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Wednesday, May 17, 2023
Question:
Hi CQD and thank you for providing great information in this forum. We are having some discussion regarding selective coordination and emergency system loads in a commercial office space. To what time level is selective coordination required? Some of the engineers I work with say selective coordination is only required to a OCPD clearing time of .01 seconds and others say it is only required to be selectively coordinated to a OCPD clearing time of .1 seconds. What is required by the
NEC®? Thanks in advance, Guy Rollins
A
Answer:
Hello Guy, thank you for your kind words and participating in the CQD. Guidance on this can be found in the definition of Selective Coordination found in Article 100 of the NEC®. Selective Coordination is achieved by choosing OCPDs that will restrict outages for the full range of available fault currents and the full range of OCPD clearing/opening times. There is no specific clearing time specified in the NEC® for emergency systems, meaning the OCPDs will need to be Selectively Coordinated for the full range of clearing times.