scope:

This paper provides information on the following topics:

1) Description of the key functions of the overcurrent protective devices (OCPDs) used in low-voltage applications for meeting selective coordination requirements per the latest edition of the NEC [18]

2) Discussion of selective coordination application information provided by manufacturers and implications for system design

3) The importance of including both phase currents, as well as ground-fault currents, for selective coordination

4) The role of the system design engineer and the necessary interaction with applicable authorities having jurisdiction (AHJ)

5) An overview of considerations for designing selectively coordinated systems

6) Background information illustrating why overlapping of time current characteristic curves does not equate to a loss of selective coordination

Purpose

The purpose of this paper is to provide guidance to engineers regarding the National Electrical Code^® (NEC) requirements for selective coordination. This paper specifically addresses how to comply with these requirements for low-voltage circuit breakers (LVCBs). LVCBs include molded-case circuit breakers (MCCBs) listed to UL 489 and low-voltage power circuit breakers (LVPCBs) listed to UL 1066. (The MCCB category also includes insulated case circuit breakers [ICCBs].)