fault Current rating.

[JdoubleU]

I am trying to find in the code what the branch circuits and equipment fault current ratings need to be. If the Main service needs to have a rating of say 100,000 amps does this mean all the branch circuit overcurrent devices need to be that as well. Also does this mean the equipment disconnects that consist of overcurrent protection need to be that as well?

 

[augie47]

All equipment must be designed with the Available Fault Current in mind. Assuming this on large currents is most often a engineering task, but, for conversations sake you can address it various ways. The system can be "fully rated" meaning all equipment has an interrupting or withstand current rating equal to your calculated available fault current. Secondly, the system can be engineered taking into account the reduction in fault current due to the wire sizes and lengths of the runs and lastly, as far as over-current devices are concerned, the manufacturers often "series rate" components so that a lower rated (AIC) breaker can me used downstream of certain higher rated breakers (a 22k 20 amp downstream of a 400 amp 65k main for example). That method is very common.
Again, the above is just conversation. A system should be engineered taking into account all details such as motor contribution, etc.

 

[jim dungar]

110.9 says protective devices must be rated for thefault current available at their line side terminals.
110.10 says equipment and circuit characteristics must be evaluated against the available short circuit current.

So,

You need to determine what the available Short Circuit Amps (SCA) are for your installation.
Simply looking at the rating of upstream equipment is not sufficient.