer, if you can supply the transformer data (voltage, kva, impedance, etc) someone here can give you a "round number" , but exact AIC levels should be a product of an engineering study.,

(sorry richxtlc , I was posting on top of you.)

 

[jim dungar]

The Short circuit current is a function of .... and the settings of the OCPDs, and the speed of the circuit breakers.

Unless a protective device is operating in a current limiting region, its settings and speed do not impact the available short circuit current. Without plotting a specific TCC, it is a fairly common practice to ignore any 'protective device' current limiting effect when determining the SCCR rating of equipment.

 

[Jraef]

We have a building with 12,470 volt switch gear in it. A contractor is putting in a new chiller in with the gear. They are replacing an old one. The electricians want to know the Available fault current of the gear. Would someone be able to help me with the steps in getting this information to the contractors

You have a few issues to clear up here with the term "Available fault current of the gear". AFC would refer to the SYSTEM, not the gear. The AFC of the system is determined as explained above, but that says nothing about the gear.

The GEAR would have an SCCR (Short Circuit Current Rating) and needs to meet or exceed the AFC of the system. If what they want to know is the SCCR rating of the gear, that is what you will need to contact the mfr about.

If the equipment was manufactured a long time ago and has no official SCCR rating, there are rules by which it can be determined. But an inspector will need to be convinced of the validity, so I would ask in advance what he is going to accept.

 

[charlie]

. . . If it is supplied by the utility, the utility should be able to give you the current at their point of supply. . .

Verify what numbers the serving electric utility is giving you. Most of the time, they are the maximum you can expect and the resulting gear can be very expensive to meet the available fault current given. If a reduction in the available fault current makes a difference, the actual available fault current can be calculated for a moment in time. Keep in mind that all distribution systems are dynamic so be a bit conservative with the number. Also, to get the calculated number, there may be a fee for the extra work to get that number.

 

[richxtlc]

The rating of the "gear" itself should be available from the gear manufacturer.
Determining the available fault current supplying the gear is an engineering function. If it is supplied by the utility, the utilty should be able to give you the current at their point of supply.
Given that number you can apply the factors that reduce the AIC (length on runs, wire sizes, etc; and current limitimng devices) and factors that can increase the AIC (such as motor contribution).
If the gear in question is fed by a transformer, if you can supply the transformer data (voltage, kva, impedance, etc) someone here can give you a "round number" , but exact AIC levels should be a product of an engineering study.,

(sorry richxtlc , I was posting on top of you.)

not a problem.