Short Circuit Current

Short Circuit Current

The National Electrical Code stipulates that equipment like fuses and circuit breakers intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage and current which is available at the line side terminals of the equipment. See NEC 110-9

Further that service equipment shall be suitable for the short circuit current available at its supply terminals.

The intent of these sections is to require the use of equipment which is so designed for its installation location that when a short circuit condition exists, neither the overcurrent protection device with an appropriate current limiting ability or other circuit components will be damaged.

Therefore the specifications of current limiting overcurrent protection is an essential consideration when designing, specifying, and installing electrical equipment. 

The calculation to arrive at available short circuit current for a specific installation is now typically required to be submitted with a riser diagram when a service change or installation is to be made.

The method shown here is referred to as the point-to-point method. There are others to include an Omic Method, a Per-Unit Method, TRON registered trademark Computer Software Method. All are to arrive at I s.c. sym RMS available short circuit symmetrical RMS current at the point of fault.

Click HERE to see the entire Cooper Bussmann pdf file booklet which explains this point to point method as well as all other methods..

Find a request for information form letter HERE.

Find the worksheet to make a calculation HERE.

Find check values for this calculation HERE.

Find a table with values for C HERE.

 

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